Pitot tube flow is measured based on air pressure passing into it and the established air density of the atmosphere at its particular elevation and air temperature. These equations are based on Bernoulli principles at moderate speeds that must be below the supersonic range. Other factors such as ice build-up or cross winds can also affect the accuracy of Pitot tube flow. Though Pitot tubes can theoretically be used to measure any fluid flow velocity, they are most often incorporated today on aircraft to determine air speed in flight. Henri Pitot is credited with the invention of the Pitot tube in 1732 during studying of the pressure of the flow of the river Seine in France, and the French scientist Henry Darcy modified its design for aircraft use in the mid 19th century.
As a form of pressure measurement, Pitot tubes do not measure average velocity, but, instead, a single point of velocity in the stream. Fluid flow velocity cannot be measured by Pitot tube flow alone on aircraft, as they also require a measurement of outside static air pressure for velocity calculations. These devices, therefore, calculate what is known as stagnation pressure, or the pressure exerted by air as it enters the Pitot tube and exits through pressure transducer connected holes at the other end. Static pressure is calculated by static ports generally mounted on the side of the fuselage of an aircraft, while Pitot tube flow is based on a Pitot tube that is often mounted on a boom that extends out from the nose of the aircraft.
With Pitot tube flow, stagnation pressure is calculated by adding the standard atmospheric static pressure to the dynamic pressure exerted on the interior of the Pitot tube. At the back of the Pitot tube are a ring of holes and a central exit hole, both connected to the pressure transducer. As air exits these holes, the differences in pressure are used by the transducer to calculated dynamic air pressure. Bernoulli's equation states that static air pressure plus dynamic air pressure equals the total air pressure, which, in this case, is the Pitot's tube's stagnation pressure.
When stagnation pressure is known as well as the local air density, Bernoulli equations can be used to calculate the velocity of the aircraft that the Pitot tube flow is passing through. While this is reliable under ideal conditions, low air speeds will often present such small changes in pressure in Pitot tube flow that the pressure transducer can often fail to accurately calculate them, resulting in erroneous velocity readings. Several fatal air accidents involving faulty Pitot tube flow readings have also occurred when they were iced over, altering the air flow, so built-in de-icing heaters are now incorporated into Pitot tubes to prevent such tragedies in the future. Adjustments can also be made for unique conditions, such as low air speed or supersonic flight, so that Pitot tubes generate accurate readings.