Gas flow measurement by the thin orifice and the classical Venturi tube

Abstract

Many applications of vacuum technology require the generation and measurement of known gas flows. For this purpose, one may use ducts with constrictions. At a constriction, the flow-dependent pressure drop occurs. The gas throughput can be obtained from the inlet and outlet pressures at the constriction, provided its characteristics are known. In the present study, the characteristics of constrictions with basic geometry, i.e. the (infinitely) thin circular orifice and the standardised (DIN 1952) classical Venturi tube were investigated. Experimental methods for measuring the gas flow through a duct are described. The characteristics of individual constrictions were carefully measured and the data quantitatively compared to theoretical calculations. The results are discussed in order to provide a better understanding of the flow phenomena and to make them applicable to thin orifices and to Venturi tubes of any size and arbitrary gas. Over the whole flow range from molecular to viscous, the thin orifice can be used. However, it causes permanent pressure loss and it only has a small aperture in relation to the duct dimensions. In the viscous range, the Venturi tube can be used successfully. Thus, it is possible to establish stable secondary flow standards if the proper constriction is chosen.