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Pitot tube, Prandtl tube

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Uploaded on May 8, 2011

Pitot tube, Prandtl tubePitot tube is used to measure dynamic pressure and flow rate of a flowing liquid or gas. The device was invented by a Frenchman Henri Pitot in the 18th century.The device looks like an incurved L-shaped tube. One of its ends is directed towards the incoming flow. Liquid inside the tube changes its position depending on the pressure difference on two ends. The liquid inside the tube can rise to a certain elevation. Its value shows a degree by which static pressure of the liquid exceeds atmospheric pressure. The value is called static head.Measuring orifice in the tube wall must be strictly perpendicular to the direction of flow. Vertical head of the liquid in a manometric tube characterizes only static pressure and does not depend on flow rate. However, you can cast the tube into the pipe and bend it at an angle of 90 degrees thus directing its feed opening towards the flow. In this way liquid will break down to a zero speed before the flow. Now the column will consist of a static head and additional elevation, called velocity head. This sum is called total head.Velocity head equals difference between height ht of the total head column and height hs. So now we can calculate flow rate.Pitot tube for gases has one of its ends opened and directed towards gas flow, while the other end is connected to a pressure measuring device.A combined instrument for measuring dynamic pressure is called Prandtl tube. It also has Pitot tube along with a pressure tube for measuring static flow pressure. The orifice that goes towards the flow income is designed for measuring total pressure.Manometer measures difference of total and static pressure. The resulting value is called a dynamic pressure. It can be used to calculate flow rate.Pitot and Prandtl tubes are widely used in those areas which require measuring flow rate, liquid or gas flow, and static pressure. They are used, for example, in gas pipelines and ventilation systems. However, they are mostly used in aviation. Not a single airplane can do without these devices since they measure both indicated and actual speed of an aircraft, as well as vertical speed and barometric flight altitude.To ensure accurate measurements it is important that the tube does not freeze. That is why it is furnished with heating elements.

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