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© Retrotec 2017
Stagnation pressure- is felt over a large area in the windward side and for several feet from the building
and is a result of the air stacking up due to the wind velocity. The wind's velocity is converted to a
pressure.
Stagnation pressures are reduced by moving the outdoor pickup point about 15 feet from the building or
away from any object that the wind will strike. Place the tube at ground level with a flat plate covering the
tube. The pressure field around the building changes from side to side and because the wind direction
varies somewhat; using two pickup points at least 20 feet apart will reduce these fluctuations.
Velocity pressure is caused by the velocity impinging in the end of the tube and converting itself into a
pressure at the tube end.
Wind velocity is always lower on the leeward side of the building. Moving pickup points away from the
windward side reduces the magnitude of wind induced pressures.
Wind velocity increases with height from the ground, so placing our pickup points at ground level reduces
the magnitude of fluctuations.
Covering the end of the tube without blocking it helps because the static pressure is reduced. Pop bottles
or other containers help. Some testers dig holes to place the tube in. All these methods work. Flat plates
on the ground work best because the openings face all directions, and the pickup point is low down. A T
works well too.
Combining these methods gives a list you can go by if Baseline pressures are above 2 Pa or you simply want
to increase repeatability. Your outdoor pressure pickup tube should:
1.
Be 15 feet from the building
2.
Terminate in a T
3.
Have 2 pickup points at least 20 feet apart attached to either side of the T
4.
Each point should be covered with a flat plate or box
These steps will reduce the magnitude and variation of the Baseline pressures your gauge will see. After
that, time averaging or long Baseline recordings will reduce the impact of these wind pressures.
Reduce uncertainty in results by taking lots of readings
Table 7 shows tests with approximate uncertainties that can result from applying various number of
Baseline points, Baseline times, and Time Averaging when taking induced pressures. As the number of
Baseline points, the Baseline time, and Time Averaging used for induced pressure readings increase, the
uncertainties decrease. For example, repeating the test using the same fan reduces the uncertainty by 5%
in each case. Typical uncertainties would be less than half of the values shown, however the table
exaggerates the uncertainty trend assuming there are errors while testing with different fans, gauges, and
test conditions.
Table 7: Test result uncertainties vary by changing the number of baseline points, the baseline time, and Time Averaging for
induced pressures.
Gauge
error
Fan
error
Baseline
points
Baseline
time
Baseline
variation
Time Averaging for
induced pressure
Uncertainty
1%
5%
1
5 s
1.5 Pa
5 s
22%
1%
5%
30
5 s
1.5 Pa
10 s
8.6%
1%
5%
30
10 s
1.5 Pa
10 s
7.9%
1%
5%
30
10 s
1.5 Pa
100 s
6.5%
1%
5%
30
20 s
0.5 Pa
100 s
5.9%
