Why do Pressure Calibration Certificates show Rising and Falling Results?

When looking at a calibration certificate you will notice that the calibration points are set out in the order they were recorded as one set of rising results followed by a set of falling results. 

The majority of pressure measurement equipment relies on the flexing of a mechanical component such as a metal diaphragm or a Bourdon Tube to sense a change in pressure.  If you compare increasing pressure points to decreasing pressure points you will discover that the readings do not match exactly.  This is due to Mechanical Hysteresis and the amount of error will vary depending on the amplitude of the cycle of pressure.

If each pressure calibration point is applied in an increasing or a decreasing sequence, the Hysteresis can be eliminated from the results leaving only the Linearity errors to be checked.  The Hysteresis can then be checked independently of linearity by comparing the rising and falling calibration data collected over a full range cycle of pressure.  If the calibration points are set randomly, there will be no way to separate Linearity and Hysteresis errors and therefore it will not be possible to quantify the contribution by each to the overall error.

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