Flow is one of the four major physical measurements in processes. Flow meters may be classified in four categories: differential pressure, velocity, mass and positive displacement or volumetric. Each category has advantages and disadvantages; however, the focus of this paper will be the differential pressure flow meters.

Differential pressure (dP) flow meters include flow elements such as the orifice plate, venturi, flow nozzle, wedge meter, cone meter and proprietary devices. The elbow flow meter, pitot and annubar are also differential type flow meters, but have a different operating principle than the others and are outside of the scope of this paper. These meters may be referred to as flow elements (FE) or primary flow elements. These flow elements are called primary because an additional or secondary device must be attached to indicate the measured differential pressure. This secondary device may be an electronic transmitter, manometer, standpipe or gage.

dP flow elements are often called “head” type, obstruction or inferential meters. The flow rate is calculated or inferred from the measured difference in pressure or head. The basic principle of operation is that an obstruction is placed in the flow stream reducing the flow area, thereby creating an increase in velocity and a decrease in the flowing pressure.