A capacitive probe for measuring the clearance between the piston and the cylinder of a gas piston gauge.

We used a capacitive technique to determine the radial clearance between the piston and cylinder of a gas piston gauge. This method determines the effective area of a piston gauge pressure standard, independent of calibration against another piston gauge or manometer, as long as the diameter of the piston is dimensionally measured. It also allows an independent determination of piston gauge distortion due to pressure. We have used this technique to measure the clearance and the distortion of six gas piston gauges of the Ruska 2465 type operating in gauge mode. Measurements were made on two gauges each in the low, middle, and high ranges. We describe the capacitance technique and the results of the measurements on the six piston gauges.