16. Measurement of Fluid Velocity With a Pitot Tube

8/13/2019 16. Measurement of Fluid Velocity With a Pitot Tube

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16. Measurement of Fluid

Velocity with a Pitot Tube

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1. Introduction

Measurement of air velocity is a common use of

pitot tubes, but they can be used to measure the

velocity of other fluids as well.

In order to determine velocity, the pitot tube

measures the difference between stagnation

pressure and static pressure. A value for the fluiddensity is also needed.

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1. Introduction (cont..)

Pitot tubes are used in a variety of applications for

measuring fluid velocity. This is a convenient,

inexpensive method for measuring velocity at a point

in a flowing fluid.

Pitot tubes (also called pitot-static tubes) are used,

for example, to make airflow measurements in HVACapplications and for aircraft airspeed measurements.

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2. Definitions

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.

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2. Definitions (cont..)

Figure 1

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Static pressure is what is commonly called simply the

pressure of the fluid. Its a measure of the amount

that fluid pressure exceeds local atmospheric

pressure.

It is measured through a flat opening that is parallel

with the fluid flow. Static pressure measurement is illustrated with the

first U-tube manometer in the diagram at the left.

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Stagnation pressure is also a measure of the amountthat fluid pressure exceeds local atmospheric

pressure, but it includes the effect of the fluidvelocity converted to pressure.

It is measured through a flat opening that isperpendicular to the direction of fluid flow and

facing into the fluid flow. Stagnation pressure (also called total pressure)

measurement is illustrated with the second U-tubemanometer in the diagram at the left.

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Dynamic 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 can also be interpreted as the pressure created by

reducing the kinetic energy to zero.

Its measurement is illustrated with the third U-tubein the diagram at the left.

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3. Relationships

The symbol, P, is often used for static pressure.

Dynamic pressure is given by the expression, V2.

The stagnation pressure is then given by the

following equation:

Pstag= P+ rV2+ gh

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3. Relationships (cont..)

Where: r is the fluid density (slugs/ft3), g 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). With the

specified units for the other parameters, pressure

will be in lb/ft2.

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4. Velocity Measurement with a PitotTube Figure 2

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4. Velocity Measurement with a PitotTube (cont..) For 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:

Pstag = P + rV2 , which can be rearranged to: V =

(2DP/r)1/2

Where P= PstagP.

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4. Velocity Measurement with a PitotTube (cont..) The pressure difference, P, (or Pstag P), can be

measured directly with a pitot tube like the third U-

tube in Figure 1, or with a pitot tube like that shown

in Figure 2. This is a concentric pitot tube. 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).

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5. Example Calculation

Consider a pitot tube being used to measure air

velocity in a heating duct. The air is at 85 oF and 16

psia. The pitot tube registers a pressure difference of

0.021 inch of water (PstagP). Calculate the velocity

of the air at that point in the duct?

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5. Example Calculation (cont..) Solution: Convert the pressure difference of 0.021

inches of water to lb/ft2 (psf) using the conversion

factor, 5.204 psf/in water. 0.021 inches of water = (0.021)(5.204) psf = 0.1093

psf

The density of air at 85oF and 16 psia can be

calculated using the ideal gas law, to be 0.002468slugs/ft3.

Now V can be calculated: V = (2P/)1/2 =[(2)(0.1093)/0.002468]1/2= 9.41 ft/sec

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6. Apparatus

Pitot-Static Tube

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7. Summary

For a fluid with known density and measured

difference between stagnation pressure and static

pressure (P), as measured with a pitot tube, the

fluid velocity can be calculated with the equation: V

= (2P/)1/2.

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END

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16. Measurement of Fluid Velocity With a Pitot Tube

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