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Gas Flow Pressure & Temperature Compensation

September 26, 2010
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David W. Spitzer
When performing pressure and temperature compensation for a differential-pressure gas flow measurement, one should:

A. Pressure and temperature compensate the differential pressure
measurement, and then take the square root.
B. Take the square root of the differential pressure measurement, and then pressure and temperature compensate
C. Pressure and temperature compensate the differential pressure
measurement with no square root
D. None of the above

Commentary
For differential-pressure flowmeters operating in the turbulent flow regime, the volumetric flow (Q) is proportional to the square root of the ratio of the differential pressure (ΔP) divided by the fluid density (ρ).

Q α (ΔP / ρ) ½

The form of this equation indicates the differential-pressure measurement is divided by the gas density, and then the square root of the result is taken. Answer B and Answer C are not correct.

A closer look reveals the density of the gas “corrects” or compensates the differential-pressure measurement. Gas density is a function of pressure and temperature and can be calculated using various techniques to include the Gas Laws, density tables, and equations of state. Afterwards, the square root of the result is taken. Therefore, Answer A is correct.

This question surfaces every so often – especially when the implementation of pressure and temperature compensation is not implemented in a flow computer. Nonetheless, it is an important question because failure to properly implement the correct calculation can result in flow errors. Further, operators and engineers may unknowingly use these flawed measurements. Investigation as to the accuracy of the flawed measurements will likely occur only when the flawed measurements yield unreasonable results in important calculations. An instrumentation specialist is often required to find the flaw in the calculations.

Additional Complicating Factors
There are many reasons for instrument users to implement gas flow pressure and temperature compensation in their control systems instead of in a flow computer. However, flow computers should be used in custody-transfer applications and considered when the computational technique used to calculate density (such as the Gas Laws or equations of state) introduces significant measurement error.

David W. Spitzer is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular "Consumer Guide" series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering consulting services for manufacturing and automation companies. He can be reached at 845 623-1830.

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