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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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2. Definitions (cont..)
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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2. Definitions (cont..)
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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2. Definitions (cont..)
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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