Differential Pressure Transmitter Performance (Part 3 of 3)

E-Zine February 2006

Click here to review Part 1
Click here to review Part 2

Differential pressure flow transmitter performance is affected by process pressure, ambient temperature, and drift (stability).

Process pressure effects are expressed as a percentage of set span per unit pressure change. As such, the pressure effects for all of the (hypothetical) differential pressure transmitters are the same.

It should be noted that the ambient temperature effects can contribute more error to the measurement than accuracy. For example, according to The Consumer Guide to Differential Pressure Flow Transmitters, the contributions of accuracy and a 25ºC ambient temperature variation for the best transmitter with a set span of 250 mbar (100 inch WC) are 0.1250 and 0.1002 mbar respectively. The temperature effect of almost all of the other transmitters is greater than the error associated with their respective accuracy specification. Further, these seemingly small fixed errors tend to produce relatively large flow errors in the lower portion of the flow range due to the nonlinear nature of the Venturi flow element.

Ambient temperature effect specifications are generally expressed as a percentage of URL per unit temperature plus a percentage of set span per unit temperature. The ambient temperature effect of the 1000 inch WC (hypothetical) transmitter is larger than that of the URL of the 250 inch WC (hypothetical) transmitter because its URL is larger. Therefore, when temperature effects are considered for a 150 inch WC range, the performance of the 250 inch WC transmitter is superior to that of the 1000 inch WC transmitter --- even though their accuracy specifications are the same.

Stability specifications are generally expressed as a percentage of URL per unit time. The error associated with the stability of the 1000 inch WC (hypothetical) transmitter is four (4) times larger than that of the URL of the 250 inch WC (hypothetical) transmitter because its URL is four (4) times larger. Therefore, when stability is considered for a 150 inch WC range, the performance of the 250 inch WC transmitter is superior to that of the 1000 inch WC transmitter --- even though their stability specifications are expressed identically (for example, 0.2% URL for 12 months) and their accuracy specifications are the same.

The overall result of this analysis shows that a "one-size fits all" strategy based solely on accuracy specifications is flawed due to the existence of other influences on differential pressure transmitter performance.

ISSN 1538-5280