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AutoQuiz: How to Choose Best Method for Level Measurement

AutoQuiz is edited by Joel Don, ISA's social media community manager.


Today's automation industry quiz question comes from the ISA Certified Automation Professional certification program. ISA CAP certification provides a non-biased, third-party, objective assessment and confirmation of an automation professional's skills. The CAP exam is focused on direction, definition, design, development/application, deployment, documentation, and support of systems, software, and equipment used in control systems, manufacturing information systems, systems integration, and operational consulting. Click this link for more information about the CAP program. The following question comes from the CAP study guide, Performance Domain III, System Design, Design, specify, and procure the hardware/software used in the system.



To measure the level of a liquid with a dielectric constant greater than 2 inches a horizontal vessel with changing composition and temperature, choose the device with BEST reproducibility:

a) radar level detector
b) ultrasonic level detector
c) capacitance level detector
d) differential pressure transmitter
e) none of the above


The fact that the tank is horizontal implies the angle of the beam will be narrow at the surface and the vessel walls will not interfere. Radar can detect the smallest change in surface level. It does not depend upon the temperature or composition of the fluid as long as the beam is narrow and the dielectric constant of the fluid is greater than two.

Ultrasonic level detectors are affected by changes in the speed of sound with temperature and are not quite as sensitive as

Capacitance level detectors are affected by changes in the dielectric constant with composition and are not quite as sensitive as radar.

Differential pressure transmitters are affected by changes in liquid density with composition and temperature and are not as sensitive as radar.

The correct answer is A, radar level detector.


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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