AutoQuiz: How to Measure the Level of a Liquid with a Dielectric Constant

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 information about the CAP program. This question is from the CAP study guide, Performance Domain III, System Design. Definition: Design, specify, and procure the hardware/software used in the system.

AutoQuiz-20160819-measuring-level-liquid-dielectric-constant

To measure the level of a liquid with a dielectric constant greater than 2 in a horizontal vessel with changing composition and temperature, the device with the best reproducibility is a:

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

   

Click Here to Reveal the Answer

The fact that the tank is horizontal implies that 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 (the ratio of the capacitance of a material to the capacitance of air) of the fluid is greater than two.

Ultrasonic level detectors react to changes in the speed of sound with temperature and are not quite as sensitive as radar.

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

Changes in liquid density with composition and temperature affect differential pressure transmitters and are not as sensitive as radar.

The best answer is A, radar level detector.

 

 

 

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