Absolute
Pressure: Gage pressure plus atmospheric pressure.
Absolute
Zero: Temperature at which thermal energy is at a minimum.
Defined as 0 Kelvin, calculated to be -273.15°C or -459.67°F.
ac:
Alternating current; an electric current that reverses its direction
at regularly recurring intervals.
Acceleration:
A change in the velocity of a body or particle with respect
to time. The parameter that an accelerometer measures (dv/dt).
Units expressed in "g".
Accelerometer:
A device which converts the effects of mechanical motion into
an electrical signal that is proportional to the acceleration
value of the motion. A sensor. A transducer.
Accuracy:
The closeness of an indication or reading of a measurement device
to the actual value of the quantity being measured. Usually
expressed as ± percent of full scale output or reading.
Acoustics:
The degree of sound. The nature, cause, and phenomena of the
vibrations of elastic bodies; which vibrations create compressional
waves or wave fronts which are transmitted through various media,
such as air, water, wood, steel, etc.
Activity
(ai): A thermodynamic term for the apparent or active concentration
of a free ion in solution. It is related to concentration by
the activity coefficient.
Activity
Coefficient (fi): A ratio of the activity of species i(ai)
to its molality (C). It is a correction factor which makes the
thermodynamic calculations correct. This factor is dependent
on ionic strength, temperature, and other parameters.
Individual
ionic activity coefficients, f+ for cation and f- for an anion,
cannot be derived thermodynamically. They can be calculated
only by using the Debye-Huckel law for low concentration solutions
in which the interionic forces depend primarily on charge, radius,
and distribution of the ions and on the dielectric constant
of the medium rather than on the chemical properties of the
ions.
Mean ionic
activity coefficient (f±) or the activity of a salt, on
the other hand, can be measured by a variety of techniques such
as freezing point depression and vapor pressure as well as paired
sensing electrodes. It is the geometric mean of the individual
ionic activity coefficients: f± = (f+n+f-n-)1/n
Adapter:
A mechanism or device for attaching non-mating parts.
ADC:
Analog-to-Digital Converter: an electronic device which converts
analog signals to an equivalent digital form, in either a binary
code or a binary-coded-decimal code. When used for dynamic waveforms,
the sampling rate must be high to prevent aliasing errors from
occurring.
Address:
The label or number identifying the memory location where a
unit of information is stored.
Aliasing:
If the sample rate of a function (fs) is less than two times
the highest frequency value of the function, the frequency is
ambiguously presented. The frequencies above (fs/2) will be
folded back into the lower frequencies producing erroneous data.
Alloy
11: A compensating alloy used in conjunction with pure copper
as the negative leg to form extension wire for platinum-platinum
rhodium thermocouples Types R and S.
Alloy
200/226: The combination of compensating alloys used with
tungsten vs. tungsten 26% rhenium thermocouples as extension
cable for applications under 200°C.
Alloy
203/225: The combination of compensating alloys used with
tungsten 3% rhenium vs. tungsten 150 rhenium thermocouples as
extension cable for applications under 200°C.
Alloy
405/426: The combination of compensating alloys used with
tungsten 5% rhenium vs. tungsten 26% rhenium thermocouples as
extension cable for applications under 870°C.
ALOMEGA®
An aluminum nickel alloy used in the negative leg of a type
K thermocouple (registered trademarks of OMEGA ENGINEERING,
INC.).
Alphanumeric:
A character set that contains both letters and digits.
ALU:
Arithmetic Logic Unit. The part of a CPU where binary data is
acted upon with mathematical operations.
Alumel:
An aluminum nickel alloy used in the negative leg of a Type
K thermocouple (Trade name of Hoskins Manufacturing Company).
Ambient
Compensation: The design of an instrument such that changes
in ambient temperature do not affect the readings of the instrument.
Ambient
Conditions: The conditions around the transducer (pressure,
temperature, etc.).
Ambient
Pressure: Pressure of the air surrounding a transducer.
Ambient
Temperature: The average or mean temperature of the surrounding
air which comes in contact with the equipment and instruments
under test.
Ammeter:
An instrument used to measure current.
Ampere
(amp): A unit used to define the rate of flow of electricity
(current) in a circuit; units are one coulomb (6.28 x 1018 electronics)
per second.
Amplifier:
A device which draws power from a source other than the input
signal and which produces as an output an enlarged reproduction
of the essential features of its input.
Amplitude
Span: The Y-axis range of a graphic display of data in either
the time or frequency domain. Usually a log display (dB) but
can also be linear.
Amplitude:
A measurement of the distance from the highest to the lowest
excursion of motion, as in the case of mechanical body in oscillation
or the peak-to-peak swing of an electrical waveform.
Analog
Output: A voltage or current signal that is a continuous
function of the measured parameter.
Analog-to-Digital
Converter (A/D or ADC): A device or circuit that outputs
a binary number corresponding to an analog signal level at the
input.
Anemometer:
An instrument for measuring and/or indicating the velocity of
air flow.
Angstrom:
Ten to the minus tenth meters (10-10) or one millimicron, a
unit used to define the wave length of light. Designated by
the symbol ‰.
Angular
Frequency: The motion of a body or a point moving circularly,
referred to as the circular frequency O which is the frequency
in cycles per second (cps) multiplied by the term (2) and expressed
in radians per second (2pf).
Anion:
A negatively charged ion (Cl-, NO3-, S2- etc.)
ANSI:
American National Standards Institute.
Anti-reset
Windup: This is a feature in a three-mode PID controller
which prevents the integral (auto reset) circuit from functioning
when the temperature is outside the proportional band.
Application
Program: A computer program that accomplishes specific tasks,
such as word processing.
ASCII:
American Standard Code for Information Interchange. A seven
or eight bit code used to represent alphanumeric characters.
It is the standard code used for communications between data
processing systems and associated equipment.
ASME:
American Society of Mechanical Engineers.
Assembler:
A program that translates assembly language instructions into
machine language instructions.
Assembly
Language: A machine oriented language in which mnemonics
are used to represent each machine language instruction. Each
CPU has its own specific assembly language.
ASTM:
American Society for Testing and Materials.
Asymmetry
Potential: The potential developed across the glass membrane
with identical solutions on both sides. Also a term used when
comparing glass electrode potential in pH 7 buffer.
Asynchronous:
A communication method where data is sent when it is ready without
being referenced to a timing clock, rather than waiting until
the receiver signals that it is ready to receive.
ATC:
Automatic temperature compensation.
Auto-Zero:
An automatic internal correction for offsets and/or drift at
zero voltage input.
Automatic
Reset: 1. A feature on a limit controller that automatically
resets the controller when the controlled temperature returns
to within the limit bandwidth set. 2. The integral function
on a PID controller which adjusts the proportional bandwidth
with respect to the set point to compensate for droop in the
circuit, i.e., adjusts the controlled temperature to a set point
after the system stabilizes.
AWG:
American Wire Gage.
Axis
of Rotation (Spin Axis): The axis of rotation (spin axis)
is that straight line about which a body rotates.
Background
Noise: The total noise floor from all sources of interference
in a measurement system, independent of the presence of a data
signal.
Backup:
A system, device, file or facility that can be used as an alternative
in case of a malfunction or loss of data.
Bandwidth:
A symmetrical region around the set point in which proportional
control occurs.
Basic
Transportation Reference: The basic transportation section
of the U.S. Government Test Specification MIL-STD-810D, Method
514.3, Paragraph I-3.2.1, Page 514.3-5. Basic transportation
defines the test profiles that have been defined for equipment
that is shipped as secured cargo; by land, by sea or by air.
The test levels are based upon land transport stress levels
because these are higher than stresses imposed by air or sea
transportation environments.
Basic:
A high-level programming language designed at Dartmouth College
as a learning tool. Acronym for Beginner's All-purpose Symbolic
Instruction Code.
Baud:
A unit of data transmission speed equal to the number of bits
(or signal events) per second; 300 baud = 300 bits per second.
BCD,
Buffered: Binary-coded decimal output with output drivers,
to increase line-drive capability.
BCD,
Parallel: A digital data output format where every decimal
digit is represented by binary signals on four lines and all
digits are presented in parallel. The total number of lines
is 4 times the number of decimal digits.
BCD,
Serial: A digital data output format where every decimal
digit is represented by binary signals on four lines and up
to five decimal digits are presented sequentially. The total
number of lines is four data lines plus one strobe line per
digit.
BCD,
Three-State: An implementation of parallel BCD, which has
0, 1 and high-impedance output states. The high-impedance state
is used when the BCD output is not addressed in parallel connect
applications.
Bearing:
A part which supports a journal and in which a journal revolves.
Beat
Frequency: Beat frequencies are periodic vibrations that
result from the addition and subtraction of two or more sinusoids.
For example, in the case of two turbine aircraft engines that
are rotating at nearly the same frequency but not precisely
at the same frequency; Four frequencies are generated:(f1) the
rotational frequency of turbine one, (f2) the rotational frequency
of turbine two, (f1 + f2) the sum of turbine rotational frequencies
one and two, and (f1 - f2) which is the difference or "beat"
frequency of turbines one and two. The difference of the two
frequencies is the lower frequency and is the one that is "felt"
as a beat or "wow" in this case.
Beryllia:
BeO (Beryllium Oxide) A high-temperature mineral insulation
material; toxic when in powder form.
Best
Fit Straight Line (BFSL): A line midway between two parallel
straight lines enclosing all output vs. pressure values.
Best Straight
Line (BSL): Terminal Base (TB) non linearity divided by 2, based
on the output deviation at the mid point.
Beta
Ratio: The ratio of the diameter of a pipeline constriction
to the unconstricted pipe diameter.
BIAS
Current: A very low-level DC current generated by the panel
meter and superimposed on the signal. This current may introduce
a measurable offset across a very high source impedance.
Binary
Coded Decimal (BCD): The representation of a decimal number
(base 10, 0 through 9) by means of a 4 bit binary nibble.
Binary:
Refers to base 2 numbering system, in which the only allowable
digits are 0 and 1. Pertaining to a condition that has only
two possible values or states.
BIOS:
Acronym for basic input/output system. The commands used to
tell a CPU how it will communicate with the rest of the computer.
Bipolar:
The ability of a panel meter to display both positive and negative
readings.
Bit:
Acronym for binary digit. The smallest unit of computer information,
it is either a binary 0 or 1.
Blackbody:
A theoretical object that radiates the maximum amount of energy
at a given temperature, and absorbs all the energy incident
upon it. A blackbody is not necessarily black. (The name blackbody
was chosen because the color black is defined as the total absorption
of light energy.)
