Quiz Corner: How to measure level of tank that contains oil and water

Which of the following technologies can be used to measure the level in a tank containing oil and water?

A. Capacitance
B. Differential pressure
C. Float/Displacer
D. Ultrasonic
E. All of the above

The contents of the tank will likely separate into two liquid phases where the oil will float on the water because its density is lower. All of these technologies can measure the level of the oil, so Answer E is correct.

Notwithstanding the answer to the stated question, the questioner should be interrogated as to whether he or she really wants to measure the level in the tank or the level of the oilwater interface — or both — understanding that people often get so engrossed in their work that they use shortcuts in their speech and writing.

If the intent is to measure the interface level, capacitance, differential pressure and float/displacer level technologies could be applicable. Ultrasonic level technology would not penetrate the liquid surface and hence measure the level, not the interface level.

Additional complicating factors

Many factors can complicate these measurements such as the presence of foam above the oil, agitation in the tank and a rag layer at the oil-water interface.

David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes LLC, which offers engineering, seminars, strategic, marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. Spitzer and Boyes is also the publisher of the Industrial Automation INSIDER. He has more than 40 years of experience and has written more than 10 books and 350 articles about flow measurement, instrumentation and process control.

Spitzer may be reached at 845-623-1830 or via spitzerandboyes.com. Click on the "Products" tab to find his Consumer Guides to various flow and level measurement technologies.