Fluid Density’s Effect on a DP Flow Control Loop

Feb. 29, 2012

David W. Spitzer How is an orifice-plate flowmeter measurement in a flow control loop affected by an increase in fluid density? A. Decreases flow measurement linearly with

David W. Spitzer

How is an orifice-plate flowmeter measurement in a flow control loop affected by an increase in fluid density?

A. Decreases flow measurement linearly with fluid density
B. Decreases flow measurement as square of fluid density
C. Decreases flow measurement as the inverse square root of fluid density
D. None of the above

Commentary
The output of a differential-pressure transmitter (Q) is proportional to the square root of the ratio of the differential pressure (ΔP) to the fluid density (ρ).

Q α ( ΔP / ρ ) ½

The flow control loop maintains the output of the differential-pressure transmitter constant, so ΔP remains constant. Fluid density is in the denominator and within the square root, so increasing fluid density will decrease the flow measurement as the inverse of the square root of fluid density. Answer C is correct.

Many differential-pressure flowmeters (especially measuring gases) are compensated for fluid density by using transmitters that measure the actual operating conditions, such as pressure and temperature. Calculations are then performed to compensate the raw differential-pressure measurement for the measured operating conditions. In theory, the compensated differential pressure would not be affected by fluid density changes. In practice, errors are introduced due to the accuracy of the compensating measurement instruments, compensation algorithms, and calculations.

David W. Spitzer is regular contributor to Flow Control magazine and a principal in Spitzer and Boyes, LLC, offering engineering, seminars, strategic marketing and distribution consulting, and expert witness services for manufacturing and automation companies. He has more than 35 years of experience and has written over 10 books and 200 articles about flow measurement, instrumentation, and process control.

Mr. Spitzer can be reached at 845 623-1830. For more information and to order his various textbooks on industrial flow measurement and process control, visit spitzerandboyes.com and click on the “Products” tab.

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