Page 30 - Instrumentation and Measurement

The Pitot tube can be used to directly measure the velocity of flow of a fluid, rather than the volume
rate of flow and consists essentially of just a small tube inserted into the fluid with an opening
pointing directly upstream (Figure 2.26). The fluid impinging on the open end of the tube is brought
to rest and the pressure difference measured between this point and the pressure in the fluid at full
flow. The difference in pressure between where the fluid is in full flow and the point where it is
stopped is due to the kinetic energy of the fluid being transformed to potential energy, this showing
up as an increase in pressure. Because kinetic energy is ½ mv2, the velocity is proportional to the
square root of the pressure difference.

FIGURE 2.26 Pitot tube.

2.5.2 Turbine Meter

The turbine flowmeter (Figure 2.27) consists of a multi-bladed rotor that is supported centrally in the
pipe along which the flow occurs. The rotor rotates as a result of the fluid flow, the angular velocity
being approximately proportional to the flow rate. The rate of revolution of the rotor can be
determined by attaching a small permanent magnet to one of the blades and using a pick-up coil. An
induced e.m.f. pulse is produced in the coil every time the magnet passes it. The pulses are counted
and so the number of revolutions of the rotor can be determined. The meter is expensive, with an
accuracy of typically about ± 0.1%. Another form uses helical screws which rotate as a result of the
fluid flow.

FIGURE 2.27 Basic principle of the turbine flowmeter.

Page | 30