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Microprocessor driven digital flow
controllers allow one to program,
record, and analyze flow rates of
various gases with a computer via
an RS-485 interface (optional
RS-232 is available).
Controllers can be programmed for
various control functions including
flow setpoint, totalizer, stop
totalizer, read totalizer, totalizer
from preset flow, stop at preset
total, auto zero, and more.
Principles of Operation
Metered gases are divided into two
laminar flow paths, one through the
primary flow conduit, and the other
through a capillary sensor tube.
Both flow conduits are designed to
ensure laminar flows and, therefore,
the ratio of their flow rates is
constant. Two precision
temperature sensing windings on
the sensor tube are heated, and
when flow takes place, gas carries
heat from the upstream to the
downstream windings. The
resultant temperature differential is
proportional to the change in
resistance of the sensor windings.
A Wheatstone bridge design is used
to monitor the temperature
dependent resistance gradient on
the sensor windings which is linearly
proportional to the instantaneous
rate of flow. The output of the
Wheatstone bridge is converted to
digital format with a 12 Bit ADC
(analog to digital converter).
An on-board microprocessor and
nonvolatile memory store all
calibration factors and directly
control a proportional
electromagnetic valve. The digital
closed loop control system
continuously compares the mass
flow output with the selected flow
rate. Deviations from the setpoint
are corrected by compensating valve
adjustments, with PID algorithm thus
maintaining the desired flow
parameters with a high degree of
accuracy. Output signals of 0 to 5
Vdc or 4 to 20 mA are generated
indicating mass molecular based
flow rates of the metered gas.
Interface
The digital interface operates
via RS485 (optional RS232) and
provides access to applicable
internal data including FLOW
SETPOINT, ACTUAL FLOW,
ZERO ADJUSTMENTS, and
LINEARIZATION TABLE
ADJUSTMENTS. The analog
interface provides 0 to 5 Vdc,
0 to 10 Vdc and 4 to 20 mA
inputs and outputs.
Auto Zero
The FMA6500 automatically nulls
the sensor zero offset whenever the
flow setpoint is below 2% of full
scale. To accommodate this feature
the control valve must fully close
under that condition. Provisions are
made to either disable, force or
store the current auto zero via
digital commands.
Totalizer
The firmware for the FMA6500
provides functions to register total
gas quantity. The total mass of gas is
calculated by integrating the actual
gas flow rate with respect to time.
Digital interface commands are
provided to:
SET the totalizer to ZERO;
START /STOP totalizing the flow;
READ the totalizer;
START the totalizer at a preset flow
STOP the flow at a preset total.
Multi-Gas Calibration
The FMA6500 is capable of storing
primary calibration data for up to 10
gases. This feature allows the same
DFC to be calibrated for multiple
gases while maintaining the rated
accuracy on each.
Conversion Factors
Conversion factors for up to 256
gases are stored in the FMA6500.
Conversion factors may be applied
to any of the ten gas calibrations via
digital interface commands.
Flow Alarms
High and Low gas flow ALARM
limits are programmed using the
digital interface. Alarm conditions
are reported via the digital interface
or can activate the contact
closure outputs.
Programmable Flow
OMEGA software supports
programmable flow modes, allowing
execution of custom programming
of up to ten steps. Various flow
configurations include ramping,
linearized increasing and
decreasing modes.
Autotune
The AUTOTUNE function allows
the FMA6500 to automatically
optimize control response for the
gas under actual process
conditions. During the AUTOTUNE
process, the instrument adjusts PID
gains for optimum step response
and determine key control valve
characteristics (only available on
units with less than 80 L/min
maximum flow).
Contact Closure
Two sets of dry contact relay
outputs are provided to actuate
user supplied equipment. These are
programmable via the digital
interface such that the relays can
be made to switch when
a specified event occurs (e.g. when
a low or high flow alarm limit is
exceeded or when the totalizer
reaches a specified value).
Valve Override
Means are provided to force the
control valve fully open (purge) or
fully closed via either the analog or
digital interfaces.
Self Diagnostics
Whenever power is first applied, the
FMA6500 runs a series of SELF
DIAGNOSTIC TESTS to ensure
that it is in optimum working
condition.
Engineering Units
The flow setpoint, measured gas
flow and associated totalizer data is
scaled directly in engineering units
via digital interface commands. The
following units of measure are
supported: % of FS, mL/min, mL/hr,
scfm, scfh, sL/min, sL/hr, lbs/hr,
lbs/min, and one user defined unit
of measure.
Leak Integrity
1 x 10-9 smL/sec of Helium maximum
to the outside environment.
Balanced Power Supply
The FMA6500 operates on ±15 Vdc.
The current requirements for the
positive and negative power
supplies are balanced such that the
current in the power supply
common connection is minimized.
Maximum power consumption is
13.5 watts at ±15 Vdc.
† Tutti gli importi indicati in EUR Nota: Comes with complete operator’s manual, software, cable (FMA65-C), and NIST Certificate.Power supply sold separately. ** Flow ranges specified are for nitrogen or air at 20 psig inlet (up to 50 SLM) or 25 psig inlet (60 to 100 SLM units) and 0 psig outlet. Add “-RS232” to model number to replace RS-485 communications, no additional cost. Add “-I” to model number for a 4 to 20 mA output instead of the standard 0 to 5 volt, no additional cost. Add “Kalrez” to model number to replace standard FKM O-rings. Add $345 to price. (OPTIONS SHOWN IN THIS "NOTE" ABOVE ARE PRICED IN US$)