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AutoQuiz: How Do You Calculate the Pressure Measurement of an Open Tube Manometer

AutoQuiz is edited by Joel Don, ISA's social media community manager.

This 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.

If an open tube manometer indicates 6.11 inches of Hg (mercury), what is the pressure measurement?

A) 2 psi
B) 3 psia
C) 17.89 psig
D) 3 psig
E) none of the above

First, recall the arrangement that constitutes an open tube manometer.

mar15

Now, recall the definitions and nomenclature of pressure.

Pabsolute = Pgauge + Patmospheric which is often noted as Pa = Pg + Patm or psia = psig + Patm.

Looking at the diagram of an open tube manometer one can see that what one is looking for to answer this question is the value for h (P2) or psig.

The next chore is to convert the units of inches of mercury to pounds per square inch (psi).

Use a conversion of 29.921 inches of Hg at 0°C to 14.696 lbf/in2 at 0°C to get 3 psig, which is answer D, the correct answer.

Be careful not to choose answer C, which is a pretty good answer for the absolute pressure (psia) but not for the gauge pressure (psig). Answer B is the correct numerical answer for the wrong parameter (psia).

About the Editor
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.

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Joel Don
Joel Don
Joel Don 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.

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