BNC:
A quick disconnect electrical connector used to inter-connect
and/or terminate coaxial cables.
Boiling
Point: The temperature at which a substance in the liquid
phase transforms to the gaseous phase; commonly refers to the
boiling point of water which is 100°C (212°F) at sea
level.
BPS:
Bits per second.
Breakdown
Voltage Rating: The dc or ac voltage which can be applied
across insulation portions of a transducer without arcing or
conduction above a specific current value.
Bridge
Resistance: See Input impedance and Output impedance.
BTU:
British thermal units. The quantity of thermal energy required
to raise one pound of water at its maximum density, 1 degree
F. One BTU is equivalent to .293 watt hours, or 252 calories.
One kilowatt hour is equivalent to 3412 BTU.
Buffer
Capacity (B): A measure of the ability of the solution to
resist pH change when a strong acid or base is added.
Buffer:
1. A storage area for data that is used to compensate for a
speed difference, when transferring data from one device to
another. Usually refers to an area reserved for I/O operations,
into which data is read, or from which data is written.2. Any
substance or combination of substances which, when dissolved
in water, produces a solution which resists a change in its
hydrogen ion concentration on the addition of an acid or alkali.
Bulb
(Liquid-in-Glass Thermometer): The area at the tip of a
liquid-in-glass thermometer containing the liquid reservoir.
Burn-In:
A long term screening test (either vibration, temperature or
combined test) that is effective in weeding out infant mortalities
because it simulates actual or worst case operation of the device,
accelerated through a time, power, and temperature relationship.
Burst
Pressure: The maximum pressure applied to a transducer sensing
element or case without causing leakage.
Burst
Proportioning: A fast-cycling output form on a time proportioning
controller (typically adjustable from 2 to 4 seconds) used in
conjunction with a solid state relay to prolong the life of
heaters by minimizing thermal stress.
Bus:
Parallel lines used to transfer signals between devices or components.
Computers are often described by their bus structure (i.e.,
S-100, IBM PC).
Byte:
The representation of a character in binary. Eight bits.
Calender-van
Dusen Equation: An equation that defines the resistance-temperature
value of any pure metal that takes the form of RT = RO(1 + AT
+ BT2) for values between the ice point (0°C) and the freezing
point of antimony (630.7°C) and the form RT = RO[1 + AT
+ BT2 + C(T-100)T2] between the oxygen point (-183.0°C)
and the ice point (0°C).
Calibration:
The process of adjusting an instrument or compiling a deviation
chart so that its reading can be correlated to the actual value
being measured.
Calorie:
The quantity of thermal energy required to raise one gram of
water 1°C at 15°C.
Cation:
A positively charged ion (Na+, H+).
Cavitation:
The boiling of a liquid caused by a decrease in pressure rather
than an increase in temperature.
Celsius
(centrigrade): A temperature scale defined by 0°C at
the ice point and 100°C at boiling point of water at sea
level.
Center
of Gravity (Mass Center): The center of gravity of a body
is that point in the body through which passes the resultant
of weights of its component particles for all orientations of
the body with respect to a uniform gravitational field.
Centripetal
Force: A force exerted on an object moving in a circular
path which is exerted inward toward the center of rotation.
Ceramic
Insulation: High-temperature compositions of metal oxides
used to insulate a pair of thermocouple wires The most common
are Alumina (Al2O3), Beryllia (BeO), and Magnesia (MgO). Their
application depends upon temperature and type of thermocouple.
High-purity alumina is required for platinum alloy thermocouples.
Ceramic insulators are available as single and multihole tubes
or as beads.
Ceramic:
Polycrystalline ferroelectric materials which are used as the
sensing units in piezoelectric accelerometers. There are many
different grades, all of which can be made in various configurations
to satisfy different design requirements.
CFM:
The volumetric flow rate of a liquid or gas in cubic feet per
minute.
Character:
A letter, digit or other symbol that is used as the representation
of data. A connected sequence of characters is called a character
string.
Charge
Sensitivity: For accelerometers that are rated in terms
of charge sensitivity, the output voltage (V)is proportional
to the charge (Q) divided by the shunt capacitance (C). This
type of accelerometer is characterized by a high output impedance.
The sensitivity is given in terms of charge; picocoulombs per
unit of acceleration (g).
Chatter:
The rapid cycling on and off of a relay in a control process
due to insufficient bandwidth in the controller.
CHROMEGA®:
A chromium-nickel alloy which makes up the positive leg of type
K and type E thermocouples (registered trademarks of OMEGA ENGINEERING,
INC.).
Clear:
To restore a device to a prescribed initial state, usually the
zero state.
Clipping:
The term applied to the phenomenon which occurs when an output
signal is limited in some way by the full range of an amplifier,
ADC or other device. When this occurs, the signal is flattened
at the peak values, the signal approaches the shape of a square
wave, and high frequency components are introduced. Clipping
may be hard, as is the case when the signal is strictly limited
at some level; or it may be soft, in which case the clipping
signal continues to follow the input at some reduced gain.
Clock:
The device that generates periodic signals for synchronization.
Closeness
of Control: Total temperature variation from a desired set
point of system. Expressed as "closeness of control" is ±2°C
or a system bandwidth with 4°C, also referred to as amplitude
of deviation.
CMR (Common-Mode
Rejection): The ability of a panel meter to eliminate the
effect of AC or DC noise between signal and ground. Normally
expressed in dB at dc to 60 Hz. One type of CMR is specified
between SIG LO and PWR GND. In differential meters, a second
type of CMR is specified between SIG LO and ANA GND (METER GND).
CMV (Common-Mode
Voltage): The AC or DC voltage which is tolerable between
signal and ground. One type of CMV is specified between SIG
LO and PWR GND. In differential meters, a second type of CMV
is specified between SIG HI or LO and ANA GND (METER GND).
Coherence
Function.: A frequency domain function computed to show
the degree of a linear, noise-free relationship between a system's
input and output. The value of the coherence function ranges
between zero and one, where a value of zero indicates there
is no causal relationship between the input and the output.
A value of one indicates the existence of linear noise-free
frequency response between the input and the output.
Color
Code: The ANSI established color code for thermocouple wires
in the negative lead is always red. Color Code for base metal
thermocouples is yellow for Type K, black for Type J, purple
for Type E and blue for Type T.
Common
Mode Rejection Ratio: The ability of an instrument to reject
interference from a common voltage at its input terminals with
relation to ground. Usually expressed in db (decibels).
Common
Mode: The output form or type of control action used by
a temperature controller to control temperature, i.e. on/off,
time proportioning, PID.
Communication:
Transmission and reception of data among data processing equipment
and related peripherals.
Compensated
Connector: A connector made of thermocouple alloys used
to connect thermocouple probes and wires.
Compensating
Alloys: Alloys used to connect thermocouples to instrumentation.
These alloys are selected to have similar thermal electric properties
as the thermocouple alloys (however, only over a very limited
temperature range).
Compensating
Loop: Lead wire resistance compensation for RTD elements
where an extra length of wire is run from the instrument to
the RTD and back to the instrument, with no connection to the
RTD.
Compensation:
An addition of specific materials or devices to counteract a
known error.
Compiler:
A program that translates a high-level language, such as Basic,
into machine language.
Complex
Function: Any mathematically defined relationship given
by the following expression:
y(x) = a(x) + ib(x)
Where: x = the real variable
a(x) = the real part of y(x)
b(x) = the imaginary part of y(x)
Complex functions
are usually expressed in terms of both their amplitude and phase.
Complex
Wave: The resultant form of a number of sinusoidal waves
that are summed together forming a periodic wave. Such waves
may be analyzed in the frequency domain to readily determine
their component parts.
Conductance:
The measure of the ability of a solution to carry an electrical
current. (See Equivalent Conductance)
Conduction:
The conveying of electrical energy or heat through or by means
of a conductor.
Confidence
Level: The range (with a specified value of uncertainty,
usually expressed in percent) within which the true value of
a measured quantity exists.
Conformity
Error: For thermocouples and RTDs, the difference between
the actual reading and the temperature shown in published tables
for a specific voltage input.
Connection
Head: An enclosure attached to the end of a thermocouple
which can be cast iron, aluminum or plastic within which the
electrical connections are made.
Constantan:
A copper-nickel alloy used as the negative lead in Type E, Type
J, and Type T thermocouples.
Continuous
Spectrum: A frequency spectrum that is characterized by
non-periodic data The spectrum is continuous in the frequency
domain and is characterized by an infinite number of frequency
components.
Control
Character: A character whose occurrence in a particular
context starts, modifies or stops an operation that effects
the recording, processing, transmission or interpretation of
data.
Control
Mode: The output form or type of control action used by
a temperature controller to control temperature, i.e., on/off,
time proportioning, PID.
Control
Point: The temperature at which a system is to be maintained.
Convection:
1. The circulatory motion that occurs in a fluid at a non-uniform
temperature owing to the variation of its density and the action
of gravity. 2. The transfer of heat by this automatic circulation
of fluid.
Coriolis
Force: A result of centripetal force on a mass moving with
a velocity radially outward in a rotating plane.
Correction
(Balancing) Plane: A plane perpendicular to the shaft axis
of a rotor in which correction for unbalance is made.
Coulomb
Sensitivity: Charge/unit acceleration, expressed in Pc/g
(charge sensitivity).
Coulomb:
A measurement of the quantity of electrical charge, usually
expressed as pico coulomb (10-12 coulombs).
Counter
Weight: A weight added to a body so as to reduce a calculated
unbalance at a desired place.
Counts:
The number of time intervals counted by the dual-slope A/D converter
and displayed as the reading of the panel meter, before addition
of the decimal point.
CPS:
Cycles per second; the rate or number of periodic events in
one second, expressed in Hertz (Hz).
CPU:
Central processing unit. The part of the computer that contains
the circuits that control and perform the execution of computer
instructions.
Critical
Damping: Critical damping is the smallest amount of damping
at which a given system is able to respond to a step function
without overshoot.
Critical
Speed: The rotational speed of the rotor or rotating element
at which resonance occurs in the system. The shaft speed at
which at least one of the "critical" or natural frequencies
of a shaft is excited.
Cryogenics:
Measurement of temperature at extremely low values, i.e., below
-200°C.
CSA:
Canadian Standards Administration.
Cure
Point: The temperature at which a normally magnetic material
goes through a magnetic transformation and becomes non-magnetic.
