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).