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Differential Pressure Transmitter Performance (Part 2 of 3)

By David W. Spitzer

E-Zine January 2006

Click here to review Part 1

Microprocessor-based differential pressure flow transmitters are generally more accurate and exhibit a larger turndown than analog differential pressure flow transmitters. As a result, fewer models (capsule sizes) are needed to measure the applicable differential pressure ranges. For example, 2 or 3 microprocessor-based transmitter models may be needed to measure over the same differential pressure ranges handled by 5 or 6 (older) analog transmitter models. As result, users could reduce their spare parts inventory costs while suppliers benefited by the savings resulting from the design and support of fewer models. From a performance perspective, transmitter accuracy specifications for each model generally apply over a range of set spans.

For example, a (hypothetical) differential pressure transmitter may have an accuracy specification of 0.10% of set span when the span is set between 10 and 100 percent of its upper range limit (URL). Assume also that the (hypothetical) differential pressure transmitter is available with URLs of 1000, 250 and 50 inches WC.

With these transmitter offerings, a 150 inch WC range can be measured accurately using the 250 or 1000 inch WC models. The set span of the 150 inch WC range is 15 percent of the URL of the 1000 inch WC transmitter and 60 percent of the URL of the 250 inch WC transmitter. The span of both transmitters is between the stipulated 10 and 100 percent of URL over which the accuracy statement applies, so both transmitters have the same accuracy specification.

As a result of this analysis, many suppliers and users applied microprocessor-based differential pressure flow transmitters using a "one-size fits all" design philosophy. For example, the following table illustrates the range selection strategy for this (hypothetical) transmitter offering.

Differential Pressure Range Transmitter URL
100 - 1000 inch WC 1000 inch WC
50 - 100 inch WC 250 inch WC
under 50 inch WC 50 inch WC

Sizing flow elements to avoid differential pressure ranges between 50 and 100 inch WC will eliminate the need for 250 inch WC transmitters.

All may seem well, but Part 3 will discuss the effects of pressure, temperature and stability on transmitter performance.

Click here to go to Part 3

ISSN 1538-5280

Spitzer and Boyes, LLC
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