Design of flowmeters
Efficient handling, utilization and disposal of fluids in engineering processes require knowledge of quantities of fluids flowing. Indirectly, this information can be obtained by stoichiometric calculations. However, precise and accurate measurements of flow quantities become essential to efficient operation. In certain physical processing like heat treatment, a gaseous atmosphere is maintained. The flow rate calculations of gaseous atmosphere is required to maintain the uniformity in the composition of the atmosphere and rate of movement of the atmosphere.

Principles of flow measurement
It is known that pressure energy can be converted into kinetic energy and vice versa. Thus, if a restriction is placed in the flow passage, the fluid would be accelerated with the corresponding decrease in pressure head. There are three different ways in which this can be practiced.   

Venturi
In the venturi, the cross sectional area of a flow passage decreases gradually in the direction of flow and attains a minimum cross section area at the throat and thereafter gradually increases further in the direction of flow. Figure shows a venturi-meter.