# ISA Interchange

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# AutoQuiz: Level Reading with a Differential Pressure Cell

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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 more information about the CCST program.

A differential pressure (d/p) cell is mounted 3 feet below the zero level in a tank. For a true level reading, the transmitter output must be:

a) calibrated for elevated zero
b) calibrated for span
c) calibrated for true zero
d) calibrated for suppressed zero
e) none of the above

If the d/p cell is not located at an elevation that corresponds to 0% level in the tank, we must calibrate it to account for the difference in elevation.

This calibration adjustment is "zero elevation" when the cell is located above the lower tap; it is "zero suppression" when the cell is located below the lower tap.

To calculate the amount of elevation or suppression, we must measure the distance between the zero reference level and the high-pressure port of the d/p cell. Taking into account that 27.68" H2O = one (1) psig, calibrate the transmitter for the distance measured in the direction necessary to result in an accurate output.

When the d/p cell is at the same elevation as the zero level in the tank, calibrate the transmitter for "true zero."

The correct answer is D, calibrated for suppressed zero.

Reference: Mike Cable, Calibration: A Technician's Guide, ISA Press.

###### Joel Don
Joel Don is the community manager for ISA and is an independent content marketing, social media and public relations consultant. Prior to his work in marketing and PR, Joel served as an editor for regional newspapers and national magazines throughout the U.S. He earned a master's degree from the Medill School at Northwestern University with a focus on science, engineering and biomedical marketing communications, and a bachelor of science degree from UC San Diego.