If a pipeline (through which a medium flows) is reduced at a particular point by a cross sectional constriction, the flow speed of the measured medium is increased at that point. According to Bernoulli's energy equation and the law of continuity, the total flow head (dynamic velocity head and static pressure head) is constant.
The increase in speed at the constriction causes a reduction in the static head.
The resulting pressure drop is called the differential pressure head; it is a measure of the flow (volume per unit of time or mass per unit of time). The flow rate q is given by: q = c√∆p The flow rate (q) is a function of the square root of the differential pressure head (∆p), where c is the coefficient of flow rate determined by the shape of the differential pressure transducer and the operating data.
The pipeline may be constricted with orifice plates.