1. Pitot Tube: If a stream of uniform velocity flows into a blunt body, the stream lines take a pattern similar to this:
Note how some move to the left and some to the right. But one, in the center, goes to the tip of the blunt body and stops. It stops because at this point the velocity is zero - the fluid does not move at this one point. This point is known as the stagnation point.
From the Bernoulli equation we can calculate the pressure at this point. Apply Bernoulli along the central streamline from a point upstream where the velocity is u1 and the pressure p1 to the stagnation point of the blunt body where the velocity is zero, u2 = 0. Also z1 = z2.
This increase in pressure which brings the fluid to rest is called the dynamic pressure.
Or converting this to head (using)
The total pressure is known as the stagnation pressure (or total pressure)
The blunt body stopping the fluid does not have to be a solid. I could be a static column of fluid. Two piezometers, one as normal and one as a Pitot tube within the pipe can be used in an arrangement shown below to measure velocity of flow.
Using the above theory, we have the equation for p2,
We now have an expression for velocity obtained from two pressure measurements and the application of the Bernoulli equation.