Current
Proportioning: An output form of a temperature controller
which provides a current proportional to the amount of control
required. Normally is a 4 to 20 milliamp current proportioning
band.
Current:
The rate of flow of electricity. The unit of the ampere (A)
defined as 1 ampere = 1 coulomb per second.
Curve
Fitting: Curve fitting is the process of computing the coefficients
of a function to approximate the values of a given data set
within that function. The approximation is called a "fit". A
mathematical function, such as a least squares regression, is
used to judge the accuracy of the fit.
Cycle
Time: The time usually expressed in seconds for a controller
to complete one on/off cycle.
Damping:
The reduction of vibratory movement through dissipation of energy.
Types include viscous, coulomb, and solid.
Data
Base: A large amount of data stored in a well-organized
manner. A data base management system (DBMS) is a program that
allows access to the information.
dB (Decibel):
20 times the log to the base 10 of the ratio of two voltages.
Every 20 dBs correspond to a voltage ratio of 10, every 10 dBs
to a voltage ratio of 3.162. For instance, a CMR of 120 dB provides
voltage noise rejection of 1,000,000/1. An NMR of 70 dB provides
voltage noise rejection of 3,162/1.
DC:
Direct current; an electric current flowing in one direction
only and substantially constant in value.
Dead
Band: 1. For chart records: the minimum change of input
signal required to cause a deflection in the pen position. 2.
For temperature controllers: the temperature band where heat
is turned off upon rising temperature and turned on upon falling
temperature expressed in degrees. The area where no heating
(or cooling) takes place.
Dead
Volume: The volume of the pressure port of a transducer
at room temperature and ambient barometric pressure.
Debug:
To find and correct mistakes in a program.
Debye-H¸ckel
Equation: Used in relating the activity coefficient (fi)
to ion strength (see Activity coefficient):where I is the ionic
strength, A and B the temperature-dependent constants (see Table
A.5), Zi the valence of the ion (i), and  the ion-size parameter
in angstroms.
Decimal:
Refers to a base ten number system using the characters 0 through
9 to represent values.
Default:
The value(s) or option(s) that are assumed during operation
when not specified.
Degree:
An incremental value in the temperature scale, i.e., there are
100 degrees between the ice point and the boiling point of water
in the Celsius scale and 180°F between the same two points
in the Fahrenheit scale.
Density:
Mass per unit of volume of a substance. I.E.: grams/cu.cm. or
pounds/cu.ft.
Derivative:
The derivative function senses the rate of rise or fall of the
system temperature and automatically adjusts the cycle time
of the controller to minimize overshoot or undershoot.
Deviation:
The difference between the value of the controlled variable
and the value at which it is being controlled.
Diaphragm:
The sensing element consisting of a membrane which is deformed
by the pressure differential applied across it.
Dielectric
Constant: Related to the force of attraction between two
opposite charges separated by a distance in a uniform medium.
Differential
Input: A signal-input circuit where SIG LO and SIG HI are
electrically floating with respect to ANALOG GND (METER GND,
which is normally tied to DIG GND). This allows the measurement
of the voltage difference between two signals tied to the same
ground and provides superior common-mode noise rejection.
Differential
Pressure: The difference in static pressure between two
identical pressure taps at the same elevation located in two
different locations in a primary device.
Differential:
For an on/off controller, it refers to the temperature difference
between the temperature at which the controller turns heat off
and the temperature at which the heat is turned back on. It
is expressed in degrees.
Digit:
A measure of the display span of a panel meter. By convention,
a full digit can assume any value from 0 through 9, a 1/2-digit
will display a 1 and overload at 2, a 3/4-digit will display
digits up to 3 and overload at 4, etc. For example, a meter
with a display span of ±3999 counts is said to be a 3-3/4 digit
meter.
Digital
Output: An output signal which represents the size of an
input in the form of a series of discrete quantities.
Digital-to-Analog
Converter (D/A or DAC): A device or circuit to convert a
digital value to an analog signal level.
DIN (Deutsche
Industrial Norm): A set of German standards recognized throughout
the world. The 1/8 DIN standard for panel meters specifies an
outer bezel dimension of 96 x 48 mm and a panel cutout of 92
x 45 mm.
DIN 43760:
The standard that defines the characteristics of a 100 ohm platinum
RTD having a resistance vs. temperature curve specified by a
= 0.00385 ohms per degree.
Discharge
Time Constant: The time required for the output-voltage
from a sensor or system to discharge 37% of its original value
in response to a zero rise time step function input. This parameter
determines a low frequency response.
Disk
Operating System (DOS): Program used to control the transfer
of information to and from a disk, such as MS DOS.
Displacement:
The measured distance traveled by a point from its position
at rest. Peak to peak displacement is the total measured movement
of a vibrating point between its positive and negative extremes.
Measurement units expressed as inches or millinches.
Dissipation
Constant: The ratio for a thermistor which relates a change
in internal power dissipation to a resultant change of body
temperature.
Dissociation
Constant (K): A value which quantitatively expresses the
extent to which a substance dissociates in solution. The smaller
the value of K, the less dissociation of the species in solution.
This value varies with temperature, ionic strength, and the
nature of the solvent.
DMA:
Acronym direct memory access. A high speed data storage mode
of the IBM PC.
Double
Precision: The degree of accuracy that requires two computer
words to represent a number. Numbers are stored with 17 digits
of accuracy and printed with up to 16 digits.
Drift:
A change of a reading or a set point value over long periods
due to several factors including change in ambient temperature,
time, and line voltage.
Droop:
A common occurrence in time-proportional controllers. It refers
to the difference in temperature between the set point and where
the system temperature actually stabilizes due to the time-proportioning
action of the controller.
Dual
Element Sensor: A sensor assembly with two independent sensing
elements.
Dual-slope
A/D Converter: An analog-to-digital converter which integrates
the signal for a specific time, then counts time intervals for
a reference voltage to bring the integrated signal back to zero.
Such converters provide high resolution at low cost, excellent
normal-mode noise rejection, and minimal dependence on circuit
elements.
Duplex
Wire: A pair of wires insulated from each other and with
an outer jacket of insulation around the inner insulated pair.
Duplex:
Pertaining to simultaneous two-way independent data communication
transmission in both direction. Same as "full duplex".
Duty
Cycle: The total time to one on/off cycle. Usually refers
to the on/off cycle time of a temperature controller.
Dynamic
(Two-Plane) Balancing Machine: A dynamic balancing machine
is a centrifugal balancing machine that furnishes information
for performing two-plane balancing.
Dynamic
Calibration: Calibration in which the input varies over
a specific length of time and the output is recorded vs. time.
Dynamic
Pressure: The difference in pressure levels from static
pressure to stagnation pressure caused by an increase in velocity.
Dynamic pressure increases by the square of the velocity.
Dynamic
Unbalance: Dynamic unbalance is that condition in which
the central principal axis is not coincident with the shaft
axis.
Echo:
To reflect received data to the sender. For example, keys depressed
on a keyboard are usually echoed as characters displayed on
the screen.
Electrical
Interference: Electrical noise induced upon the signal wires
that obscures the wanted information signal.
Electrode
Potential (E): The difference in potential established between
an electrode and a solution when the electrode is immersed in
the solution.
Electrode:
See Isopotential point.
Electrolyte:
Any substance which, when in solution will conduct an electric
current. Acids, bases, and salts are common electrolytes.
Electromotive
Force (emf): The potential difference between the two electrodes
in a cell. The cell emf is the cell voltage measured when no
current is flowing through the cell. It can be measured by means
of a pH meter with high input impedance.
Electronic
Industries Association (EIA): A standards organization specializing
in the electrical and functional characteristics of interface
equipment.
EMF:
Electromotive force. A rise in (electrical) potential energy.
The principal unit is the volt.
EMI:
Electromagnetic interference.
Emissivity:
The ratio of energy emitted by an object to the energy emitted
by a blackbody at the same temperature. The emissivity of an
object depends upon its material and surface texture; a polished
metal surface can have an emissivity around 0.2 and a piece
of wood can have an emissivity around 0.95.
End Point
(Potentiometric): The apparent equivalence point of a titration
at which a relatively large potential change is observed.
End Points:
The end points of a full scale calibration curve.
Endothermic:
Absorbs heat. A process is said to be endothermic when it absorbs
heat.
Enthalpy:
The sum of the internal energy of a body and the product of
its volume multiplied by the pressure.
Environmental
Conditions: All conditions in which a transducer may be
exposed during shipping, storage, handling, and operation.
Eprom:
Erasable Programmable Read-Only Memory. The PROM can be erased
by ultraviolet light or electricity.
Equilibrium
Constant: The product of the concentrations (or activities)
of the substances produced at equilibrium in a chemical reaction
divided by the product of concentrations of the reacting substances,
each concentration raised to that power which is the coefficient
of the substance in the chemical equation.
Equitransference:
Equal diffusion rates of the positively and negatively charged
ions of an electrolyte across a liquid junction without charge
separation.
Equivalent
Conductance (l): Equivalent conductance of an electrolyte
is defined as the conductance of a volume of solution containing
one equivalent weight of dissolved substances when placed between
two parallel electrodes 1 cm apart, and large enough to contain
between them all of the solution. l is never determined directly,
but is calculated from the specific conductance (Ls). If C is
the concentration of a solution in gram equivalents per liter,
then the concentration of a solution in gram equivalents per
liter, then the concentration per cubic centimeter is C/1000,
and the volume containing one equivalent of the solute, is,
therefore, 1000/C.
Error
Band: The allowable deviations to output from a specific
reference norm. Usually expressed as a percentage of full scale.
Error:
The difference between the value indicated by the transducer
and the true value of the measurand being sensed. Usually expressed
in percent of full scale output.
Eutectic
Temperature: The lowest possible melting point of a mixture
of alloys.
Excitation:
The external application of electrical voltage current applied
to a transducer for normal operation.
Exothermic:
Gives off heat. A process is said to be exothermic when it releases
heat.
Expansion
Factor: Correction factor for the change in density between
two pressure measurement areas in a constricted flow.
Explosion-proof
Enclosure: An enclosure that can withstand an explosion
of gases within it and prevent the explosion of gases surrounding
it due to sparks, flashes or the explosion of the container
itself, and maintain an external temperature which will not
ignite the surrounding gases.
Exposed
Junction: A form of construction of a thermocouple probe
where the hot or measuring junction protrudes beyond the sheath
material so as to be fully exposed to the medium being measured.
This form of construction usually gives the fastest response
time.
Fahrenheit:
A temperature scale defined by 32° at the ice point and
212° at the boiling point of water at sea level.
