How to fix a pitot tube

A pitot tube (impact tube) is primarily used for measuring the speed or velocity of a fluid, both gas and liquid. The instrument is named after Henri Pitot, who worked on the measurement of water flow in the early eighteenth century. In principle, if an open-ended tube faces the flow of the fluid a pressure will build up and increase with the velocity of flow. If a pressure gauge is attached to the outlet of the tube, it will indicate fluid velocity.

The tube facing the fluid is known as the head and is required to sense a total pressure, which is the addition of the surrounding (static) pressure and the pressure due to flow (dynamic pressure). The face of the tube creates an obstacle to the flow; this is known as a stagnation point which creates a stagnation pressure, and it requires careful design to make the stagnation point give the required pressure.

To derive an indication of velocity a pressure difference must be measured between the total pressure and the surrounding static pressure, which is sensed by static vents. In a simple system the vents are holes pierced in the side of the outer tube assembly, but in large installations such as aircraft, the airflow is complicated and therefore static vents areplaced at a number of positions and the average pressure is taken.

Total pressure is directly dependent upon the density of the fluid, which is directly dependent upon absolute temperature (temperature measured in Kelvins, where 0° C is 273 Kelvins: 273 K). The error on measured velocity due to temperature change is approximately 0.2 per cent per °C in air and therefore compensation must be included for high accuracy.

A typical design is the L-shaped tube, which is satisfactory where the fluid flow is straight and not swirling; that is, there is no turbulence. This design will begin to give significant error if the flow deviates more than about ±10° but, by shaping the inlet hole, flow angles of up to ±60° are acceptable. A general-purpose pitot-static tube would be about 0.3 in. (8 mm) in diameter and have a smooth surface. The nose must be carefully tapered or shaped both to give minimum disturbance to flow and to achieve a good total pressure.

The basic tube is usually metallic, either nonferrous or stainless steel and the mounting and surrounding assembly is often plastic or of a fibrous material. In conditions where damage may occur a protective shield may be added but this must not interfere with the flow. The nose may also be hardened to prevent abrasive action.