David W. Spitzer

A primary flow element generates 0-150 inches of water column differential at maximum flow. Presuming similar design and manufacture, which of the following capsules, with the following (hypothetical) maximum spans, should be selected?

A. 0-130 inches of water column
B. 0-150 inches of water column
C. 0-200 inches of water column
D. 0-250 inches of water column

The differential-pressure flow transmitter should be calibrated for 150 inches of water column. Even though it might be possible to calibrate Transmitter A to 0-150 inches, the maximum capsule span should be larger than the calibration. Transmitter A will likely not be accurate.

The maximum span for Transmitter B matches the primary flow element requirement, while Transmitters C and D can be calibrated to 0-150 inches of water column. Therefore, Transmitters B, C and D could legitimately be selected.

However, it would be appropriate to gather more information about the application in order to select the best transmitter. For example, will the process be expanded to (say) double its existing capacity? If so, Transmitter D might be the best selection so as not to have to purchase a new transmitter during the expansion.

Given this, the maximum capsule span should be higher than the calibrated span, and assuming the transmitters are of similar manufacture and design, superior flowmeter performance is usually achieved by selecting the capsule that is as close as practical to the calibrated span. In this application, Transmitter B should be selected because it is equal to the calibration.

Additional Complicating Factors
The above analysis is quite cursory in nature. More detailed analysis of the accuracy, temperature effect, and pressure effect should be performed to quantify the improved accuracy of Transmitter B. Further, complication will occur when comparing differential pressure transmitters of different design and manufacture.

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.