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Chapter 9 Control System Instrumentation Figure 9 3 A typical process transducer Transducers and Transmitters Figure 9 3 illustrates the general configuration of a measurement transducer it typically consists of a sensing element combined with a driving element transmitter 1 Chapter 9 Transducers for process measurements convert the magnitude of a process variable e g flow rate pressure temperature level or concentration into a signal that can be sent directly to the controller The sensing element is required to convert the measured quantity that is the process variable into some quantity more appropriate for mechanical or electrical processing within the transducer Standard Instrumentation Signal Levels Before 1960 instrumentation in the process industries utilized pneumatic air pressure signals to transmit measurement and control information almost exclusively These devices make use of mechanical force balance elements to generate signals in the range of 3 to 15 psig an industry standard 2 Since about 1960 electronic instrumentation has come into widespread use Sensors Chapter 9 The book briefly discusses commonly used sensors for the most important process variables See text Transmitters A transmitter usually converts the sensor output to a signal level appropriate for input to a controller such as 4 to 20 mA Transmitters are generally designed to be direct acting In addition most commercial transmitters have an adjustable input range or span For example a temperature transmitter might be adjusted so that the input range of a platinum resistance element the sensor is 50 to 150 C 3 Chapter 9 In this case the following correspondence is obtained Input Output 50 C 4 mA 150 C 20 mA This instrument transducer has a lower limit or zero of 50 C and a range or span of 100 C For the temperature transmitter discussed above the relation between transducer output and input is 20 mA 4 mA D Tm mA T 50 C 4 mA D D 150 C 50 C mA D 0 16 D T C 4 mA C 4 The gain of the measurement

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