Trend Recorders
Vertical strip-chart recorders head this category, embracing many types that are commonly designed for permanent installation and are normally mounted vertically-usually in panel cut-outs for process measurements but also on relay racks as for laboratory measurements. Size is one distinction of this class, with chart widths ranging from 100 mm to 10 inches (250 mm) or more.

Figure 7-2:Typical Circular Chart Recorder Trace

  Other terms used to describe vertical strip-chart recorders include electronic, hybrid, and multi-point. Certain models can record up to 32 channels, although common designs for process measurements normally do not exceed 12. Instruments in this class are designed to plot variables over long periods of time, utilizing paper take-up rolls or trays for Z-fold paper. Chart speeds, programmable in most cases, normally range from 1 cm/min to 30 cm/min.
  This category contains many variations on the basic principle of strip-chart recording shown in Figure 7-1. One extensive catalog reference covers 29 different models; a majority of these are for the measurement of process variables, but there are also numerous designs customized for scientific and laboratory applications.
  The impact of the microprocessor is obvious in many strip-chart recorder models, with digital storage of data points a common advanced feature. Another result of new technology is the ability to handle a variety of variables, such as flow, along with temperature and pressure.

Flatbed recorders offer a portable, flexible alternative to permanently mounted units.

  Another result of microprocessor technology is the hybrid recorder, which provides high-speed recording of up to 30 recording channels as well as digital measurement data. On these units, the digital information is sometimes printed on the left margin of the chart in up to six colors. Typically, some form of keyboard on the front panel (or accessible with the door open) is used for programming of chart speed, alarm setpoints, full-scale range, and other features. Some of the features of strip-chart process recorders can include:

    1 to 4-pen continuous writing and up to 24-point dot printing;
    Memory card for recorder setup and data storage; and,
    Digital display, horizontal bar graph, and digital totalizer for each channel.

  In large case, 250-mm recorders, additional features may include:

    Interactive, menu-driven programming;
    Trend printing with user-assignable colors; and,
    Ability to print up to 60 analog input channels.

  Flatbed and portable versions also are available, more suited for laboratory or test uses. Some include signal input modules which plug into compartments on the recorder so that, for example, they can accommodate the direct input from a thermocouple. Some models have as many as six such plug-in input modules and are referred to as modular recorders.
  One recorder with a capability to handle 1,600 samples/second employs a thermo-array recording system, with 200 dots per inch (dpi) resolution on a 200-mm wide thermally sensitive chart. Referred to as therographic recording, this method uses a linear array of fixed thermal-printing elements (stylii) across the chart width. A heated stylus affects the heat sensitive paper to produce the chart record. The chart grid system is generated by the printer.
  A portable, benchtop model that handles 10 or 20 channels uses a printing technique described as raster scan. With this printing method, the recorder can handle high-speed scanning of 20 points/second and high-speed recording of 50 points/second.

Many trend recorders include a digital display of the variable in question as well as a long term paper record.

  A highly-specialized "transient" recorder with a 170-mm wide thermally sensitive paper chart is offered to record signals for which standard recorders are too slow. It offers digital sampling and storage up to 1 MHz. It has a modular design that allows expansion to 30 channels. Its 16 K memory per channel can store up to 16,000 samples and its standard analog output can directly interface to an oscilloscope or strip-chart recorder.

Flatbed Recorders
For testing and other laboratory work, flatbed recorders are offered with a variety of switch-selectable, full-scale ranges to accommodate different inputs. Designed to meet various application needs, they typically offer a range of chart speeds from centimeters/minute to centimeters/hour. A typical slow speed is 1 cm/hr; a fast speed generally is 60 cm/min.
  Most flatbed recorders, including X-Y types, record with ink, using fiber-tipped, disposable cartridges. Chart width is typically 100 mm; larger models range from 200 to 250 mm. Notable is the instrument's speed-typically 0.4 seconds full-scale response for this class.
  Offerings in various flatbed models include:

    Ability to accept a variety of inputs, such as from thermocouples and RTDs, ac/dc voltage/current, and relative humidity-with available inputs depending on the model;
    Two and three pens with pen offset compensation. (One model has four channels with a choice of display modes: measured data, bar graph, or range data);
    Widely adjustable chart speeds. One portable, battery-operated model for recording temperature and relative humidity comes with chart paper pre-printed for chart capacity of 1, 7, and 32 days;     Built-in 3.5-in. digital (LCD) display (on a one-channel model) indicating true value of input;
    Protocol printing on chart to permanently record measurement range, chart speed, etc.;
    Built-in remote control of paper advance, pen lift (automatically lifts the pen after 30 seconds if chart is not advancing), and event marker; and,
    One model type, called a "function recorder," is offered with up to three channels, each of which can have its own plug-in input module to equip the recorder for specific inputs types.

  More sophisticated units add RS-232 serial or IEEE-488 parallel computer interface to handle data output and remote configuration, 3.5-in. disk drive for data logging, and digital data print-out. XY Recorders
XY recorders, though usually flatbed in design, are really a class by themselves. Most employ a potentiometric pen-positioning system, but some newer models feature a digital servo system. The instrument's basic function is to plot the relationship of two input variables with no regard for time. Input signals to the recorder can be analog or digital, with suitable conditioning as required. Common sizes for the chart paper are 8.5 by 11 and 11 by 17 inches.
  In addition to features as described above for flatbed recorders, options available in certain X-Y recorders include:

    Ability to also plot either the X or Y inputs versus time. Such a capability is expressed as X-Y, Y-t and X-t recording. The mode to be used is selectable on the instrument's front adjustment panel.
    An additional servo-actuated measuring element and recording pen yields an XX'Y or XYY' recorder that can plot the relationship among three variables.
    To distinguish different chart records plotted on the same chart, one design with a digital plotter can use eight different color pens (disposable felt tipped). Pen change is selected manually or via RS-232C interface.
    Recorder responsiveness is expressed in a "slewing" speed along each axis. The speed may be, say, 0.6 meter/second for the X-axis and 0.8 m/s for the Y-axis.

  Some thermal-array recorders also include an X-Y recording mode. Such instruments offer much higher real-time frequency response and the ability to print many Y channels against a single X channel. Some thermal-array recorders include a memory X-Y mode, so that the user can capture high-speed transients for later X-Y replay.

Circular Chart Recorders
Basic characteristics of this type of recorder were covered with reference to Figure 7-2. In this class, too, the microprocessor has had its beneficial effects. Recent developments include circular chart recorders that use blank circular chart paper, printing the scales and traces for up to four channels, as recording progresses.

Today's circular chart recorders come equipped with microprocessors and offer an array of display, recording, and control functionality.

  One of the newer types uses heat-sensitive paper on which a single stylus printhead produces up to four analog traces and also prints alphanumeric chart data and time lines. The instrument is user configurable via English language prompts. Another unit can be programmed to print the equivalent of 10-inch, 11-inch, or 12-inch diameter charts. It is fully configurable from a PC or a keypad and prints straight time lines (not curved, as conventional recorders have used for many years).
  Other capabilities that the microprocessor has brought to the circular-chart recorder include:

    Ability to handle a variety of inputs such as all common types of thermocouples, RTDs, dc voltages and currents;
    Control outputs (including PID) and adjustable alarms settings;
    Flow totalization, including data logging of totals; and
    Retransmission options.

  One model, custom-designed for measuring and recording temperature and relative humidity, uses a double-sided, 200-mm (8-inch) chart. Available chart capacities are 1, 7, or 32 days. Here again, an embedded microprocessor affords capabilities such as digital display with user-selectable read-out of temperature in ¡F or ¡C, relative humidity, and alarm setpoints.