Method of measuring the change in volume of a diaphragm bellows used in a volume displacer of a constant-pressure gas flowmeter (with a practical guide)

Construction of constant-pressure gas flowmeters utilizing directly driven bellows and diaphragm bellows as volume displacers were described in recent decades. They have important advantages: accuracy, reliability, easy operation and maintenance.Difficult is to scale sufficiently accurately the changes in volume at compression, to evaluate the uncertainty of this measurement, and to include all the emerging uncertainties into generated throughput uncertainty budget.A compensation method has been proposed for precise measurement of this dependence. This paper presents guides for the implementation of it. The requirements that must be met to achieve a very low uncertainty are described. The method was used for measuring two diaphragm bellows of different sizes as example illustrating achievable values. The attained uncertainty of measurement is analyzed and a procedure is described for determination of the volume of the displaced gas and its uncertainty when using a directly driven diaphragm bellows in a constant-pressure gas flowmeter.