Abstract
A constant pressure type flowmeter was developed to generate precise gas flow for calibrations of vacuum gauges by an orifice flow method and standard leaks by a comparison method. The flow rate is derived from the constant pressure and the volume reduction rate. For accurate measurement of volume reduction, a relation of the bellows length and its volume was measured as mounted in the flowmeter. Volume change at 25 mm was 32.19 ml. Measurement uncertainty is 0.24%. In order to keep the pressure in the main chamber constant, a reference chamber was installed to detect pressure change in the main chamber by the differential pressure between two chambers. Pressure was kept constant within 0.01% during the operation of flowmeter. By changing the initial pressure in the flowmeter from 0.16 Pa to 5000 Pa for nitrogen, flow rate of 4×10−9∼8×10−4 Pa m3/s was generated with a relative standard uncertainty of 5.7%∼0.36%. A gas is effused through a metal capillary attached to the main chamber. Repeated conductance measurements of the capillary for 540 days showed that the gas flow rate measurements was stable within∼0.1%.
