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.

Today’s automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. AutoQuiz-20150501Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. This question is from the Level I study guide, Domain 1, Calibration. Level I represents a professional who has a five-year total of education, training, and/or experience.

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

 

Click Here to Reveal the Answer

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

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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).

InTech uses 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).

 

 

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