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
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