Measurement of flow rate and quantity is the oldest of all measurement of process variables in the field of instrumentation. It is made for determining the proportions and the amount of materials flowing in or out of a process. For the purpose of cost accounting, such measurement is often required to be made for stream, water and gas services. Without flow measurements, plant process would be almost impossible.

Various kinds of flowmeters available are

• Variable Head or Differential flowmeter.
  This is the oldest and most widely used method of industrial flow measurement. The flow meter measures volume rather than the mass flow rates, but the mass flow rates can be calculated or computed easily by knowing or sensing temperatures and pressures.
• Variable area Flowmeter.
  In the differential flowmeter, the flow restriction is of fixed size and the pressure differential across it changes with the flow rate; whereas in the variables area flow meters, the size of the restrictions is adjusted by an amount necessary to keep the pressure differential constant when the flow rate changes, and the amount of adjustment required is proportional to the flow rate. Basically, there are two types of variable- area flowmeter Rotameters and Cylinder Piston type flowmeter.
• Magnetic flowmeters
• Turbine flowmeter
• Thermal flowmeters
• Vortex flowmeter
• fluidic flowmeter
• ultrasonic flowmeter