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should be avoided, both upstream and downstream. These locations include any obstruction or change such as valves, elbows,
flow splits, pumps, fans, etc. To increase the accuracy of pressure measurement in a dynamic system, allow at least 10 pipe /
duct diameters downstream of any change or obstruction and at least 2 pipe / duct diameters upstream. In addition the pipe / duct
diameter should be much larger than the diameter of the Pitot tube. The pipe / duct diameter should be at least 30 times the Pitot
tube diameter. Flow straighteners can also be used to minimize any variations in the direction of the flow. Also, when using a Pitot
tube, it is recommended that the static pressure tap be aligned in the same plane as the total pressure tap. On the Pitot–static tube,
the difference in location is assumed to be negligible.
Flow–through pipes and ducts will result in a velocity field and dynamic pressure field that are non–uniform. At the wall of any
duct or pipe there exists a no–slip boundary due to friction. This means that at the wall itself the velocity of the fluid is zero. Figure
5 shows an imaginary velocity distribution in a duct. The shape of the distribution will depend on the fluid conditions, system flow
and pressure. In order to accurately determine the average dynamic pressure across a duct section, a series of total pressure
readings must be taken across the duct. These pressure measurements should be taken at different radii and clock positions
across the cross section of a round duct or at various width and height locations for a rectangular duct. Once this characterization
has been performed for the duct , a correlation can be easily made between the total pressure measurement at the center of the
duct relative to the average duct total pressure. This technique is also used to determine the velocity profile within the duct.
Velocity
Distribution
Static Pressure Tap
Pitot Tube
B
A
(A) Static Pressure
Probe
(B) Total Pressure
Pitot Tube
(C) Combination Static Pressure
and Total Pressure Pitot Tube
(Pitot–Static Tube)
Flow
Flow
Flow
B
B
A
A
Ps
Ps
Po
Po
Figure 5. Static and Total Pressure Measurements Within a Dynamic Fluid System.
Figure 6. Types of Pressure Probes
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n
.