Calculating for Calibration & Conversion-Factor Inconsistency  

David W. Spitzer, P.E

Accurate flow measurement often entails careful attention to detail. Errors can be introduced when calculations in different parts of the flow measurement system are performed using different standard conditions. What is the approximate effect on performance of the flow measurement system when a differential-pressure flowmeter is calibrated in inches of water column at 20 C and the flow computer uses a flow conversion factor of 27.70728 inches WC per PSI?

A. 0.074 percent higher

B. 0.037 percent higher

C. No effect

D. 0.037 percent lower

E. 0.074 percent lower


The height of water at 20 C that generates one PSI of pressure is 27.7276 inches. The flow computer uses a conversion factor of 27.70728 inches. Inconsistency between the differential-pressure transmitter calibration and the flow computer conversion factor will result in measurement error, so Answer C is not correct.

The flow computer uses a lower conversion factor than that used for calibration. Therefore, a transmitter that is calibrated at (say) 100 inches of water column at 20 C will be calibrated at a pressure of 3.607 PSI (100 / 27.7226). The flow computer will interpret 3.607 PSI as 99.940 inches of water column (3.607 * 27.70728). Therefore, the flow computer will measure lower so Answers A and B are not correct.

The difference in the density is related to the ratio of the water columns generated. In this application, this difference is approximately 0.073 percent (100*(27.7276 - 27.70728) / 27.70728). However, the flow varies as the square root of the differential pressure, so the effect on the flow measurement is approximately half of this amount or approximately 0.037 percent. Therefore, Answer D is correct.

Additional Complicating Factors

This error could have been avoided by using pressure units, such as kPa or mbar, for all differential-pressure measurements and calculations. This would entail recalculation of the flowmeter element differential-pressure calculation and subsequent recalibration if necessary. Reconfiguration of the flow computer to utilize the same pressure units that were used in the flow element calculation is dependent upon the sophistication of the flow computer and may or may not be possible.

David W. Spitzer, P.E., 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 is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.






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