AutoQuiz: How to Calculate the Output Signal Value of a Pneumatic Transmitter

AutoQuiz is edited by Joel Don, ISA’s community manager.

Today’s automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for information about the CCST program. This question comes from the Level I study guide, Domain 3, Troubleshooting. Level I represents a professional who has a five-year total of education, training, and/or experience.

If the range of a 3-15 psi pneumatic transmitter is 50-350 degrees F, what is the output signal value, in psi, at 198 degrees F?

a) 5.7
b) 7.2
c) 8.9
d) 12.6
e) none of the above

   

Click Here to Reveal the Answer

There is a straightforward linear relationship between the units of pounds  per square inch (psi) and degrees F (Fahrenheit). Therefore, one (1) psi equals 25 degrees F. The calculation for this is ((350-50) degrees F) / ((15-3) psi) = 25 degrees F / one psi.
To find the output signal, use the given temperature-198-and subtract the lower limit-50-from it to get 148 degrees. So, the output signal will be 148 degrees F above the lower output signal of 3 psi. Convert the 148 degrees F to psi by dividing 148 by 25, which equals 5.92 psi. Add 5.92 psi to the lower limit of 3 psi and get 8.92 psi.

The best answer is C, 8.9.

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