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Spitzer and Boyes compares the accuracy of multi-variable and single-variable differential pressure (DP) transmitters.

One instrumentation paradigm occasionally quoted is that multi-variable differential pressure transmitters that measure pressure, temperature and differential pressure are more accurate than three individual transmitters measuring the same parameters.

This was once true, as early multivariable designs used technology that was significantly better than that which was incorporated into the single-variable transmitters available at the time.

However, improvements have been made to single-variable transmitters over the years to the extent that it is now questionable whether multi-variable transmitters offer better performance than single-variable transmitters.

Part of this issue arises from the flexibility of single-variable transmitters.

For example, a multi-variable transmitter may be capable of measuring pressure up to 1,000lb/in2a.

However, measuring the flow of 25# steam with this transmitter will likely introduce more measurement error than a similar pressure transmitter with an upper-range limit of around 100lb/in2a.

This is because some performance specifications are related to the upper-range limit of the transmitter.

Typically, the pressure error associated with a transmitter with a high upper-range limit is higher than the pressure error associated with a transmitter with a lower upper-range limit.

An additional aspect of this issue raises the question whether existing multi-variable transmitters have better performance specifications than the best pressure, temperature and differential pressure transmitters currently available.

A preliminary examination of some offerings indicates that the best single-variable transmitters can offer better performance than multi-variable transmitters in some cases.

This does not imply that single-variable transmitters are superior to multi-variable transmitters or that one transmitter type should be preferred over another.

They should be examined in the context of the application at hand.

Best-in-class performance may not be the prime objective of a measurement, so a multi-variable transmitter may be the proper selection because it offers fewer parts with fewer taps at a lower cost, while meeting the requirements of the application.

However, the paradigm that multi-variable transmitters are superior to single-measurement transmitters should be questioned when best-in-class performance is the objective of a flow measurement system.

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