Ferrule:
A compressible tubular fitting that is compressed onto a probe
inside a compression fitting to form a gas-tight seal.
Field
Balancing Equipment: An assembly of measuring instruments
for performing balancing operations on assembled machinery which
is not mounted in a balancing machine.
Field
of View: A volume in space defined by an angular cone extending
from the focal plane of an instrument.
File:
A set of related records or data treated as a unit.
Filling
Solution: A solution of defined composition to make contact
between an internal element and a membrane or sample. The solution
sealed inside a pH glass bulb is called an internal filling
solution. This solution normally contains a buffered chloride
solution to provide a stable potential and a designated zero
potential point. The solution which surrounds the reference
electrode internal and periodically requires replenishing is
called the reference filling solution. It provides contact between
the reference electrode internal and sample through a junction.
Firmware:
Programs stored in PROMs.
Flag:
Any of various types of indicators used for identification of
a condition or event; for example, a character that signals
the termination of a transmission.
Floppy
Disk: A small, flexible disk carrying a magnetic medium
in which digital data is stored for later retrieval and use.
Flow
Rate: Actual speed or velocity of fluid movement .
Flow:
Travel of liquids or gases in response to a force (i.e. pressure
or gravity).
Flowmeter:
A device used for measuring the flow or quantity of a moving
fluid.
FM Approved:
An instrument that meets a specific set of specifications established
by Factory Mutual Research Corporation.
FM:
Factory Mutual Research Corporation. An organization which sets
industrial safety standards.
Forced
Vibration: Vibration of a system caused by an imposed force.
Steady-state vibration is an unchanging condition of periodic
or random motion.
FORTRAN:
Formula Translation language. A widely used high-level programming
language well suited to problems that can be expressed in terms
of algebraic formulas. It is generally used in scientific applications.
FPM:
Flow velocity in feet per minute.
FPS:
Flow velocity in feet per second.
Freezing
Point: The temperature at which the substance goes from
the liquid phase to the solid phase.
Frequency
Modulated Output: A transducer output which is obtained
in the form of a deviation from a center frequency, where the
deviation is proportional to the applied stimulus.
Frequency
of Vibration: The number of cycles occurring in a given
unit of time. RPM - revolutions per minute. CPM- cycles per
minute.
Frequency
Output: An output in the form of frequency which varies
as a function of the applied input.
Frequency,
Natural: The frequency of free (not forced) oscillations
of the sensing element of a fully assembled transducer.
Frequency:
The number of cycles over a specified time period over which
an event occurs. The reciprocal is called the period.
Full
Bridge: A Wheatstone bridge configuration utilizing four
active elements or strain gages.
Full
Scale Output: The algebraic difference between the minimum
output and maximum output.
Functions:
Three mode PID controller. A timeproportioning controller with
integral and derivative functions. The integral function automatically
adjusts the system temperature to the set point temperature
to eliminate droop due to the time proportioning function.
g:
The force of acceleration due to gravity equal to 32.1739 ft/sec2
or 386 in./sec2.
Gage
Factor: A measure of the ratio of the relative change of
resistance to the relative change in length of a piezoresistive
strain gage.
Gage
Length: The distance between two points where the measurement
of strain occurs.
Gage
Pressure Transducer: A transducer which measures pressure
in relation to the ambient pressure.
Gage
pressure: Absolute pressure minus local atmospheric pressure.
Gain:
The amount of amplification used in an electrical circuit.
Galvanometer:
An instrument that measures small electrical currents by means
of deflecting magnetic coils.
GPH:
Volumetric flow rate in gallons per hour.
GPM:
Volumetric flow rate in gallons per minute.
Ground:
1. The electrical neutral line having the same potential as
the surrounding earth. 2. The negative side of DC power supply.
3. Reference point for an electrical system.
Grounded
Junction: A form of construction of a thermocouple probe
where the hot or measuring junction is in electrical contact
with the sheath material so that the sheath and thermocouple
will have the same electrical potential.
Half
Bridge: Two active elements or strain gages.
Half-Duplex:
One way at a time data communication; both devices can transmit
and receive data, but only one at a time.
Handshake:
An interface procedure that is based on status/data signals
that assure orderly data transfer as opposed to asynchronous
exchange.
Hardcopy:
Output in a permanent form (usually a printout) rather than
in temporary form, as on disk or display terminal.
Hardware:
The electrical, mechanical and electromechanical equipment and
parts associated with a computing system, as opposed to its
firmware or software.
Head
Loss: The loss of pressure in a flow system measured using
a length parameter (i.e., inches of water, inches of mercury).
Head
Pressure: Pressure in terms of the height of fluid, P =
yrg, where r = fluid density and y = the fluid column heights.
Expression of a pressure in terms of the height of fluid, r
= yrg, where r is fluid density and y = the fluid column height.
g = the acceleration of gravity.
Heat
Sink: 1. Thermodynamic. A body which can absorb thermal
energy. 2. Practical. A finned piece of metal used to dissipate
the heat of solid state components mounted on it.
Heat
Transfer: The process of thermal energy flowing from a body
of high energy to a body of low energy. Means of transfer are:
conduction; the two bodies contact. Convection; a form of conduction
where the two bodies in contact are of different phases, i.e.
solid and gas. Radiation: all bodies emit infrared radiation.
Heat
Treating: A process for treating metals where heating to
a specific temperature and cooling at a specific rate changes
the properties of the metal.
Heat:
Thermal energy. Heat is expressed in units of calories or BTU's.
Hertz
(Hz): Units in which frequency is expressed. Synonymous
with cycles per second.
Hexadecimal:
Refers to a base sixteen number system using the characters
0 through 9 and A through F to represent the values. Machine
language programs are often written in hexadecimal notation.
Hold:
Meter HOLD is an external input which is used to stop the A/D
process and freeze the display. BCD HOLD is an external input
used to freeze the BCD output while allowing the A/D process
to continue operation.
Hooke's
Law: Defines the basis for the measurement of mechanical
stresses via the strain measurement. The gradient of Hooke's
line is defined by the ratio of which is equivalent to the Modulus
of Elasticity E (Young's Modulus).
Host:
The primary or controlling computer in a multiple part system.
Hydrogen
Ion Activity (aH+): Activity of the hydrogen ion in solution.
Related to hydrogen ion concentration (CH+) by the activity
coefficient for hydrogen (f H+).
Hysteresis
(Electrode Memory): When an electrode system is returned
to a solution, equilibrium is usually not immediate. This phenomenon
is often observed in electrodes that have been exposed to the
other influences such as temperature, light, or polarization.
Hysteresis:
The difference in output when the measurand value is first approached
with increasing and then with decreasing values. Expressed in
percent of full scale during any one calibration cycle. See
also Deadband.
Icon:
A graphic functional symbol display. A graphic representation
of a function or functions to be performed by the computer.
ICP:
Integrated Circuit Piezoelectric; term sometimes used to describe
an accelerometer with built-in electronics.
Impedance:
The total opposition to electrical flow (resistive plus reactive).
Infrared:
An area in the electromagnetic spectrum extending beyond red
light from 760 nanometers to 1000 microns (106 nm). It is the
form of radiation used for making non-contact temperature measurements.
Initial
Unbalance: Initial unbalance is that unbalance of any kind
that exists in the rotor before balancing.
Input
Impedance: The resistance measured across the excitation
terminals of a transducer.
Input
Impedance: The resistance of a panel meter as seen from
the source. In the case of a voltmeter, this resistance has
to be taken into account when the source impedance is high;
in the case of an ammeter, when the source impedance is low.
Input
Resistance (Impedance): The input resistance of a pH meter
is the resistance between the glass electrode terminal and the
reference electrode terminal. The potential of a pH-measuring
electrode chain is always subject to a voltage division between
the total electrode resistance and the input resistance.
Insulated
Junction: See Ungrounded Junction
Insulation
Resistance: The resistance measured between two insulated
points on a transducer when a specific dc voltage is applied
at room temperature.
Integral:
A form of temperature control. See Automatic Reset, #2
Interchangeability
Error: A measurement error that can occur if two or more
probes are used to make the same measurement. It is caused by
a slight variation in characteristics of different probes.
Interface:
The means by which two systems or devices are connected and
interact with each other.
Internal
Reference electrode (Element): The reference electrode placed
internally in a glass electrode.
Interpreter:
A system program that converts and executes each instruction
of a high-level language program into machine code as it runs,
before going onto the next instruction.
Interrupt:
To stop a process in such a way that it can be resumed.
Intrinsically
Safe: An instrument which will not produce any spark or
thermal effects under normal or abnormal conditions that will
ignite a specified gas mixture.
Ionic
Mobility: Defined similarly to the mobility of nonelectrolytic
particles, viz., as the speed that the ion obtains in a given
solvent when influenced by unit power.
Ionic
Strength: The weight concentration of ions in solution,
computed by multiplying the concentration of each ion in solution
(C) by the corresponding square of the charge on the ion (Z)
summing this product for all ions in solution and dividing by
2:ionic strength - 1/2 _ Z2 C.
IPTS-48:
International Practical Temperature Scale of 1948. Fixed points
in thermometry as specified by the Ninth General Conference
of Weights and Measures which was held in 1948.
IPTS-68:
International Practical Temperature Scale of 1968. Fixed points
in thermometry set by the 1968 General Conference of Weights
and Measures.
ISA:
Instrument Society of America.
Isolation:
The reduction of the capacity of a system to respond to an external
force by use of resilient isolating materials.
Isopotential
Point: A potential which is not affected by temperature
changes. It is the pH value at which dE/dt for a given electrode
pair is zero. Normally, for a glass electrode and SCE reference,
this potential is obtained approximately when immersed in pH
7 buffer.
Isothermal:
A process or area that is a constant temperature.
Joule:
The basic unit of thermal energy.
Journal:
A journal is that part of a rotor that is in contact with or
supported by a bearing in which it revolves.
Junction:
The point in a thermocouple where the two dissimilar metals
are joined.
K:
When referring to memory capacity, two to the tenth power (1024
in decimal notation).
Kelvin:
Symbol K. The unit of absolute or thermodynamic temperature
scale based upon the Celsius scale with 100 units between the
ice point and boiling point of water. 0°C = 273.15K (there
is no degree (°) symbol used with the Kelvin scale).
Kilowatt
(kw): Equivalent to 1000 watts.
Kilowatt
Hour (kwh): 1000 watthours. Kilovolt amperes (kva): 1000
volt amps.
