Piston manometer as an absolute standard for vacuum gage calibration in the range 10 to 700 microtorr

Total pressure in a calibration chamber is determined by measuring the force on a disk suspended in an orifice in the baseplate of the chamber. The disk forms a narrow annular gap with the orifice. A continuous flow of calibration gas passes through the chamber and annulus to a downstream pumping system. The ratio of pressures on the two faces of the disk exceeds 100:1, so that chamber pressure is substantially equal to the product of disk area and net force on the disk. This force is measured with an electrodynamometer that can be calibrated in situ with dead weights. Probable error in pressure measurement is plus or minus (0.5 microtorr + 0.6 percent).