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The Importance of Proper Orifice Sizing and Proper Calibration

The uncertainty and error caused by low differential recordings on chart or electronic flow meters is depicted in the following chart and table. One can see error and uncertainty increases exponentially with lower differential recordings. The data below assumes an error of plus or minus 1/10th (0.1) of 1 inch water column pressure. The data suggests differential recordings should be kept above 20 inches in order to maintain an "interpretation" error of less than ¼ of one percent, even with an electronic flow meter where the transducer error is 1/10th (0.1) of 1 inch water column pressure. When you apply this to a chart the uncertainty is multiplied several times. When a chart record is produced there is calibration and chart paper error, and again more error when the chart integrator operator attempts to interpret the recorded line. It is best to stay away from the lower range of a meter by properly sizing the orifice plate.