Kinetic
Energy: Energy associated with mass in motion, i.e., 1/2
rV2 where r is the density of the moving mass and V is its velocity.
KVA:
Kilovolt amperes (1000-volt amps).
Lag:
1. A time delay between the output of a signal and the response
of the instrument to which the signal is sent. 2. A time relationship
between two waveforms where a fixed reference point on one wave
occurs after the same point of the reference wave.
Laminar
Flow: Streamlined flow of a fluid where viscous forces are
more significant than inertial forces, generally below a Reynolds
number of 2000.
Large
Scale Integration (LSI): The combining of about 1,000 to
10,000 circuits on a single chip. Typical examples of LSI circuits
are memory chips and microprocessor.
Latent
Heat: Expressed in BTU per pound. The amount of heat needed
(absorbed) to convert a pound of boiling water to a pound of
steam.
Leakage
Rate: The maximum rate at which a fluid is permitted or
determined to leak through a seal. The type of fluid, the differential
Limits of Error: A tolerance band for the thermal electric response
of thermocouple wire expressed in degrees or percentage defined
by ANSI specification MC-96.1 (1975).
Least-squares
Line: The straight line for which the sum of the squares
of the residuals (deviations) is minimized.
Life
Cycle: The minimum number of pressure cycles the transducer
can endure and still remain within a specified tolerance.
Limits
of Error: A tolerance band for the thermal electric response
of thermocouple wire expressed in degrees or percentage defined
by ANSI specification MC-96.1 (1975).
Linearity:
The closeness of a calibration curve to a specified straight
line. Linearity is expressed as the maximum deviation of any
calibration point on a specified straight line during any one
calibration cycle.
Liquid
Junction Potential: The potential difference existing between
a liquid-liquid boundary. The sign and size of this potential
depends on the composition of the liquids and the type of junction
used.
Load
Impedance: The impedance presented to the output terminals
of a transducer by the associated external circuitry.
Load:
The electrical demand of a process expressed as power (watts),
current (amps) or resistance (ohms).
Logarithmic
Scale: A method of displaying data (in powers of ten) to
yield maximum range while keeping resolution at the low end
of the scale.
Loop
Resistance: The total resistance of a thermocouple circuit
caused by the resistance of the thermocouple wire. Usually used
in reference to analog pyrometers which have typical loop resistance
requirements of 10 ohms.
LS-TTL
Compatible: For digital input circuits, a logic 1 is obtained
for inputs of 2.0 to 5.5 V which can source 20 µA, and a logic
0 is obtained for inputs of 0 to 0.8 V which can sink 400 µA.
For digital output signals, a logic 1 is represented by 2.4
to 5.5 V with a current source capability of at least 400 µA;
and a logic 0 is represented by 0 to 0.6 V with a current sink
capability of at least 16 MA. "LS" stands for low-power Schottky.
LS-TTL
Unit Load: A load with LS-TTL voltage levels, which will
draw 20 µA for a logic 1 and -400 µA for a logic 0.
LSD (Least-Significant
Digit): The rightmost active (non-dummy) digit of the display.
M:
Mega; one million. When referring to memory capacity, two to
the twentieth power (1,048,576 in decimal notation).
Machine
Language: Instructions that are written in binary form that
a computer can execute directly. Also called object code and
object language.
Mandrel
(Balancing Arbor): An accurately machined shaft on which
work is mounted for balancing.
Manual
Reset (Adjustment): The adjustment on a proportioning controller
which shifts the proportioning band in relationship to the set
point to eliminate droop or offset errors.
Manual
Reset (Switch): The switch in a limit controller that manually
resets the controller after the limit has been exceeded.
Mass
Flow Rate: Volumetric flowrate times density, i.e. pounds
per hour or kilograms per minute.
Mass
Storage: A device like a disk or magtape that can store
large amounts of data readily accessible to the central processing
unit.
Maximum
Elongation: The strain value where a deviation of more than
±5% occurs with respect to the mean characteristic (diagram
of resistance change vs strain).
Maximum
Excitation: The maximum value of excitation voltage or current
that can be applied to the transducer at room conditions without
causing damage or performance degradation beyond specified tolerances.
Maximum
Operating Temperature: The maximum temperature at which
an instrument or sensor can be safely operated.
Maximum
Power Rating: The maximum power in watts that a device can
safely handle.
Mean
Ionic Activity Coefficient: See Activity coefficient.
Mean
Temperature: The average of the maximum and minimum temperature
of a processequilibrium.
Measurand:
A physical quantity, property, or condition which is measured.
Measuring
Junction: The thermocouple junction referred to as the hot
junction that is used to measure an unknown temperature.
Mechanical
Hysteresis: The difference of the indication with increasing
and decreasing strain loading, at identical strain values of
the specimen.
Medium
Effect (f m): For solvents other than water the medium effect
is the activity coefficient related to the standard state in
water at zero concentration. It reflects differences in the
electrostatic and chemical interactions of the ions with the
molecules of various solvents. Solvation is the most significant
interaction.
Melting
Point: The temperature at which a substance transforms from
a solid phase to a liquid phase.
Membrane:
The pH-sensitive glass bulb is the membrane across which the
potential difference due to the formation of double layers with
ion-exchange properties on the two swollen glass surfaces is
developed. The membrane makes contact with and separates the
internal element and filling solution from the sample solution.
Method
of Correction: A procedure whereby the mass distribution
of a rotor is adjusted to reduce unbalance, or vibration due
to unbalance, to an acceptable value. Corrections are usually
made by adding material to, or removing it from, the rotor.
Mica:
A transparent mineral used as window material in high-temperature
ovens.
Microamp:
One millionth of an ampere, 10-6 amps, µA.
Microcomputer:
A computer which is physically small. It can fit on top of or
under a desk; based on LSI circuitry, computers of this type
are now available with much of the power currently associated
with minicomputer systems.
Micron:
One millionth of a meter, 10-6 meters.
Microvolt:
One millionth of a volt, 10-6 volts.
Mil:
One thousandth of an inch (.001").
Milliamp:
One thousandth of an amp, 10-3 amps, symbol mA.
Millimeter:
One thousandth of a meter, symbol mm.
Millivolt:
Unit of electromotive force. It is the difference in potential
required to make a current of 1 millampere flow through a resistance
of 1 ohm; one thousandth of a volt, symbol mV.
Mineral-insulated
Thermocouple: A type of thermocouple cable which has an
outer metal sheath and mineral (magnesium oxide) insulation
inside separating a pair of thermocouple wires from themselves
and from the outer sheath. This cable is usually drawn down
to compact the mineral insulation and is available in diameters
from .375 to .010 inches. It is ideally suited for high-temperature
and severe-duty applications.
Minor
Scale Division: On an analog scale, the smallest indicated
division of units on the scale.
Modem:
Modulator/Demodulator. A device that transforms digital signals
into audio tones for transmission over telephone lines, and
does the reverse for reception.
Molality:
A measure of concentration expressed in mols per kilogram of
solvent.
Monovalent
Ion: An ion with a single positive or negative charge (H+,
C1-).
Motherboard:
The pc board of a computer that contains the bus lines and edge
connectors to accommodate other boards in the system. In a microcomputer,
the motherboard contains the microprocessor and connectors for
expansion boards.
Mounting
Error: The error resultant from installing the transducer,
both electrical and mechanical.
MSD (Most-Significant
Digit): The leftmost digit of the display.
Mueller
Bridge: A high-accuracy bridge configuration used to measure
three-wire RTDthermometers.
Multiplex:
A technique which allows different input (or output) signals
to use the same lines at different times, controlled by an external
signal. Multiplexing is used to save on wiring and I/O ports.
N/C (No
Connection): A connector point for which there is no internal
connection.
NBS:
National Bureau of Standards.
NEC:
National Electric Codes.
Negative
Temperature Coefficient: A decrease in resistance with an
increase in temperature.
NEMA-4:
A standard from the National Electrical Manufacturers Association,
which defines enclosures intended for indoor or outdoor use
primarily to provide a degree of protection against windblown
dust and rain, splashing water, and hose-directed water.
NEMA-7:
A standard from the National Electrical Manufacturers Association,
which defines explosion-proof enclosures for use in locations
classified as Class I, Groups A, B, C or D, as specified in
the National Electrical Code.
NEMA-12:
A standard from the National Electrical Manufacturers Association,
which defines enclosures with protection against dirt, dust,
splashes by non-corrosive liquids, and salt spray.
NEMA-Size
Case: An older US case standard for panel meters, which
requires a panel cutout of 3.93 x 1.69 inches.
Nernst
Equation: A mathematical description of electrode behavior:
E is the total potential, in millivolts, developed between the
sensing and reference electrodes; Ex varies with the choice
of electrodes, temperature, and pressure: 2.3RT/nF is the Nernst
factor (R and F are constants, n is the charge on the ion, including
sign, T is the temperature in degrees Kelvin), and ai is the
activity of the ion to which the electrode is responding.
Nernst
Factor (S, Slope); The term 2.3RT/nF is the Nernst equation,
which is equal (at T = 25°C) to 59.16 mV when n = 1 and
29.58 mV when n - 2, and which includes the sign of the charge
on the ion in the term n. The Nerst factor varies with temperature.
Network:
A group of computers that are connected to each other by communications
lines to share information and resources.
Nibble:
One half of a byte.
Nicrosil/Nisil:
A nickel chrome/nickel silicone thermal alloy used to measure
high temperatures. Inconsistencies in thermoelectric voltages
exist in these alloys with respect to the wire gage.
NMR (Normal-Mode
Rejection): The ability of a panel meter to filter out noise
superimposed on the signal and applied across the SIG HI to
SIG LO input terminals. Normally expressed in dB at 50/60 Hz.
Noise:
An unwanted electrical interference on the signal wires.
Normal
(axial) Stress: The force per unit area on a given plane
within a body a = F/A
Normal
Hydrogen Electrode: A reversible hydrogen electrode (Pt)
in contact with hydrogen gas at 1 atmosphere partial pressure
and immersed in a solution containing hydrogen ions at unit
activity.
Normal-mode
Rejection Ratio: The ability of an instrument to reject
interference usually of line frequency (50-60 Hz) across its
input terminals.
NPT:
National Pipe Thread.
Null:
A condition, such as balance, which results in a minimum absolute
value of output.
O.D.:
Outside diameter.
Octal:
Pertaining to a base 8 number system.
Offset:
The difference in temperature between the set point and the
actual process temperature. Also, referred to as droop.
ofhc:
Oxygen-free high-conductivity copper. The industrial designation
of the pure copper used in a Type T thermocouple.
