Zero stability and calibration results for a group of capacitance diaphragm gages

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Abstract
The lowest pressure which may be measured by a capacitance diaphragm gage is established by instabilities in the gage zero. In the systematic study of the stability of 14 different temperature-controlled gages we have observed that most of the instabilities are correlated with changes in room temperature. However, the magnitude of the changes for different gages differed by three orders of magnitude, underscoring the necessity for users to evaluate the stability of their own gages. We also observed small but steady zero drifts which extended over weeks, as well as occasional large, usually discontinuous, zero shifts that were precipitated by fast temperature changes. The most stable gage we observed changed by less than one part per million (ppm) of full range over periods of days; at the other extreme we have observed gages to change by 1000 ppm within an hour. At higher pressures the accuracy of gage readings will be determined in part by the long-term stability of the gage calibration factor. The calibration records for 17 gages, for which we have two or more calibrations separated by intervals on the order of 1 year, show shifts ranging from essentially no change to about 2% with an average value of 0.45%. With one exception, these do not appear as a steady drift with time, but as random shifts between calibrations.

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