A constant pressure gas flowmeter using a directly driven diaphragm bellows as a volume displacer was studied. This flowmeter is perfectly vacuum-sealed, does not contain elastomers and liquids that would prevent outgassing at elevated temperatures and can achieve a smaller ratio of the final volume to the displaced volume than when bellows are used, so that the uncertainty in generating small flow rates can be small. As, when a bellows is used in the volume displacer, the displaced volume cannot be calculated from the geometric dimensions and its dependence on the displacement is non-linear, a sensitive method had to be developed for measuring this dependence. The uncertainty in measuring the displaced volume by this method and the achieved uncertainty of the flowmeter were determined.

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The pressure differences between the positions for gauge connection in a chamber for gauge comparison were measured and evaluated in two ways: (a) by the mutual interchange of two ionization gauges at the positions; and (b) by readings of one ionization gauge at various positions and of a spinning rotor gauge at a fixed position. The pressure in the chamber was set up by simultaneous admission and pumping of the gas. Differences of the order of a few percent were observed, probably caused by imperfect adjustment of the gas-introducing jet. Those differences are specific to the given apparatus, they depend on its adjustment, the geometry and some other parameters. The two methods are suitable for checking and adjusting the apparatus.

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