Criteria for a differential pressure instrument

Diameter of conduit Ratio of diameters Reynolds number	Each type of differential pressure instrument is subject to specific limiting values for the internal diameter of the conduit (see column 1 in the selection table), for the diameter ratio β (for a specific orifice plate) exceeds the permissible limit, it is preferable to chose a nozzle which, under the same conditions, will require a lower diameter ratio β.
Pressure losses	For a given differential pressure Dp, the pressure losses are 4 to 6 times less in classic Venturi tubes and Venturi nozzles, than in orifice plates and nozzles .
*Lenght of straight pipe to be provided upstream and downstream of the instrument*	For reliable meauserement to be guaranteed, the fluid must enter the primary element free of turbolence. This can be achieved by providing suitable lenghts of straight pipe.
*Type of fluid*	When abrasive or corrosive fluid are measured, the flowrate may vary over tume, due to wearing of the edge. Deposits left in the nozzleand in Venturi tube can have a considerable influence on the flowrate coefficient α. These modifications to the flowrate over time are usually pronounced at first, afterwards they attenuate until stabilization.
*Accuracy*	The error tolerance for the flowrate coefficient are defined individually for each differential pressure instrument (see columns 7 and 8 in the selection table).
*Cost and manufacture*	Orifice plates are less expensive and more simple than all other types of differential pressure flowrate measurement devices.