Pitot Tube
What is it & how does it work?:
The Pitot static tube system is an ingenious device used by airplanes and boats for measuring forward speed. The device is really a differential pressure gauge and was invented by Henri Pitot in 1732. An example of an air pressure gauge is a tire pressure gauge. The Pitot measures a fluid velocity by converting the kinetic energy of the flow into potential energy. A Pitot-static tube is a flow velocity meter, which is capable of measuring fluid velocities as a localized point (as opposed to an averaged velocity across a larger section). A schematic of a Pitot tube is shown below.
Figure 1. Typical Pitot Static Tube
The conversion from potential to kinetic energy takes place at the stagnation point, located at the Pitot tube entrance (see the schematic below, Figure 2). A pressure higher than the free-stream (i.e. dynamic) pressure results from the kinematic to potential conversion. This "static" pressure is measured by comparing it to the flow's dynamic pressure with a differential manometer.
Figure 2. Cross-section of a Typical Pitot Static Tube
Converting the resulting differential pressure measurement into a fluid velocity depends on the particular fluid flow regime the Pitot tube is measuring. Specifically, for an incompressible flow, the Bernoulli equation describes the relationship between the velocity and pressure along a streamline,
Figure 3. Generalization for calculating velocity using the Bernoulli equation
To measure the static pressure in a fluid flow, it is normal to make a small hole in the boundary wall of the flow and to connect the hole to a pressure measuring device - a manometer being the traditional instrument used.
To measure the total pressure, it is normal to employ a device known as a Pitot tube. This is a thin tube that can be pointed directly into the flow such that it is aligned exactly with the local streamlines. The other end of the tube is connected to a manometer (or other pressure measuring device). The streamline that meets the end of the tube within the flow is brought to rest - because there is no actual flow through the tube/manometer system - and therefore all the dynamic pressure is converted to static pressure. The sum of these two forms of static pressure is known as the stagnation pressure or total pressure.
To measure the dynamic pressure, the most common device (and the simplest and cheapest) used is a Pitot-static tube. This is a combination of the two techniques described above within one instrument. It consists of two thin concentric tubes bent into an L-shape; the inner tube has an open end which is pointed into the flow (as described above when measuring total pressure), while the outer tube is sealed and streamlined at its end but has a number of small holes around its circumference some way back from the end. The two tubes are connected across a differential pressure-measuring device (again, commonly a U-tube manometer), and the difference in pressure is the dynamic pressure.
Where it’s used:
- Hostile environments. e.g. flue stacks
- Combustion efficiency tests
- Research testing
- HVAC commissioning
- Heat exchanger balancing
- Process Control lines.
- Marine and aeronautical speedometers
