Pitot tube on an Airbus A321.įrench engineer Henri Pitot invented the pitot tube in 1732 after he was assigned to measure the water flow in the Seine river in Paris. As such, several essential flight instruments and flight systems rely on them for data. Because air is considered a fluid in physics, aircraft use a pitot tube as a sensor that provides vital data necessary for a safe flight by measuring air pressure.įlight data such as airspeed, altitude, and rate of climb stems from the pitot tubes and associated systems.
What Is a Pitot Tube?Ī pitot tube is an instrument used to measure the velocity of a flowing fluid. Onboard flight computers use this pressure to calculate the airspeed.Įrrors and failures in the pitot tube system are considered potentially safety-critical and have contributed to several accidents and crashes over the years. This pressure increases as the airplane moves faster and vice versa. As the aircraft moves, air rams into the tube, creating pressure. Now V can be calculated: V = (2ΔP/ρ)1/2 = 1/2 = 9.41 ft/sec Summaryįor a fluid with known density and measured difference between stagnation pressure and static pressure (ΔP), as measured with a pitot tube, the fluid velocity can be calculated with the equation: V = (2ΔP/ρ)1/2.They work by measuring the air pressure in a small metal tube mounted outside the aircraft, pointing into the airflow. (See the article, “ Use the Ideal Gas Law to Find the Density of Air at Different Pressures and Temperatures,” for more information.) The density of air at 85oF and 16 psia can be calculated using the ideal gas law, to be 0.002468 slugs/ft3. Solution: Convert the pressure difference of 0.021 inches of water to lb/ft2 (psf) using the conversion factor, 5.204 psf/in water.Ġ.021 inches of water = (0.021)(5.204) psf = 0.1093 psf Calculate the velocity of the air at that point in the duct? The pitot tube registers a pressure difference of 0.021 inches of water (Pstag – P). Example CalculationĬonsider a pitot tube being used to measure air velocity in a heating duct. The inner tube has a stagnation pressure opening (perpendicular to the fluid flow) and the outer tube has a static pressure opening (parallel with the fluid flow). The pressure difference, Δp, (or Pstag – P), can be measured directly with a pitot tube like the third U-tube in the figure above, or with a pitot tube like that shown in the figure at the right. Pstag = P + ½ ρV2, which can be rearranged to: V = (2ΔP/ρ)1/2 Velocity Measurement with a Pitot Tubeįor pitot tube measurements and calculations, the reference plane is taken to be at the height of the pitot tube measurements, so the equation for stagnation pressure becomes: With the specified units for the other parameters, pressure will be in lb/ft2. Where: ρ is the fluid density (slugs/ft3), γ is the specific weight of the fluid (lb/ft3), h is the height above a specified reference plane (ft), V is the average velocity of the fluid (ft/sec). The stagnation pressure is then given by the following equation: Dynamic pressure is given by the expression, ½ ρV2.
The symbol, P, is often used for static pressure. Static Pressue, Stagnation Pressue and Dynamic Pressure - Relationships Its measurement is illustrated with the third U-tube in the diagram at the left. It can also be interpreted as the pressure created by reducing the kinetic energy to zero. Stagnation pressure (also called total pressure) measurement is illustrated with the second U-tube manometer in the diagram at the left.ĭynamic pressure (also called velocity pressure) is a measure of the amount that the stagnation pressure exceeds static pressure at a point in a fluid. It is measured through a flat opening that is perpendicular to the direction of fluid flow and facing into the fluid flow. Stagnation pressure is also a measure of the amount that fluid pressure exceeds local atmospheric pressure, but it includes the effect of the fluid velocity converted to pressure.