A PRECISION GAS FLOWMETER FOR VACUUM METROLOGY

In vacuum metrology gas flow meters are used to define very low pressures (10(-2)-10(-7) Pa) by the continuous (so-called 'dynamic') expansion method and to measure leak rates and pumping speeds. They have to deliver a pure gas flow with a well-known throughput over many decades. An all-metal constant pressure flowmeter in use at the PTB is described. It generates throughputs ranging from 10(-4) to 10(-8) Pa m3 s-1 with uncertainties (2sigma) of 0.3-1.5% as received from a rigorous and detailed uncertainty analysis. The low range can be extended by the constant conductance method down to 5 x 10(-12) Pa m3 s-1 (uncertainty at this limit: +/- 9%), the high range by the constant volume method up to 1 Pa m3 s-1 (+/- 5%).