Flow rate measurement by an orifice in a slowly reciprocating gas flow

It is shown that small density changes can give rise to misinterpretation of flow rate signals in unsteady (reciprocating) flows. Basically a flow rate measured at some point A cannot simply be assigned to a remote point B. Depending on the way of plotting a hysteresis appears which, in fact, does not exist. Unsteady conservation of mass is applied to a volume and orifice flow system to obtain an equation which explains and predicts the apparent hysteresis. The equation in dimensionless form contains a key parameter β which holds the flow determining quantities. Experiments are conducted with respect to a wide spread of β. It is shown that the equation predicts reality quite well.