Measurement of volume change induced by a bulging of a diaphragm inside a differential type capacitance diaphragm gauge

A diaphragm gauge measures differential pressure by bulging of a thin diaphragm inside a metal housing. A volume change induced by its bulging is evaluated for a full scale 133 Pa CDG (differential type) by using Boyle's law. It was observed that below its full scale, the bulged volume is proportional to the pressure as predicted by the elastic theory. The ratio of the bulged volume to the pressure is obtained to be 0.000131 ml/Pa with an uncertainty of 1.4% (k = 1) at a line pressure of about 100 kPa. The bulged volume is increased with an increase of the differential pressure above 133 Pa. At a differential pressure of 40 kPa, the diaphragm is seemed to be touching to a guard electrode and the corresponding maximum bulged volume is estimated to be 0.18 ml.