Ohmeter:
An instrument used to measure electrical resistance.
On/off
Controller: A controller whose action is fully on or fully
off.
Open
Circuit: The lack of electrical contact in any part of the
measuring circuit. An open circuit is usually characterized
by rapid large jumps in displayed potential, followed by an
off-scale reading.
Operating
System: A collection of programs that controls the overall
operation of a computer and performs such tasks as assigning
places in memory to programs and data, processing interrupts,
scheduling jobs and controlling the overall input/output of
the system.
Operational
pH: The determination of sample pH by relating to pH measurements
in a primary standard solution. This relationship assumes that
electrode errors such as sensitivity and changes in asymmetry
potential can be disregarded or compensated for, provided the
liquid junction potential remains constant between standard
and sample.
Optical
Isolation: Two networks which are connected only through
an LED transmitter and photoelectric receiver with no electrical
continuity between the two networks.
Outboard
Rotor: A two-journal rotor which has its center of gravity
between the journals.
Output
Impedance: The resistance as measured on the output terminals
of a pressure transducer.
Output
Noise: The RMS, peak-to-peak (as specified) ac component
of a transducer's dc output in the absence of a measurand variation.
Output:
The electrical signal which is produced by an applied input
to the transducer.
Overshoot:
The number of degrees that a process exceeds the set point temperature
when coming up to the set point temperature.
Parallax:
An optical illusion which occurs in analog meters and causes
reading errors. It occurs when the viewing eye is not in the
same plane, perpendicular to the meter face, as the indicating
needle.
Parallel
Transmission: Sending all data bits simultaneously. Commonly
used for communications between computers and printer devices.
Parity:
A technique for testing transmitting data. Typically, a binary
digit is added to the data to make the sum of all the digits
of the binary data either always even (even parity) or always
odd (odd parity).
Peltier
Effect: When a current flows through a thermocouple junction,
heat will either be absorbed or evolved depending on the direction
of current flow. This effect is independent of joule I2 R heating.
Perfectly
Balanced Rotor: A rotor is perfectly balanced when its mass
distribution is such that it transmits no vibratory force or
motion to its bearings as a result of centrifugal forces.
Peripheral:
A device that is external to the CPU and main memory, i.e.,
printer, modem or terminal, but is connected by the appropriate
electrical connections.
pH Junctions:
The Junction of a reference electrode or combination electrode
is a permeable membrane through which the fill solution escapes
(called the liquid junction).
pH(S)
(Standard pH Scale): The conventional standard pH scale
established on the basis that an individual ionic activity coefficient
can be calculated from the Debye-H¸ckel law for primary buffers.
Phase
Difference: The time expressed in degrees between the same
reference point on two periodic waveforms.
Phase
Proportioning: A form of temperature control where the power
supplied to the process is controlled by limiting the phase
angle of the line voltage.
Phase:
A time based relationship between a periodic function and a
reference. In electricity, it is expressed in angular degrees
to describe the voltage or current relationship of two alternating
waveforms.
PID:
Proportional, integral, derivative. A three mode control action
where the controller has time proportioning, integral (auto
reset) and derivative rate action.
Piezoelectric
Accelerometer: A transducer that produces an electrical
charge in direct proportion to the vibratory acceleration.
Piezoresistance:
Resistance that changes with stress.
Pixel:
Picture element. Definable locations on a display screen that
are used to form images on the screen. For graphic displays,
screens with more pixels provide higher resolution.
Plane
Separation: Of a balancing machine, is the operation of
reducing the correction plane interference ratio for a particular
rotor.
Platinel:
A non-standard, high temperature platinum thermocouple alloy
whose thermoelectric voltage nearly matches a Type K thermocouple
(Trademark of Englehard Industries).
Platinum
6% Rhodium: The platinum-rhodium alloy used as the negative
wire in conjunction with platinum-30% rhodium to form a Type
B thermocouple.
Platinum
10% Rhodium: The platinum-rhodium alloy used as the positive
wire in conjunction with pure platinum to form a Type S thermocouple.
Platinum
13% Rhodium: The platinum-rhodium alloy used as the positive
wire in conjunction with pure platinum to form a Type R thermocouple.
Platinum
30% Rhodium: The platinum-rhodium alloy used as the positive
wire in conjunction with platinum 6% rhodium to form a Type
B thermocouple.
Platinum
67: To develop thermal emf tables for thermocouples, the
National Bureau of Standards paired each thermocouple alloy
against a pure platinum wire (designated Platinum 2 prior to
1973, and currently Platinum 67). The thermal emf's of any alloy
combination can be determined by summing the "vs. Pt-67" emf's
of the alloys, i.e., the emf table for a Type K thermocouple
is derived from the Chromel vs. Pt-67 and the Alumel vs. Pt-67
values.
Platinum:
A noble metal which in its pure form is the negative wire of
Type R and Type S thermocouples.
Poisson
Ratio: The ratio between the strain of expansion in the
direction of force and the strain of contraction perpendicular
to that force v = -Et/E1.
Polarity:
In electricity, the quality of having two oppositely charged
poles, one positive one negative.
Polarization:
The inability of an electrode to reproduce a reading after a
small electrical current has been passed through the membrane.
Glass pH electrodes are especially prone to polarization errors
caused by small currents flowing from the pH meter input circuit
and from static electrical charges built up as the electrodes
are removed from the sample solution, or when the electrodes
are wiped.
Port:
A signal input (access) or output point on a computer.
Positive
Temperature Coefficient: An increase in resistance due to
an increase in temperature.
Potential
Energy: Energy related to the position or height above a
place to which fluid could possibly flow.
Potentiometer:
1. A variable resistor often used to control a circuit. 2. A
balancing bridge used to measure voltage.
Power
Supply: A separate unit or part of a circuit that supplies
power to the rest of the circuit or to a system.
PPM:
Abbreviation for "parts per million," sometimes used to express
temperature coefficients. For instance, 100 ppm is identical
to 0.01%.
Primary
Device: Part of a flowmeter which is mounted internally
or externally to the fluid conduit and produces a signal corresponding
to the flowrate and from which the flow may be determined.
Primary
Standard (NBS): The standard reference units and physical
constants maintained by the National Bureau of Standards upon
which all measurement units in the United States are based.
Primary
Standards: Aqueous pH buffer solutions established by the
National Bureau of Standards within the 2.5 to 11.5 pH range
of ionic strength less than 0.1 and which provide stable liquid
junction potential and uniformity of electrode sensitivity.
Principal
Axes: The axes of maximum and minimum normal stress.
Probe:
A generic term that is used to describe many types of temperature
sensors.
Process
Meter: A panel meter with sizeable zero and span adjustment
capabilities, which can be scaled for readout in engineering
units for signals such as 4-20 mA, 10-50 mA and 1-5 V.
Program:
A list of instructions that a computer follows to perform a
task.
Prom:
Programmable read-only memory. A semiconductor memory whose
contents cannot be changed by the computer after it has been
programmed.
Proof
Pressure: The specified pressure which may be applied to
the sensing element of a transducer without causing a permanent
change in the output characteristics.
Proportioning
Band: A temperature band expressed in degrees within which
a temperature controller's time proportioning function is active.
Proportioning
Control Mode: A time proportioning controller where the
amount of time that the relay is energized is dependent upon
the system's temperature.
Proportioning
Control plus Derivative Function: A time proportioning controller
with derivative function. The derivative function senses the
rate at which a system's temperature is either increasing or
decreasing and adjusts the cycle time of the controller to minimize
overshoot or undershoot.
Proportioning
Control plus Integral: A two-mode controller with time proportioning
and integral (auto reset) action. The integral function automatically
adjusts the temperature at which a system has stabilized back
to the setpoint temperature, thereby eliminating droop in the
system.
Proportioning
Control with Integral and Derivative Functions: Three mode
PID controller. A time proportioning controller with integral
and derivative functions. The integral function automatically
adjusts the system temperature to the set point temperature
to eliminate droop due to the time proportioning function. The
derivative function senses the rate of rise or fall of the system
temperature and automatically adjusts the cycle time of the
controller to minimize overshoot or undershoot.
Protection
Head: An enclosure usually made out of metal at the end
of a heater or probe where connections are made.
Protection
Tube: A metal or ceramic tube, closed at one end into which
a temperature sensor is inserted. The tube protects the sensor
from the medium into which it is inserted.
Protocol:
A formal definition that describes how data is to be exchanged.
PSIA:
Pounds per square inch absolute. Pressure referenced to a vacuum.
PSID:
Pounds per square inch differential. Pressure difference between
two points.
PSIG:
Pound per square inch gage. Pressure referenced to ambient air
pressure.
PSIS:
Pounds per square inch standard. Pressure referenced to a standard
atmosphere.
Pulse
Width Modulation: An output in the form of duty cycle which
varies as a function of the applied measurand.
Radiation:
See Infrared
Random
Access Memory (RAM): Memory that can be both read and changed
during computer operation. Unlike other semi-conductor memories,
RAM is volatile-if power to the RAM is disrupted or lost, all
the data stored is lost.
Range:
Those values over which a transducer is intended to measure,
specified by its upper and lower limits.
Rangeability:
The ratio of the maximum flowrate to the minimum flowrate of
a meter.
Rankine
(°R): An absolute temperature scale based upon the
Fahrenheit scale with 180° between the ice point and boiling
point of water. 459.67°R = 0°F.
Rate
Action: The derivative function of a temperature controller.
Rate
time: the time interval over which the system temperature
is sampled for the derivative function.
Ratiometric
Measurement: A measurement technique where an external signal
is used to provide the voltage reference for the dual-slope
A/D converter. The external signal can be derived from the voltage
excitation applied to a bridge circuit or pick-off supply, thereby
eliminating errors due to power supply fluctuations.
Read
Only Memory (ROM): Memory that contains fixed data. The
computer can read the data, but cannot change it in any way.
Real
Time: The time interval over which the system temperature
is sampled for the derivative function.
Record:
A collection of unrelated information that is treated as a single
unit.
Recovery
Time: The length of time which it takes a transducer to
return to normal after applying a proof pressure.
Redox
Potential: The potential developed by a metallic electrode
when placed in a solution containing a species in two different
oxidation states.
Reference
Junction: The cold junction in a thermocouple circuit which
is held at a stable known temperature. The standard reference
temperature is 0°C (32°F). However, other temperatures
can be used.
Reference
Mark: Any diagnostic point or mark which can be used to
relate a position during rotation of a part to its location
when stopped.
