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© Retrotec 2017
Measure a negative pressure
1.
Connect a pressure tube to the red (negative, “Reference”) port of the gauge.
2.
Place the end of the tube into a glass of water, about 1 inch below the surface.
3.
The gauge will display around -1.00 in WC, or -249 Pa or -5.2 psf. Notice the negative sign.
Figure 72: The gauge displaying a negative pressure (-250.6 Pa).
Why static and fluctuating pressures occur, causing bias
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.
Managing fluctuating pressures created by wind
There are two types of pressure that wind creates, stagnation pressure and velocity pressure.
