Sensors & Transmitters

A sensor element measures a process variable: flow rate, temperature, pressure, level, pH, density, composition, etc. Much of the time, the measurement is inferred from a second variable: flow and level are often computed from pressure measurements, composition from temperature measurements.

A transducer is a device that receives a signal and retransmits it in a different form. For example, we've discussed I/P transducers that convert a current signal to pneumatic form. Most industrial sensors act to detect process variables in the form of a position or voltage change, and hence most sensors also function as transducers. For example, a thermocouple represents a temperature change as a voltage change, while a displacer represents a level change as a change in position of a rotating element.

If the sensor element does not produce a signal suitable for transmission through the plant, an additional transducer element is needed. This combined sensor/transducer device is typically called a transmitter, at least in industrial settings. Laboratory equipment manufacturers are likely to refer to the combined device as a transducer.

Signals produced by sensor measurement and transducer transmission are processed by the control system. They may be displayed on control panels as indicators, stored by recorders, or used by alarms or switches. Standard symbols for these devices consist of two letter groups -- the first indicates the process variable, the second the control function. So:

• PI - pressure indicator
• TC - temperature controller
• LAH - level alarm high
• FS - flow switch
• PRC - pressure recorder/controller
• TIC - temperature indicator/controller

A sensor device which provides local readout only is usually referred to as a gage. Local pressure gages and level gages (sight glasses) are very common.

When acquiring a transmitter, certain properties are important. These include:

• Accuracy -- the difference between the measured value obtained by the sensor and the true value
• Repeatability -- the difference between the sensed values obtained for the same true value
• Rangeability -- the ratio of large to small readings that maintains accuracy
• Sensor Dynamics -- the time constant of the sensor; how long it takes to detect and transmit a changed value

The users of a transmitter must periodically calibrate the device. This is done by using the sensor to measure some fixed standard and adjusting its settings to ensure accuracy and repeatability.

Users of a sensor/trasmitter typically specify three values:

• the zero is the measurement value corresponding to minimum signal (20 C set to produce 4 mA)
• The range specifies the boundaries of an operating region. This term is used loosely and so it is important to distinguish between
  • the instrument range which is characteristic of the device and set by tolerances, materials of construction, etc. (0 to 500 C can be seen without mechanical failure)
  • the operating range or calibrated range which the device is set to detect (for example, 20 C to 200 C)
• The span is the size of the calibrated operating region (180 C)

The zero of the transmitter will be set to 0 inches.
The span of the transmitter will be set to 24 inches.
Thus, the "calibrated" or operating range of the transmitter will be 0-24 inches.

Next, consider the case where the level is expected to fall between 4 and 30 inches.

The zero will be set to 4 inches.
The span will be set to 26 inches.
The calibrated range will be 4-30 inches; the instrument range will remain 0-40 inches.

Sensor Response

The key factor in sensor response is the measured variable -- pressure and flow measurements are usually fast, temperature slow, and composition slowest.

Sensor time constants are usually on the order of a few seconds, except for composition. The transmitter time constant adds less than 1 second for an electronic signal.

Usually, we want a linear relationship between the measured variable and the signal transmitted. Adjustments must be made for nonlinearities; most notably by using square root extractors in flow measurement.

References:

1. Luyben, Process Modeling, Simulation and Control for Chemical Engineers (2nd Edition), McGraw-Hill, 1990, pp. 205-13.
2. Marlin, T.E., Process Control: Designing Processes and Control Systems for Dynamic Performance, McGraw-Hill, 1995, pp. 234-39.
3. Riggs, J.B., Chemical Process Control (2nd Edition), Ferret Publishing, 2001, pp. 65-67, 77-78.

R.M. Price
Original: 10/15/93
Modified: 3/16/96, 2/24/97, 3/23/98; 4/30/2003

Copyright 1998, 2003 by R.M. Price -- All Rights Reserved