Differential Pressure Measurement | Air Leak Testing

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Where do I place the exterior pressure pickup tubes?

Answer

The exterior pressure reference for the differential pressure measurement across the door panel is provided by the red tube.  The exterior pickup location must be chosen to minimize the influence of wind, sun and atmospheric pressure on the differential measurement, or measurements must be taken to allow correction for these influences.  Each standard defines what is expected in terms of the exterior reference pressure measurement.  Use Table 2 to determine where best to locate your red tube depending on the standard procedure you are following.

Table 2:  Locations for Exterior Reference Pressure Pickup as required by the various Standard Procedures

Exterior pressure pick-up locations from Standards
ASTM 1 tube across the middle of each façade (NOT at corners of the building)
Manifold and average all pressure readings using a manifold (averaged ver 10s)
If > 3 stories, measured at more than 1 height
ATTMA Measured at the lowest floor level of the building
Located "some distance away" from the building envelope, out of the way of fan airflow and sheltered from wind
CGSB Calm conditions - 1 pressure measurement outside the building is ok
Windy - min of 4 measurements on each façade, manifold
Gusty winds - use wind damping kit (capillary tubes, averaged over 5s)
EN13829-FR Measure at the bottom floor level, but if tall building, measure at the top as well
Keep exterior pressure taps out of the sun, and fitted to a T-pipe or connected to a perforated box to protect from wind
USACE Min 1 exterior pressure tap required, but if bias pressures high, use more
Interior pressure gauge references tied together in a manifold to read 1 pressure reading

Figure 3

Figure 28 typically recommended locations for exterior pressure pickups

Even though most standards recommend pressure pickup placement as shown above, this is seldom done because the fluctuations are not large enough to warrant the time.  In fact, results may be improved by multiple pickups or in some cases may be made worse. Sometimes one exterior pickup is sufficient.

 

See section 5.8 "Enclosure pressure measurement disturbances" for more details on how to avoid pressure fluctuations.

 

 

5.8        Enclosure pressure measurement disturbances

Induced enclosure pressures are the difference between outdoors and inside the measured zone. The outdoor pressure pickup point(s) will be affected by wind. Steady winds create pressure that can be subtracted from readings but wind is never steady and it's the fluctuations that cause problems. Generally, on the windward side of the building impacts the ends of pressure tubes creating a positive pressure due to the wind velocity being stopped by the tube. Positive pressures will also result from the overall air movement being stopped by the windward side of the building creating a positive pressure field that can extend 5 to 20 feet from the building. To overcome these effects, it is best to use pickup point(s) away from the direct impact of wind.

The impact of the wind can be best measured by the effect on the gauge. First, extend the exterior pressure pickup point of the tube away from the fan's airstream and about 5 feet from the building. Monitor the gauge for ten minutes. If the gauge reading is above 2 Pa, insert a T in the tube end and cover the tube end with a flat sheet. If still above 2 Pa, T the tube and add two equal length tubes sufficient to be placed in two wind free locations on opposite sides of the building. It may be necessary to T these tubes once more to create 4 pressure pickup points, again with equal length tubes.

Ensure the tube ends are not in contact with water since that will seal them off.

Red tubes are always run through the Door Panel and connected to the Red port on the gauge. If testing from inside the building, red tubes will be run outdoors. If testing from outside the building, a red tube runs through the panel but Blue tubes are used to pick up the outdoor pressure. This is done to ensure, the gauge always reads the pressure in the building with the correct sign, namely, if the building is being depressurized, the gauges will read negative. If pressurized, the gauges will read positive. This works whether the tester and gauges are indoors or outdoors.

Passing more than one tube through the enclosure is possible and an approach that is sometimes used but in general there is seldom an advantage to doing this. As long as the exterior pressure pickup used is in a favorable location, it will adequately represent the outdoor pressure since the outdoor pressure will be unaffected by the test fans since outdoors is an unlimited sink for airflow just like it is when for the grounding of electrical circuits.

Running multiple blue tubes indoors is important because we must ensure all areas in the zone have similar test pressures.