The most important class of flow meter design is based on the fluid that is either accelerated or retarded at the measuring section by reducing the flow area, and cause change in the kinetic energy. This change is measured by recording the pressure difference produced. Fluid flow in volume base as the measuring unit of m3/hr i.e. volume of fluid passes out in an hour.
This classification includes measuring instruments like:
A small element of fluid is brought to rest at an orifice situated at right angles to the direction of flow. The flow rate is then obtained from the difference between the impact and the static pressure. With this instrument, the velocity of the fluid is measured directly by a small filament of fluid.
Fluid is accelerated at a sudden constriction (the orifice) and the pressure developed is then measured. This is a relatively cheap and reliable instrument though the overall pressure drop is high because most of the kinetic energy of the fluid at the orifice is wasted.
Fluid is gradually accelerated to a throat and gradually retarded as the flow channel is expanded to the pipe size. A high proportion of the kinetic energy is thus recovered but the instrument is expensive and bulky.
It allows the fluid to flow with increasing velocity gradually until to the end of the throat of the instrument but expansion to pipe diameter is sudden as with an orifice. This is an expensive instrument because of the accuracy required over the inlet section.
Notch or weir:
Fluid flows over the weir so that its kinetic energy is measured by determining the head of the fluid flowing above the weir. This instrument is used in open-channel flow and extensively in tray towers where the height of the weir is adjusted to provide the necessary liquid depth for a given flow. Each of these devices will now be considered in more detail together with some less common and special purpose meters.