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.
The level measurement installation in the post image infers level by measuring:
A) variable capacitance
B) hydrostatic head
C) positive displacement
D) thermal conductivity
E) none of the above
Click Here to Reveal the Answer
This level measurement apparatus operates by using a constant source of pressurized air or gas coming from the right-side supply. This air bubbles through the visual flow indicator and enters the conduit that leads to the vessel − on the left − whose height we wish to measure. In the vessel, the pressure to force the air down and through the pipe such that it bubbles out is equal to the pressure of the height of the liquid in that vessel from the bottom end of the pipe.
Height translates to level.
Answers A and D use electrical terms, and there is no sign of electrical metering in the diagram. Thus, one could eliminate answers A and D outright. Positive displacement C is a tricky one to eliminate in that we have volumes of gas and liquid here. But it is not the correct answer, and there is no sign of displacement measurement in the diagram.
Hydrostatic head is the pressure that a height of liquid creates above a given point.
The correct answer is B.
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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.