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January 2009

ISA Certified Control Systems Technician (CCST) program

Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

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. 

CCST question

Approximately what is the pressure, in psi, at the bottom of a 20 ft tank filled to 100% of capacity with a fluid that has a specific gravity of 1.5?

A. 8.7

B. 12.99

C. 5

D. 35

CCST answer

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:

P_absolute = P_gauge + P_atmosphere

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.