Therefore the flow velocity at the throat of the venturi and the volumetric flow rate are directly
proportional to the square root of the differential pressure.
The pressures at the upstream section and throat are actual pressures, and velocities from
Bernoullis equation without a loss term are theoretical velocities. When losses are considered
in the energy equation, the velocities are actual velocities. First, with the Bernoulli equation (that
is, without a head-loss term), the theoretical velocity at the throat is obtained. Then by
multiplying this by the venturi factor (Cv), which accounts for friction losses and equals 0.98 for
most venturis, the actual velocity is obtained. The actual velocity times the actual area of the
throat determines the actual discharge volumetric flow rate.
The pressure drop, P1-P2, across the venturi can be used to measure the flow rate using a U-tube
manometer as shown in Figure 6. The reading, R, of the manometer is proportional to the
pressure drop and thus the velocity of the fluid.