Retrotec DM-2 - Manual (Page 33)

Page 33 of 72
©Retrotec Inc. 2012
5 Reduce Bias pressures, static or fluctuating
Wind blowing across the tip of a tube will cause a significant pressure to appear on the gauge. High
fluctuations of bias (baseline) pressures on the gauge (greater than 2 Pa) are a good indication that
wind may be a large source of error.
Another thing to notice when the wind is blowing is that it’s very difficult to establish and maintain the
test pressure. In windy conditions, the readings on the house/room gauge will fluctuate. For example,
when trying to establish 50 Pa, the wind will cause that pressure to go up to 55 Pa and down to 45 Pa,
making it very difficult to take a reading.
There are other reasons a large Bias or Baseline pressure can appear on the gauge and these should be
eliminated first before assuming wind is causing the problem. If the tube is being moved by the wind,
a large rapidly fluctuating pressure will appear but can easily be eliminated by taping or tying down the
tube. Next, a large and steady pressure of 10 to 70 Pa will appear if the tube end has touched water
and a drop has sealed off the end. Stepping on the tube, or pinching the tube will induce a Bias
pressure, so ensure that tubes are not pinched and are located away from walkways.
If air inside the exterior heats up due to the Sun shining on it and if the tube end is above or below the
point where it leaves the enclosure, a stack pressure will build up inside the tube that will create a
steady pressure that may increase as the tube gets hotter.
Reduce fluctuating pressures created by wind
There are two types of pressure that wind creates, stagnation pressure and velocity pressure.
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: