Abstract Precisely known gas flows are required in various applications, as, for example, in generation of uhv calibration pressures, testing of vacuum pumps, and quantification of leak rates. Commercial thermoelectrical flow meters are convenient instruments, but neither fundamental nor available for small throughputs. Laboratory flow meters based on the volumetric method at constant pressure are well proven primary instruments. The present paper describes an advanced version of such an instrument with easy operation. In the normal operation mode as a gas source, the experimental setup is very compact, consisting only of a vernier leak valve, a pressure gauge, and an orifice. The throughput qpV is derived from the orifice conductance C and the pressure p according to the relation qpV = pC. This simple setup takes advantage of the stability of the orifice conductance with time. The tedious procedure of calibrating the orifice conductance has to be performed only once for each orifice and is accomplished as usual with a volume displacement unit. Design features and operating performance of the instrument are described. Using an orifice with 100 μm nominal diameter and two interchangeable pressure gauges (capacitance diaphragm gauges with 10 and 1000 mbar FS) the usable throughput range amounts to about 10−5 − 1 mbar l s−1 with a relative uncertainty of about 2% (2σ).
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