Velocity Measurement of Fluid with Pitot Tube

The arrangement for measuring fluid velocity using a Pitot tube is shown in Figure 18.5. The Pitot tube consists of bent tube of small diameter with a rounded nose. The Pitot tube is connected to one limb of a U tube manometer. The other limb of the manometer is connected to a tap made on the tube wall. The tube tap and the nose of the Pitot tube are roughly in the same plane. It is assumed that the wall tap senses the static pressure p of the fluid while the Pitot tube senses the stagnation pressure p_o of the fluid. From Bernoulli principle, we get

Figure 18.5 Velocity Measurement with Pitot Tube

where ρ is the density (constant in the case of low speed flow) of the fluid whose velocity is being measured. In case of gas flow the temperature also needs to be measured since the density is a function of static pressure and temperature. With ρ_m as the density of the manometer liquid, the pressure difference is given by,