Power and Energy Measurement Units and Techniques - Energy Conversion and Management - page 17

Fig. 2.3 Pitot tube

• Venturi tube (see Fig. 2.2c ). This meter uses a Venturi nozzle which permits
flow measurement with low differential pressure values. Then shorter straight
pipes upstream and downstream are required, generally less than 15 D .
Notice that the error of head meters is generally less than 2 %, but increases as
the meter works in the lowest part of the scale because of the squared relation-
ship between flow and differential pressure. Care must be taken not to oversize
the full-scale capacity on the basis of the real flow rate; a good degree of
accuracy is obtained from full capacity down to 25 % (working range 4:1).
4. Pitot tube (see Fig. 2.3 ). This operates on the principle that the total pressure p t ,
detected at a small orifice facing into the flow, is the sum of static and dynamic
pressures. The static pressure p s can be detected by a measurement orthogonal to
the direction of the flow. The dynamic pressure is then obtained by subtraction.
Notice that straight pipes are required upstream and downstream in order to
avoid turbulence near the point of measurement.
Fluids without solid matter are required. Flow speed must be relatively high,
because low speed involves a differential pressure too slight to measure easily.
A squared relationship exists between flow and differential pressure, as in head
meters.
The basic operating formula is as follows:

p

p t q ¼ α K v S ð p s Þ=ρ