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This newsletter is e-mailed by McCrometer monthly to subscribers worldwide. It consists of flow measurement information of interest for professionals in this field.

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McCrometer September 2002 Newsletter

Errors From Differential Pressure Transmitters

Flowmeters are only as good as the instrumentation that reads their output. In the case of differential pressure (DP) meters, this includes the DP Transmitter. Here, we look at the sources of errors associated with measuring pressure using an electronic DP transmitter.

There are four major sources of errors with this type of transmitter: Repeatability; ambient temperature effect; line pressure effect; and stability. The first three are independent of each other, while the fourth error (stability) is dependent on time.
Repeatability, also known as 'reference accuracy', is the uncertainty from the spread in the data obtained when repeating readings under identical flow conditions.
Ambient temperature shift is due to a change from the temperature at which the transmitter was calibrated to another ambient temperature.
Static line pressure effect is due to a change from the calibration pressure to another working pressure. It effects the zero and span of a transmitter, however at low DP values, the effect on the span is very small.
Lastly, stability or 'drift' indicates a transmitter's ability to be consistent over a period of time.

The good news is that errors due to line pressure, ambient temperature and drift effects can all be eliminated by the sound engineering practice of periodically 'zeroing' at the working line pressure and ambient temperature. But errors of repeatability are a little more complex.

DP transmitters have a designated Upper Limit Range or 'URL' that states the device's largest readable DP. The transmitter can be set to any value at or under this URL, and is called the 'span'. Often the uncertainty of the DP reading is described in terms of 'percentage of span'. This can be considerably different to the 'percentage of reading' especially at low differential pressures. For example, if a URL of 250-inch water column (WC) is spanned to 100-inch WC and the uncertainty is said to be 1% of span (ie 1-inch WC), when you take a reading of 10-inch WC the actual uncertainty is 10% not 1%. Therefore you should take care when estimating overall metering uncertainties.

To find out more about McCrometer's line of flowmeters and using them for optimum flow measurement, contact McCrometer at 951-652-6811 or visit our web-site at www.mccrometer.com <http://www.mccrometer.com>.
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Copyright 2002 McCrometer