When the fluid enters the pitot tube, it is forced to come to a stop. They are an important part of an airplane, where they are used to provide the pilot with in-flight velocity measurements. Now V can be calculated: V = (2ΔP/ρ)1/2 = 1/2 = 9.Pitot tubes are small, right angle shaped tubes that can measure the velocity of a fluid. (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.
Static pressure measurement is illustrated with the first U-tube manometer in the diagram at the left. It is measured through a flat opening that is parallel with the fluid flow. It’s a measure of the amount that fluid pressure exceeds local atmospheric pressure. Static pressure is what is commonly called simply the pressure of the fluid. Understanding of the terms, static pressure, stagnation pressure and dynamic pressure is very helpful in the explanation of pitot tubes, so they are being defined in this section.
Static Pressure, Stagnation Pressure and Dynamic Pressure - Definitions Pitot tubes (also called pitot-static tubes) are used, for example, to make airflow measurements in HVAC applications and for aircraft airspeed measurements. This is a convenient, inexpensive method for measuring velocity at a point in a flowing fluid. Pitot tubes are used in a variety of applications for measuring fluid velocity.
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