A pitot tube is a device which is inserted in the pipe line . There are two openings in the pitot tube. One opening is facing the flow direction which measures static plus the velocity head. The other opening is in the other direction which measures static head only. The difference in these two is measured by a device reading differential pressure such as differential pressure transmitters. This difference is the dynamic head , square root of which is directly proportional to the velocity at the point where is pitot tube is inserted.

Normally the point of insertion is either the pipe line centre or the point of average velocity. The point of average velocity is not easy to locate and moreover could shift with the changes in Reynolds numbers. The centre is fixed but it is the point of maximum velocity from which the average velocity has to be computed. The flow is the product of average velocity and the cross sectional area of the pipe. Thus if the pipe is slightly elliptical, there will be error in the flow measurement. This is valid for all devices measuring flow based on the velocity measurement.

The averaging pitot measures velocity at four different locations in the pipe line and averages the same to get the average velocity. The pipe line cross section is divided in four areas having equal flow and thus the points at which the velocity is measured are located on the basis of average velocity in these four areas.

Thus averaging pitot tube can measure the flow with better accuracy i.e. between 1 to 2 % depending on other factors such as the uniformity of pipe cross section , straight lengths available etc.

The advantages of the averaging pitot tube are that a reasonable accuracy is achieved at relative low cost and low permanent pressure drops thus resulting in low energy wastage. They are suitable for clear fluids ( liquids , gases and steam ) and for Reynolds numbers above 10000
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