David W. Spitzer

The accuracy of an orifice plate differential pressure flowmeter that is fabricated and installed per applicable standards can be on the order of 0.60 percent of rate. The stated accuracy of differential pressure transmitters is usually a few times lower, so its contribution to measurement accuracy is neglected in many applications. However, a more detailed investigation may reveal another side to the performance of the differential pressure transmitter. The accuracy of an orifice plate differential pressure flowmeter that is fabricated and installed per applicable standards can be on the order of 0.60 percent of rate. The stated accuracy of differential pressure transmitters is usually a few times lower, so its contribution to measurement accuracy is neglected in many applications.

However, a more detailed investigation may reveal another side to the performance of the differential pressure transmitter.Assume that a differential pressure transmitter has an accuracy of 0.1 percent of calibrated span. If a differential pressure flowmeter system measures 0-100 lpm, what is the approximate flow measurement error introduced by the differential pressure transmitter at 25 lpm?

A. 0.2 percent
B. 0.4 percent
C. 0.8 percent
D. 1.6 percent

Commentary
The span of the differential pressure transmitter is 100 percent of the full scale range (0-100 lpm). For ease of calculation, assume that the differential pressure produced by 100 lpm corresponds to 100 units, and that the factory-calibrated span is 200 units. Therefore, the transmitter is re-ranged at the factory to produce an output of 0-100 units. The accuracy of the transmitter is 0.001*200, or 0.2 units. This error band of 0.2 units remains the same, regardless of the flow rate. (Note that the error band will increase if the calibrated span is more than 200 units, and decrease if it is less than 200 units.)

Differential pressure flowmeters generally operate in the turbulent flow regime where the differential pressure across the flowmeter is proportional to the square of the flow rate. The expression “double the flow… four times the differential” can be applied at 25 lpm, where the differential pressure produced at 25 percent of flow is 100*0.252, or 6.25 units.

With the differential pressure error band of 0.2 units, the differential pressure transmitter will measure a differential pressure of 6.25 units between 6.05 and 6.45 units. These flow rates correspond to 24.6 and 25.4 lpm respectively. The flow measurement accuracy expressed as a percentage of rate is (25-24.6)/25, or 1.6 percent (Answer D). Note that the error introduced by the differential pressure transmitter significantly exceeds the accuracy of the orifice plate flowmeter at this flow rate.

Additional Complicating Factors
The accuracy of differential pressure transmitters increase when their operating pressure and temperature vary from calibrated conditions. Often, day-to-night and seasonal ambient temperature changes can introduce errors that exceed the reference accuracy of the transmitter, and hence degrade the performance of the flow measurement system.

David W. Spitzer is a regular contributor to Flow Control with more than 35 years of experience in specifying, building, installing, startup, troubleshooting and teaching process control instrumentation. Mr. Spitzer has written over 10 books and 150 technical articles about instrumentation and process control, including the popular “Consumer Guide” series that compares flowmeters by supplier. Mr. Spitzer is a principal in Spitzer and Boyes LLC, offering engineering, expert witness, development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.