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AutoQuiz: How to Measure Pressure in a Tank

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.


Approximately what is the pressure, in psi, at the bottom of a 20-foot (6.1 meter) tank filled to 100% of capacity with a fluid that has a specific gravity of 1.5?

a) 8.7255
b) 12.9964
c) 5.6521
d) 35.1423
e) none of the above

One needs to have some conversion factors or tables ready to answer this question. Also, assume that psi, pounds per square inch, is in fact psig, pounds per square inch gauge. As an aside, recall that:

Pabsolute = Pgauge + Patmosphere

Nearly all of our calculations in the plant are psig. That is why in this problem we do not worry about the weight of the atmosphere on top of the 20 ft tank of water. A pressure gauge reads zero because it disregards the atmospheric pressure, which is constant at all points where we operate.

The specific gravity (SG) of a substance is the ratio of the density of the substance to the density of water. Therefore, the SG of water is one (1). The substance in this problem is 1.5 times as dense as water.

Using dimensional analysis and knowing from conversion tables that 33.9 ft H2O equals one atmosphere equals 14.696 lbf /in2 (psi), we perform the calculation.

The best answer is B, 12.99.

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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