– this simple device relies on the water moving at different
1. Bend meter – this simple device relies on the water moving at different
velocities on the inside and outside of a bend. From the energy equation we know that this difference in velocity will correspond to a measurable pressure difference.
2. Venturi meter – this is a classic flow measurement device, where water is passed through a narrow throat (throat diameter = 38 mm) in order to induce a higher velocity. Pressure is measured upstream and in the throat, to give the two values corresponding to high and low velocity.
3. Pitot tube – this device is useful for measuring high flows as it results in minimal disruption to the flow. A tube is inserted into the flow such that the water at the entrance of the tube stagnates. The pressure measured in this tube (the stagnation pressure) is equal to the pressure head plus velocity head. By comparing against a measurement of pressure head (static pressure) taken in the main flow field nearby, the difference between these measurements will be the velocity head.
4. Orifice meter – this is like a crude version of a Venturi meter and follows exactly the same principles by forcing water through a smaller section. The only physical difference between the two devices is that the orifice is simply a flat plate (perpendicular to the direction of the flow) with a hole in it (orifice diameter 26 mm), whereas a Venturi contains a gradual contraction and expansion. The result of this is that the orifice meter induces a far great pressure difference (and head loss) for a given velocity.