In almost all liquid flow measurement instruments, the rate of flow is calculated by measuring the liquid’s velocity or the change in kinetic energy. The velocity of liquid depends on the differential pressure which is forcing the liquid through a pipe or conduit. Since the cross-sectional area of pipe is known and it remains constant, the average velocity is an indication of the flow rate. In such cases, the basic relationship for determining the liquid’s flow rate is given by:
Q = V x A
Where Q is the liquid flow through the pipe, V is the average velocity of the flow and A is the cross-sectional area of the pipe.