Note the circular form of the paper chart, allowing the pen to draw a trace as the circular chart slowly spins. The following image shows an illustration of a Bristol brand recording pressure gauge found on page 562 of Cassier’s Magazine volume 8, published in 1895. Paper chart recorders are a form of instrumentation with a long history. A temperature indicating recorder for the nuclear reactor system shown previously would be designated as a “TIR” accordingly. Recorders usually have indications built into them for showing the instantaneous value of the instrument signal(s) simultaneously with the historical values, and for this reason are usually designated as indicating recorders. The numerical display of this indicator uses LCD technology rather than red-glowing LEDs, in order to use less electrical power:Īnother common “auxiliary” instrument is the recorder (sometimes specifically referred to as a chart recorder or a trend recorder), the purpose of which is to draw a graph of process variable(s) over time. The following photograph shows a field-mounted indicator, operating directly from the electrical power available in the 4-20 mA loop. Indicators may also be used in “field” (process) areas to provide direct indication of measured variables if the transmitter device lacks a human-readable indicator of its own. It is directly wired in series with the same 4-20 milliamp current signal sent to the gate actuator.Ī less sophisticated style of panel-mounted indicator shows only a numeric display, such as this unit shown here: This particular indicator shows the position of a flow-control gate in a wastewater treatment facility, both by numerical value (98.06%) and by the height of a bargraph (very near full open – 100%). This allows us to display the reactor temperature in as many locations as we desire, since there is no absolute limit on how far we may conduct a DC milliamp signal along copper wires.Ī numerical-plus-bargraph indicator appears in this next photograph, mounted in the face of a metal panel inside of a control room: There is nothing preventing us from connecting multiple indicators, at multiple locations, to the same 4 to 20 milliamp signal wires coming from the temperature transmitter. The temperature transmitter is built to withstand the radiation, though, and it transmits a 4 to 20 milliamp electronic signal to an indicating recorder located on the other side of a thick concrete wall blocking the reactor’s radiation, where it is safe for human occupancy. It would be unsafe for human beings to approach the nuclear reactor when it is in full-power operation, due to the strong radiation flux it emits. An example where remote indication would be practical is shown here, in a nuclear reactor temperature measurement system: Moreover, indicators may be located far from their respective transmitters, providing readouts in locations more convenient than the location of the transmitter itself. An indicator gives a human operator a convenient way of seeing what the output of the transmitter is without having to connect test equipment (pressure gauge for 3-15 PSI, ammeter for 4-20 mA) and perform conversion calculations. Quite often process transmitters are not equipped with readouts for whatever variable they measure: they just transmit a standard instrument signal (3 to 15 PSI, 4 to 20 mA, etc.) to another device. One common “auxiliary” instrument is the indicator, the purpose of which is to provide a human-readable indication of an instrument signal. However, other instruments exist to perform useful functions for us. Transmitters, controllers, and control valves are respective examples of each instrument type. So far we have just looked at instruments that sense, control, and influence process variables.
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