KING FAHD UNIVERSITY CHEMICAL ENGINEERING COURSE NOTES (Simulation)-Chapter_9

KING FAHD UNIVERSITY CHEMICAL ENGINEERING COURSE NOTES (Simulation)-Chapter_9

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1 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).
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2 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.
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3 Chapter 9 Since about 1960, electronic instrumentation has come into widespread use. Sensors 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.
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4 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 ( 29 ( 29 ( 29 20 mA 4 mA mA 50 C 4 mA 150 C 50 C mA 0.16 C 4 mA C m T T T - = - + - = - o o o o o
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5 Chapter 9 The gain of the measurement element
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This note was uploaded on 09/22/2009 for the course CHEMICAL CHE 401 taught by Professor Dr.muhammadal-arfaj during the Spring '09 term at King Fahd University of Petroleum & Minerals.

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KING FAHD UNIVERSITY CHEMICAL ENGINEERING COURSE NOTES (Simulation)-Chapter_9

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