March 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
A plant has a water tank mounted on top of an 80-feet platform. The tank is 30 feet high. What is the level of water in the tank if a differential pressure transmitter mounted on the bottom of the tank reads 277 inches of water?
A. 10 feet
B. 17 feet
C. 23 feet
D. 30 feet
CCST answer
A pressure unit is the ratio of a force to the area on which the force acts. The units of pressure are force units divided by area units like Newtons per square meter (N/m2) and pounds-force per square inch (lbf/in2 or psi).
In addition to being expressible as a force per unit area, a pressure may express as a “head” of a particular fluid, like water, as it does in this problem. You may have heard the expression “we’ve got a good head of steam,” which comes from the days locomotives were steam engines and this expression meant the pressure in the engine was high and thus, so was the locomotive’s speed.
“Head” means the height of the fluid that would exert a given pressure at the base of the column that contains that fluid.
Therefore, we can speak of 14.7 psi (atmospheric pressure) as equivalent to the force 33.9 feet of water would exert at the bottom.
The pressure transmitter reads 277 inches of water. Divide this by 12 to translate inches to feet, the units of the answer. The best answer is C, 23 feet.
The 80-feet high platform does not matter, and the 30 of the 30-feet deep tank is another number that does not matter to the actual calculation, though of course the depth of the tank has to be more than 23 feet for the problem to make sense.
Nicholas Sheble (nsheble@isa.org) writes and edits Certification Review.
Read questions answered by our experts or join the email list.
