Pressure recovery in orifice-type meters

It should be noted that all these restricted flow type meters are primarily intended for pipe sizes greater than 50 mm, and when used on smaller tubes they must be individually calibrated. 

In the meters so far described the area of the constriction or orifice is constant and the drop in pressure is dependent on the rate of flow. In the variable area meter, the drop in pressure is constant and the flowrate is a function of the area of the constriction. 

A typical meter of this kind, which is commonly known as a rotameter 6.21), 

This meter may thus be considered as an orifice meter with a variable aperture, and the formulae already derived are therefore applicable with only minor changes. Both in the orifice-type meter and in the rotameter the pressure drop arises from the conversion of pressure energy to kinetic energy and from frictional losses which are accounted for in the coefficient of discharge. The pressure difference over the float — AP, is given by  

If the area of the annulus between the float and tube is A2 and the cross-sectional area of the tube is A, then from equation 6.19  

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