Reference
Plane: Any plane perpendicular to the shaft axis to which
an amount of unbalance is referred.
Refractory
Metal Thermocouple: A class of thermocouples with melting
points above 3600°F. The most common are made from tungsten
and tungsten/rhenium alloys Types G and C. They can be used
for measuring high temperatures up to 4000°F (2200°C)
in non-oxidizing, inert, or vacuum environments.
Register:
A storage device with a specific capacity, such as a bit, byte
or word.
Relay
(Mechanical): An electromechanical device that completes
or interrupts a circuit by physically moving electrical contacts
into contact with each other.
Relay
(Solid State): A solid state switching device which completes
or interrupts a circuit electrically with no moving parts.
Remote:
Not hard-wired; communicating via switched lines, such as telephone
lines. Usually refers to peripheral devices that are located
a site away from the CPU.
Repeatability:
The ability of a transducer to reproduce output readings when
the same measurand value is applied to it consecutively, under
the same conditions, and in the same direction. Repeatability
is expressed as the maximum difference between output readings.
Reserved
Word: A word that has a defined function in the language,
and cannot be used as a variable name.
Residual
(Final) Unbalance: Residual unbalance is that unbalance
of any kind that remains after balancing.
Resistance
Ratio Characteristic: For thermistors, the ratio of the
resistance of the thermistor at 25°C to the resistance
at 125°C.
Resistance
Temperature Characteristic: A relationship between a thermistor's
resistance and the temperature.
Resistance:
The resistance to the flow of electric current measured in ohms
(1/2) for a conductor. Resistance is function of diameter, resistivity
(an intrinsic property of the material) and length.
Resolution:
The smallest detectable increment of measurement. Resolution
is usually limited by the number of bits used to quantize the
input signal. For example, a 12-bit A/D can resolve to one part
in 4096 (2 to the 12 power equals 4096).
Resonant
Frequency: The measurand frequency at which a transducer
responds with maximum amplitude.
Response
Time (time constant): The time required by a sensor to reach
63.2% of a step change in temperature under a specified set
of conditions. Five time constants are required for the sensor
to stabilize at 600 of the step change value.
Response
Time: The length of time required for the output of a transducer
to rise to a specified percentage of its final value as a result
of a step change of input.
Reynolds
Number: The ratio of inertial and viscous forces in a fluid
defined by the formula Re = rVD/µ, where: r = Density of fluid,
µ = Viscosity in centipoise (CP), V = Velocity, and D = Inside
diameter of pipe.
RFI:
Radio frequency interference.
Rheostat:
A variable resistor.
Rigid
Rotor: A rotor is considered rigid when it can be corrected
in any two (arbitrarily selected) planes [see "Correction (Balancing)Plane"]
and after that correction, its unbalance does not significantly
exceed the balancing tolerances (relative to the shaft axis)
at any speed up to maximum operating speed and when running
under conditions which approximate closely to those of the final
supporting system.
Rise
Time: The time required for a sensor or system to respond
to an instantaneous step function, measured from the 10% to
90% points on the response waveforms.
Room
Conditions: Ambient environmental conditions under which
transducers must commonly operate.
Root
Mean Square (RMS): Square root of the mean of the square
of the signal taken during one full cycle.
Rotor:
A rotor is a rotating body whose journals are supported by bearings.
RTD:
Resistance temperature detector.
Salt
Bridge: The salt bridge of a reference electrode is that
part of the electrode which contains the filling solution to
establish the electrolytic connection between reference internal
cell and the test solution. Auxiliary Salt Bridge: A glass tube
open at oneend to receive intermediate electrolyte filling solution,
and the reference electrode tip and a junction at the other
end to make contact with the sample.
Salt
Effect (fx): The effect on the activity coefficient due
to salts in the solution.
SAMA:
Scientific Apparatus Makers Association. An association that
has issued standards covering platinum, nickel, and copper resistance
elements (RTDs).
SCE:
Saturated calomel electrode.
SCR:
Ssilicone controlled rectifier.
Scroll:
To move all or part of the screen material up to down, left
or right, to allow new information to appear.
Secondary
Device: A part of the flowmeter which receives a signal
proportional to the flowrate, from the primary device, and displays,
records and/or transmits the signal.
Secondary
Standard: pH buffer solutions which do not meet the requirements
of primary standard solutions but provide coverage of the pH
range not covered by primary standards. Used when the pH value
of the primary standard is not close to the sample pH value.
Seebeck
Coefficient: The derivative (rate of change) of thermal
EMF with respect to temperature normally expressed as millivolts
per degree.
Seebeck
Effect: When a circuit is formed by a junction of two dissimilar
metals and the junctions are held at different temperatures,
a current will flow in the circuit caused by the difference
in temperature between the two junctions.
Seebeck
EMF: The open circuit voltage caused by the difference in
temperature between the hot and cold junctions of a circuit
made from two dissimilar metals.
Self
Heating: Internal heating of a transducer as a result of
power dissipation.
Sensing
Element: That part of the transducer which reacts directly
in response to the input.
Sensitivity
Shift: A change in slope of the calibration curve due to
a change in sensitivity.
Sensitivity:
The minimum change in input signal to which an instrument can
respond.
Sequential
Access: An access mode in which records are retrieved in
the same order in which they were written. Each successive access
to the file refers to the next record in the file.
Serial
transmission: Sending one bit at a time on a single transmission
line. Compare with parallel transmission.
Set Point:
The temperature at which a controller is set to control a system.
Settling
Time: The time taken for the display to settle within one
digit final value when a step is applied to the meter input.
Shear
Modulus: The ratio of the shear stress and the angular shear
distortion.
Shear
Stress: Where normal stress is perpendicular to the designated
plane, shear stress is parallel to the plane.
Shearing
Strain: A measure of angular distortion also directly measurable,
but not as easily as axial strain.
Sheath
Thermocouple: A thermocouple made out of mineral-insulated
thermocouple cable which has an outer metal sheath.
SI:
System Internationale. The name given to the standard metric
system of units.
Signal
Conditioner: A circuit module which offsets, attenuates,
amplifies, linearizes and/or filters the signal for input to
the A/D converter. The typical output signal conditioner is
+2 V dc.
Signal
Conditioning: To process the form or mode of a signal so
as to make it intelligible to, or compatible with, a given device,
including such manipulation as pulse shaping, pulse clipping,
compensating, digitizing, and linearizing.
Signal:
An electrical transmittance (either input or output) that conveys
information.
Single
Precision: The degree of numeric accuracy that requires
the use of one computer word. In single precision, seven digits
are stored, and up to seven digits are printed. Contrast with
double precision.
Single-ended
Input: A signal-input circuit where SIG LO (or sometimes
SIG HI) is tied to METER GND. Ground loops are normally not
a problem in AC-powered meters, since METER GND is transformer-isolated
from AC GND.
Single-Plane
(Static) Balancing Machine: A single plane balancing machine
is a gravitational or centrifugal balancing machine that provides
information for accomplishing single plane balancing.
Slope
(Electrode Sensitivity, Span): See Nernst factor.
Smallest
Bending Radius: The smallest radius that a strain gage can
withstand in one direction, without special treatment, without
suffering visible damage.
Software:
Generally, programs loaded into a computer from external mass
storage but also extended to include operating systems and documentation.
Solvation:
Ions in solution are normally combined with at least one molecule
of solvent. This phenomenon is termed solvation.
Source
Code: A non-executable program written in a high-level language.
A compiler or assembler must translate the source code into
object code (machine language) that the computer can understand
and process.
Span
Adjustment: The ability to adjust the gain of a process
or strain meter so that a specified display span in engineering
units corresponds to a specified signal span. For instance,
a display span of 200°F may correspond to the 16 mA span
of a 4-20 mA transmitter signal.
Span:
The difference between the upper and lower limits of a range
expressed in the same units as the range.
Spare:
A connector point reserved for options, specials, or other configurations.
The point is identified by an (E#) for location on the electrical
schematic.
Specific
Gravity: The ratio of mass of any material to the mass of
the same volume of pure water at 4°C.
Specific
Heat: The ratio of thermal energy required to raise the
temperature of a body 1° to the thermal energy required
to raise an equal mass of water 1°.
Spectral
Filter: A filter which allows only a specific band width
of the electromagnetic spectrum to pass, i.e., 4 to 8 micron
infrared radiation.
Spectrum
Analysis: Utilizing frequency components of a vibration
signal to determine the source and cause of vibration.
Spectrum:
The resolving of overall vibration into amplitude components
as a function of frequency.
Spot
Size: The diameter of the circle formed by the cross section
of the field of view of an optical instrument at a given distance.
Spurious
Error: Random or erratic malfunction.
SSR:
Solid state relay (see relay, solid state).
Stability:
The quality of an instrument or sensor to maintain a consistent
output when a constant input is applied.
Stagnation
Pressure: The sum of the static and dynamic pressure.
Standard
Electrode Potential (E0): The standard potential E0 of an
electrode is the reversible emf between the normal hydrogen
electrode and the electrode with all components at unit activity.
Standardization:
a process of equalizing electrode potentials in one standardizing
solution (buffer) so that potentials developed in unknown solutions
can be converted to pH values.
Static
Calibration: A calibration recording pressure versus output
at fixed points at room temperature.
Static
Error Band: The error band applicable at room temperature.
Static
Pressure: Pressure of a fluid whether in motion or at rest.
It can be sensed in a small hole drilled perpendicular to and
flush with the flow boundaries so as not to disturb the fluid
in any way.
Static
Unbalance: Static unbalance is that condition of unbalance
for which the central principal axis is displayed only parallel
to the shaft axis
Steady
Flow: A flow rate in the measuring section of a flow line
that does not vary significantly with time.
Steady
State Vibration: That condition of vibration induced by
an unchanging continuing periodic force.
Stiffness:
The ratio of the force required to create a certain deflection
or movement of a part expressed as (Force/deflection) lbs/in
or grams/cm.
Stop
Bit: A signal following a character or block that prepares
the receiving device to receive the next character or block.
Strain
Gage: A measuring element for converting force, pressure,
tension, etc., into an electrical signal.
Strain:
The ratio of the change in length to the initial unstressed
reference length.
String:
A sequence of characters.
Strouhal
Number: A nondimensional parameter important in vortex meter
design defined as: s = Fh/V where f = frequency, V = velocity,
and h = a reference length
Super
Cooling: The cooling of a liquid below its freezing temperature
without the formation of the solid phase.
Super
Heating: 1. The heating of a liquid above its boiling temperature
without the formation of the gaseous phase. 2. The heating of
the gaseous phase considerably above the boiling-point temperature
to improve the thermodynamic efficiency of a system.
