Retrotec DM32 - Manual (Page 65)

Page 65 of 94
©Retrotec Inc. 2014
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Whip the end of the tube around to remove excess water.
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If this doesn’t work (in extreme cases), water can be blown out from the other end. Be sure to not
have the gauge connected when blowing water from the tube.
8.14 Check if large fixed errors are caused by sun heating the tubes
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.
8.15 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 is 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.
8.15.1. 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.
8.15.2. Use Time Averaging feature to reduce the effects of wind
Variable readings caused by changing input pressures can be rectified by increasing the Time Averaging to
20 seconds or more. Quadrupling the Time Averaging typically halves the pressure variation due to the
effects of wind. Be aware that if the fan is increasing or decreasing in speed, it will take 20 seconds at least
for the gauge to register its reading. For example, if during the initial 10 seconds there was no building
pressure whatsoever, and during the second 10 seconds there was 50 Pa, then the gauge will average that
to 25 Pa. The rule of thumb here is to wait for at least double the time average period before a reading is
taken.
[Time Avg] exercise to practice reducing the effects of wind
1. Use a fan to create an air stream.
2. Set Time Averaging to “1 s”.
3. Place a tube, connected to the positive port, in the path of the air stream.
4. The gauge will display a pressure that fluctuates. Adjust the "wind" speed until the gauge is
reading between 2 and 5 Pa.
5. Adjust Time Averaging to 20 seconds.
6. Notice that after 20 seconds, the gauge reading fluctuates significantly less.
8.15.3. Let Time Averaging take effect before making readings
While the Time Averaging feature is useful to help minimize the effects of wind, it can also cause problems
if not used correctly. Whenever Time Averaging is on, it is important to wait at least twice the time
averaging period before taking a reading.