PFT Air Infiltration Measurement Technique
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The airtightness of a building can be determined by using several methods. The PFT (PerFluorocarbon tracer gas) technique and building pressurization (blower door) test both provide information about air leakage and energy loss.
A building pressurization or blower door test locates air infiltration by exaggerating the defects in the building shell. However, this type of test only measures air infiltration at the time of the test. It does not take into account changes in atmospheric pressure, weather, wind velocity, or any activities of the occupants that may affect air infiltration rates over a period of time.
The Brookhaven National Laboratory developed the PFT technique to measure changes over time (a few hours to several months) when determining a building's air infiltration rate. While this test cannot locate exact points of infiltration, it does reveal long-term infiltration problems.
The PFT technique uses two pencil-size devices. One, the emitter, gives off a small amount of a colorless, odorless, and harmless perfluorocarbon gas. The other, the receiver, absorbs some of that vapor from the average concentration of the gas in the room. That average concentration is proportional to the building tightness—the tighter the building, the higher the concentration.
The emitters and receivers are located according to written instructions provided by Brookhaven. The ventilation measurement begins by uncapping the receiver. Because the rate at which the emitter releases gas is temperature-sensitive, the user needs to provide an average temperature estimate during the procedure for the test to be most accurate. At the end of the test period, the receiver is recapped and sent, along with the temperature readings and room volumes, to the Tracer Technology Center at Brookhaven National Laboratory for analysis, where an expert must analyze the results.
For a single zone measurement, the emitter's rate (e.g., nL/h) divided by the receiver's average concentration (e.g., nL/m^3) gives the room's ventilation rate (e.g., m^3/h). Dividing by the room volume (e.g., m^3) gives the familiar Air Changes per Hour ACH (e.g., h^-1). The Tracer Technology Center sends the customer a computerized test report that includes the average air infiltration rate.
Emitters are available with six different perfluorocarbon gases, making it possible to conduct simultaneous tests in six different areas or zones of a building. Every zone measured requires one receiver. Zones may be physically distinct areas, such as the basement, first and second floors of a house, or distinct heating zones in a zone-heated building. Utilizing more than one gas makes it possible to determine not only the air infiltration rate for each zone but also the airflow pattern between zones, such as between the basement and the first floor. This, in turn, makes it possible to determine air leakage into and out of each zone.
The PFT technique can also determine the infiltration rates of other gases, such as radon. This is done by placing different emitters in the soil next to the foundation, below the basement floor, and in the house. Calculating the rate of infiltration may help detect a potential radon hazard if you live in an area with large radon concentrations.
In addition, the PFT technique can be used to quantify pollutant source strengths in multizone buildings if the user provides pollutant concentration data in each zone taken at the same time as the PFT measurement. In this way, the user will know which location contains the pollutant source even though pollutant is present in all zones.
The cost of the PFT testing varies. There is a minimum per project/site base charge of approximately $1,500. There are additional charges based on the number of gas emitters, rooms, and area of building. For homes, a typical cost per measurement period can be less than $100 for 1-zone tests to up to $400 for a 4-zone test when the base charge is spread over a large number of measurements. A per case fee structure is usual.
