Measuring Flow Rates with a Pitot Tube

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A pitot tube is used to determine the velocity of a fluid flowing through a pipe by measuring the difference in pressure between two points in the pipe. Bernoulli's equation is then used to determine fluid velocity. In this Demonstration, you can vary either the manometer fluid height (green) or the velocity of the fluid in the pipe (blue) to see how these two variables are related. Use a slider to vary the density of the fluid in the manometer. The intensity of the color of the manometer fluid changes with density.

Contributed by: Jon Barbieri and Rachael L. Baumann (September 2014)
Additional contributions by: Garret D. Nicodemus
(University of Colorado Boulder, Department of Chemical and Biological Engineering)
Open content licensed under CC BY-NC-SA


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Details

Pitot tubes are used to measure the velocity of a fluid moving through a pipe by taking advantage of the fact that the velocity at the height of the bend in the tube (stagnation point) is zero. Some kinetic energy density of the fluid flowing through the pipe is converted into pressure, resulting in a change in manometer height. Bernoulli's equation is used to calculate the velocity of the bulk fluid in the pipe by using this pressure difference in the pitot tube:

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All terms on the left side represent the stagnation point (entrance of the pitot tube); here is the stagnation pressure and is the velocity of fluid in the pipe at point 1. All terms on the right side refer to point 2, a point upstream from the pitot tube. The two points that are being evaluated are at the same height, so and drop out. Thus we obtain the simplified form of Bernoulli's equation:

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The equation for the difference in pressure in a manometer is substituted into the simplified Bernoulli equation:

,

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This equation can be rearranged and used to solve for fluid velocity or difference in height of the fluids in the manometer:

,

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Here is the static pressure of fluid in the pipe, and are the densities of the fluid in the pipe and manometer fluid, is specific gravity of fluid in the pipe, is the gravitational constant and is the difference in height of the manometer fluid.



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