In subsonic conditions, the pressure difference sensed by the pitot-static system is proportional to velocity squared; that is, every time the velocity is doubled the pressure difference increases by a factor of four. As velocity increases toward the speed of sound in the fluid, shock waves begin to build up until supersonic conditions are reached, where the pressure can in some circumstances decrease with increased velocity. At these speeds, there is a need for special tube shapes.
At low air velocities, the pressures are extremely small, for example, at 2 mph (about 3 ft/sec,1 m/s) the pressure is only about 0.0001 psi (0.007 millibars); at 200 mph (100 m/s) this increases to about 1 psi (69 millibars). Thus, for faster-moving vehicles such as aircraft, the pitot becomes a practicable method of air speed indication.
To obtain accurate calibrations, the pilot system must be placed in a fluid flow which is directed accurately along the axis of the tube, without disturbance or turbulence. A common method is to use a whirling arm rig, which consists of a rotating arm, perhaps 10 to 20 ft (3 to 6 m) long, whose outer end travels in a circular trough. The pitot is mounted on the outer end of the arm and, although the travel is circular, the radius is large enough for the fluid flow to be considered axial with the pitot. The wind tunnel is another method of calibration and can be most useful for higher-speed air flow. The tunnel can be rectangular or octagonal and can range from a few inches to many feet across with a length which is many times the aperture. The shape Above: Highly Maneuverable Aircraft Technology (HiMAT) in the form of a U.S. aircraft. Pitot tubes are employed to provide signals for computing airspeed.
Some sort of pressure-measuring instrument is required to indicate the pressure difference sensed by the pitot-static system. The liquid manometer is widely used for experimental purposes, where the pressure may be conveniently fed to each end of the classic U-tube configuration.
Differential pressure gauges are more practicable for many purposes, for example, the aircraft air speed indicator; in this case the linkage within the gauge is mechanically designed to compensate for the Square Law (where pressure is proportional to the square of velocity of the fluid) and provide readings on a linear scale. Where an electric signal is required for control or computation, a differential pressure transducer would be used.