Retrotec DM-2 - Manual (Page 49)

Page 49 of 72
©Retrotec Inc. 2012
To learn how Time Averaging can cause error
1. Create a pressure on the positive port of Channel A (Input).
2. Set Time Averaging to one minute.
3. Wait until the pressure reading becomes stable. After one minute, Channel A should display
the created pressure.
4. Remove the tube from Channel A, eliminating the pressure.
5. Notice that the gauge continues to display a pressure on Channel A, which slowly decreases.
After one minute, the gauge should display no pressure difference.
If a reading is taken from the gauge before the full time averaging period has passed, then the
displayed pressure may not be accurate.
9.4 Check if large fixed errors are caused by pinched tubes
A pinched pressure tube can create a very high reading on the gauge, which is erroneous. In other
cases, a pinched tube will prevent the pressure measurement from reaching the gauge at all. In cases
where tubes get stepped on, they are seldom sealed off completely, but will still cause the gauge
readings to jump around.
Figure 15: Errors in pressure reading from a pinched tube.
It is possible to complete a Blower Door test with a pinched tube. It will appear as if the fan is creating
a pressure in the building, but in fact the results are way off. If the gauge begins to display an
unusually high or unexplained pressure, check the tubing to ensure that it has not been compressed in
any way.
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Be careful that no one steps on the tubes, or creates a kink in any of the lines while performing a
test. If this happens, wait a few seconds for the effects to subside before taking any readings from
the gauge.
9.5 Check if large fixed errors are caused by water in the tube
Water can also cause significant errors if a drop is pulled into the pressure tubes. It is very easy to get
a water drop inside the end of a tube. Simply drag it over a shallow puddle of water, and a drop of
water will get pulled into the tube by capillary action. Notice in the Figure 16 how one drop of water in
the end of the tube is creating 190 Pascal pressure. It is possible to complete a Blower Door test with a
water drop in the end of the tube. It will appear as if the fan is creating a pressure in the building, but