Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.
Which of the following is true of most pressure measurement methods?
A. They are not able to measure a small differential pressure.
B. The sensor matches the digital or analog signal conditioning and transmission.
C. They are sensitive to volume but not temperature.
D. They measure pressure by sensing the deflection of the diaphragm.
The correct answer is D, they measure pressure by sensing the deflection of the diaphragm. For most pressure applications, changes in pressure are detected by the change in deflection of a measuring diaphragm.
The deflection is converted into an electrical signal (voltage) by a piezoelectric or capacitance device. The small electrical current is converted to a standard signal (e.g., 4-20 mA or a digital signal) by a transmitter. Therefore, answer B is not correct.
Answer A is not correct, because pressure sensors can measure very small pressure changes (inches of water) and in some cases, millimeters of water.
Pressure measurement devices are not affected by volume, since they are measuring force over an area only. Many pressure sensors are sensitive to temperature (capillary tubes are filled with fluids that can expand with temperature). Therefore, answer C is not correct.
Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.
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