Surge
Current: A current of short duration that occurs when power
is first applied to capacitive loads or temperature dependent
resistive loads such as tungsten or molybdenum heaters-usually
lasting no more than several cycles.
Suspension
Effect: The source of error due to varied reference liquid
junction potential depending upon whether the electrodes are
immersed in the supernatant fluid or deeper in the sediment.
Normally encountered with solutions containing resins or charged
colloids.
Syntax:
The rules governing the structure of a language.
Tape:
A recording media for data or computer programs. Tape can be
in permanent form, such as perforated paper tape, or erasable,
such as magnetic tape. Generally, tape is used as a mass storage
medium, in magnetic form, and has a much higher storage capacity
than disk storage, but it takes much longer to write or recover
data from tape than from a disk.
PFA:
A fluorocarbon polymer used for insulation of electrical wires
.
Telecommunication:
Synonym for data communication. The transmission of information
from one point to another.
TEMPCO:
Abbreviation for "temperature coefficient": the error introduced
by a change in temperature. Normally expressed in %/°C
or ppm/°C.
Temperature
Error: The maximum change in output, at any measurand value
within the specified range, when the transducer temperature
is changed from room temperature to specified temperature extremes.
Temperature
Range, Compensated: The range of ambient temperatures within
which all tolerances specified for Thermal Zero Shift and Thermal
Sensitivity Shift are applicable (temperature error).
Temperature
Range, Operable: The range of ambient temperatures, given
by their extremes, within which the transducer may be operated.
Exceeding compensated range may require recalibration.
Terminal:
An input/output device used to enter data into a computer and
record the output.
Terminal Base:
Deviation at the mid point from a straight line between the lowest and
highest readings (typically 0 and 100%) as a percentage of span.
Thermal
Coefficient of Resistance: The change in resistance of a
semiconductor per unit change in temperature over a specific
range of temperature.
Thermal
Conductivity: The property of a material to conduct heat
in the form of thermal energy.
Thermal
emf: See Seebeck emf
Thermal
Expansion: An increase in size due to an increase in temperature
expressed in units of an increase in length or increase in size
per degree, i.e. inches/inch/degree C.
Thermal
Gradient: The distribution of a differential temperature
through a body or across a surface.
Thermal
Sensitivity Shift: The sensitivity shift due to changes
of the ambient temperature from room temperature to the specified
limits of the compensated temperature range.
Thermal
Zero Shift: An error due to changes in ambient temperature
in which the zero pressure output shifts. Thus, the entire calibration
curve moves in a parallel displacement.
Thermistor:
A temperature-sensing element composed of sintered semiconductor
material which exhibits a large change in resistance proportional
to a small change in temperature. Thermistors usually have negative
temperature coefficients.
Thermocouple
Type Material
(ANSI Symbol)
J Iron/Constantan
K CHROMEGA®/ALOMEGA®
T Copper/Constantan
E CHROMEGA®/Constantan
R Platinum/Platinum 13% Rhodium
S Platinum/Platinum 10% Rhodium
B Platinum 6% Rhodium/Platinum30% Rhodium
G* Tungsten/Tungsten 26% Rhenium
C* Tungsten 5% Rhenium/Tungsten 26% Rhenium
D* Tungsten 3% Rhenium/Tungsten 150 Rhenium
*Not ANSI symbols.
Thermocouple:
The junction of two dissimilar metals which has a voltage output
proportional to the difference in temperature between the hot
junction and the lead wires (cold junction) (refer to Seebeck
emf).
Thermopile:
An arrangement of thermocouples in series such that alternate
junctions are at the measuring temperature and the reference
temperature. This arrangement amplifies the thermoelectric voltage.
Thermopiles are usually used as infrared detectors in radiation
pyrometry.
Thermowell:
A closed-end tube designed to protect temperature sensors from
harsh environments, high pressure, and flows. They can be installed
into a system by pipe thread or welded flange and are usually
made of corrosion-resistant metal or ceramic material depending
upon the application.
Thomson
Effect: When current flows through a conductor within a
thermal gradient, a reversible absorption or evolution of heat
will occur in the conductor at the gradient boundaries.
Transducer
Vibration: Generally, any device which converts movement,
either shock or steady state vibration, into an electrical signal
proportional to the movement; a sensor.
Transducer:
A device (or medium) that converts energy from one form to another.
The term is generally applied to devices that take physical
phenomenon (pressure, temperature, humidity, flow, etc.) and
convert it to an electrical signal.
Transient
Vibration: A temporary vibration or movement of a mechanical
system.
Transitional
Flow: Flow between laminar and turbulent flow, usually between
a pipe Reynolds number of 2000 and 4000.
Transmitter
(Two-Wire): 1. A device which is used to transmit data from
a sensor via a two-wire current loop. The loop has an external
power supply and the transmitter acts as a variable resistor
with respect to its input signal. 2. A device which translates
the low level output of a sensor or transducer to a higher level
signal suitable for transmission to a site where it can be further
processed.
Triac:
A solid state switching device used to switch alternating current
wave forms.
Triboelectric
Noise: The generation of electrical charges caused by layers
of cable insulation. This is especially troublesome in high
impedance accelerometers.
Triple
Point (Water): The thermodynamic state where all three phases,
solid, liquid, and gas may all be present in equilibrium. The
triple point of water is .01°C.
Triple
Point: The temperature and pressure at which solid, liquid,
and gas phases of a given substance are all present simultaneously
in varying amounts.
True
RMS: The true root-mean-square value of an AC or AC-plus-DC
signal, often used to determine power of a signal. For a perfect
sine wave, the RMS value is 1.11072 times the rectified average
value, which is utilized for low-cost metering. For significantly
non-sinusoidal signals, a true RMS converter is required.
TTL Unit
Load: A load with TTL voltage levels, which will draw 40
µA for a logic 1 and -1.6 mA for a logic 0.
TTL-Compatible:
For digital input circuits, a logic 1 is obtained for inputs
of 2.0 to 5.5 V which can source 40 µA, and a logic 0 is obtained
for inputs of 0 to 0.8 V which can sink 1.6 mA. For digital
output signals, a logic 1 is represented by 2.4 to 5.5 V with
a current source capability of at least 400 µA; and a logic
0 is represented by 0 to 0.6 V with a current sink capability
of at least 16 mA.
TTL:
Transistor-to-transistor logic. A form of solid state logic
which uses only transistors to form the logic gates.
Turbulent
Flow: When forces due to inertia are more significant than
forces due to viscosity. This typically occurs with a Reynolds
number in excess of 4000.
Typical:
Error is within plus or minus one standard deviation (±1%) of
the nominal specified value, as computed from the total population.
UL:
Underwriters Laboratories, Inc. An independent laboratory that
establishes standards for commercial and industrial products.
Ultraviolet:
That portion of the electromagnetic spectrum below blue light
(380 nanometers).
Unbalance:
That condition which exists in a rotor when vibratory force
or motion is imparted to its bearings as a result of centrifugal
forces.
Unbalance
Tolerance: The unbalance tolerance with respect to a radial
plane (measuring plane or correction plane) is that amount of
unbalance which is specified as the maximum below which the
state of unbalance is considered acceptable.
Undershoot:
The difference in temperature between the temperature a process
goes to, below the set point, after the cooling cycle is turned
off and the set point temperature.
Ungrounded
Junction: A form of construction of a thermocouple probe
where the hot or measuring junction is fully enclosed by and
insulated from the sheath material.
Union:
A form of pipe fitting where two extension pipes are joined
at a separable coupling.
Vacuum:
Any pressure less than atmospheric pressure.
Velocity:
The time rate of change of displacement; dx/dt.
Vibration
Error Band: The error recorded in output of a transducer
when subjected to a given set of amplitudes and frequencies.
Vibration
Error: The maximum change in output of a transducer when
a specific amplitude and range of frequencies are applied to
a specific axis at room temperature.
Viscosity:
The inherent resistance of a substance to flow.
Volt:
The (electrical) potential difference between two points in
a circuit. The fundamental unit is derived as work per unit
charge-(V = W/Q). One volt is the potential difference required
to move one coulomb of charge between two points in a circuit
while using one joule of energy.
Voltage:
An electrical potential which can be measured in volts.
Voltmeter:
An instrument used to measure voltage.
Volume
Flow Rate: Calculated using the area of the full closed
conduit and the average fluid velocity in the form, Q = V x
A, to arrive at the total volume quantity of flow. Q = volumetric
flowrate, V = average fluid velocity, and A = cross sectional
area of the pipe.
Watt
Density: The watts emanating from each square inch of heated
surface area of a heater. Expressed in units of watts per square
inch.
Wheatstone
Bridge: A network of four resistances, an emf source, and
a galvanometer connected such that when the four resistances
are matched, the galvanometer will show a zero deflection or
"null" reading.
Window:
In computer graphics, a defined area in a system not bounded
by any limits; unlimited "space" in graphics.
Word:
Number of bits treated as a single unit by the CPU. In an 8-bit
machine, the word length is 8 bits; in a sixteen bit machine,
it is 16 bits.
Working
Standard: A standard of unit measurement calibrated from
either a primary or secondary standard which is used to calibrate
other devices or make comparison measurements.
Write:
To record data in a storage device or on a data medium.
Young's
Modulus: Young's Modulus (the Modulus of Elasticity) is
equivalent to the ratio of normal stress to strain.
Zero
Adjustment: The ability to adjust the display of a process
or strain meter so that zero on the display corresponds to a
non-zero signal, such as 4 mA, 10 mA, or 1 V dc. The adjustment
range is normally expressed in counts.
Zero
Offset: 1. The difference expressed in degrees between true
zero and an indication given by a measuring instrument. 2. See
Zero Suppression
Zero
Point: The electrical zero point where zero millivolts would
be displayed. Used in conjunction with the slope control to
provide a narrower range calibration.
Zero
Power Resistance: The resistance of a thermistor or RTD
element with no power being dissipated.
Zero
Suppression: The span of an indicator or chart recorder
may be offset from zero (zero suppressed) such that neither
limit of the span will be zero. For example, a temperature recorder
which records a 100° span from 400° to 500° is
said to have 400° zero suppression.
Zero
Voltage Switching: The making or breaking of circuit timed
such that the transition occurs when the voltage wave form crosses
zero voltage; typically only found in solid state switching
devices.
Zooming:
In computer graphics, causing an object to appear smaller or
larger by moving the window and specifying various window sizes.
