The constant pressure gas flowmeter must meet the requirements of perfect vacuum seal and low gassing of inner surfaces. In addition, low ratio of invariable volume to variable volume is to be achieved at flowmeter for very low gas throughputs. The volume displacer is a key element of a constant pressure gas flowmeter. So far, dynamically sealed pistons, bellows compressed by means of a liquid and directly compressed bellows have been used in construction.
The possibility of using directly compressed diaphragm bellows as the volume displacer in the construction of a precise constant pressure gas flowmeter was investigated. This arrangement is of similar advantages (a perfect vacuum seal, no oil fill is required in the volume displacer) and disadvantages (the volume of displaced gas cannot be calculated from the geometric dimensions) as that using directly compressed bellows, but, better ratio of invariable volume to variable volume can be achieved.
Inevitable conditions for using the diaphragm bellows are good reproducibility of the curves volume versus degree of compression and a method how to determine these curves very accurately ahead.
Two volume displacers with diaphragm bellows of the displaced volume approximately 100 cm3 and 2 cm3 were designed. Best ratios of invariable volume to variable volume 0.3 and 6 respectively were achieved.
A method of measurement the curves volume versus degree of compression by means of auxiliary volume displacer with piston was developed and the curves have been measured under various differences between inner and ambient pressure in the range 0-100 kPa. These curves are non linear and slightly but notably dependent on differences between inner and ambient gas pressure. Nevertheless the curves are at a given pressure difference well reproducible and thus a directly compressed diaphragm bellows can be used for the design of a volume displacer in a constant pressure gas flowmeter.
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