Retrotec DM-2 - Manual (Page 46)

Page 46 of 72
©Retrotec Inc. 2012
to the fan, but is in the path of the fan's air flow, can swing back and forth. This swinging motion can
easily cause fluctuations in the measured pressure.
Outdoor pressure tubes need to be sheltered from the wind. In spite of standards that require tubes
be attached to walls, this strategy does not work particularly well for limiting wind related fluctuations.
What does seem to work is taking the outdoor pressure pickup tube 25 feet away from the building,
and setting it in a flat area. Cover the end of the tube with a heavy flat sheet of half-inch plywood or
place the tube into a box, for example, to reduce wind fluctuations.
It's never a good idea to leave a tube hanging in the air, either between two points, or out of a
window. The pressure tube should sit securely on the ground and be kept still as much as possible.
The effects of a moving tube are very similar to the effects of wind on a tube, which is described below.
To see the effect of a moving tube
1. Connect a pressure tube (10 feet or longer) to the blue port of the gauge.
2. Set Time Averaging to one second.
3. Wave the end of the tube through the air. Notice the fluctuations.
4. Hold the end of the tube still, and swing the middle of the tube. Notice the fluctuations.
Figure 13: The effect of a moving tube on pressure readings.
9.2 Check if wind is causing fluctuating pressure
Wind blowing across the tip of a tube can cause significant error, even if the tube is not moving. 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 the test pressure.
In windy conditions, 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.
9.2.1 Use the [@ Pressure] key to reduce the effects of wind
Variable readings caused by changing input pressures can be rectified by using the [@ Pressure] key,
which will extrapolate the reading to the desired pressure regardless of the actual pressure being
experienced at the moment. This does not solve the problem completely, but it does make it easier to
take a reading.