What is PITOT โ€“ TUBE

PITOT โ€“ TUBE

A Pitot tube is a simple device used for measuring the velocity of flow. The basic principle used in this is that if the velocity of flow at a particular point is reduced to zero, which is known as stagnation point, the pressure there is increased due to conversion of the kinetic energy in to pressure energy and by measuring the increase in pressure energy at this point , the velocity of flow may be determined.ย 

Pitot tube
Pitot tube

Simplest form of a pitot tube consists of a glass tube, large enough for capillary effects to be negligible and bent at right angles. A single tube of this type is used for measuring the velocity of flow in an open channel. The tube is dipped vertically in the flowing stream of fluid with its open end A directed to face the flow and other open end projecting above the fluid surface in the stream. The fluid enters the tube and the level of theย fluid in the tube exceeds that of the fluid surface in the surrounding stream.ย 

This is so becauseย the end A of the tube is a stagnation point, where the fluid is at rest, and the fluid approaching end A divides at this point and passes around tube. Since at stagnation point the kinetic energy is converted in to pressure energy, the fluid in the tube rises above the surrounding fluid surface by a height, which corresponds to the velocity of flow of fluid approaching end A of the tube. The pressure at the stagnation point is known as stagnation pressure.

Consider a point 1 slightly upstream of end A and lying along the same horizontal plane in the flowing stream of velocity V. Now if the point 1 and A are at a vertical depth of hO from the free surface of fluid and h is the height of the fluid raised in the pitot tube above the free surface of the liquid. Then by applying Bernoulliโ€Ÿs equation between the point 1 and A, neglecting loss of energy, we get

Pitot tube

This equation indicates that the dynamic pressure head h is proportional to the square of the velocity of flow close to end A.

Thus the velocity of flow at any point in the flowing stream may be determined by dipping the Pitot tube to the required point and measuring the height โ€žhโ€Ÿ of the fluid raised in the tube above the free surface. The velocity of flow given by the above equation (1) is more than actual velocity of flow as no loss of energy is considered in deriving the above equation.

When the flow is highly turbulent the Pitot tube records a higher value of h, which is higher than the mean velocity of flow. In order to take in to account the errors due to the above factors, the actual velocity of flow may be obtained by introducing a co-efficient C or CV called Pitot tube co-efficient. So the actual velocity is given by

Pitot tube

When a pitot tube is used for measuring the velocity of a flow in a pipe, the Pitot tube may be inserted in a pipe. Since pitot tube measures the stagnation pressure head (or the total head) at its dipped end, the static pressure head is also required to be measured at the same section, where tip of pitot tube is held, in order to determine the dynamic pressure head โ€žhโ€Ÿ. For measuring the static pressure head a pressure tap is provided at this section to which a piezo meter may be connected. Alternatively a dynamic pressure head may also be determined directly by connecting a suitable differential manometer between the pitot tube and pressure tap.

Pitot tube

Where P1 and P2 are the pressure intensities at points 1 and 2, V is velocity of flow at point 1 and ๐œ” is the specific weight of the fluid flowing through the pipe. P1 is the static pressure and P2 is the stagnation pressure. The equation for the pressure through the manometer in meters of water may be written as,

Pitot tube


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