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Written by Robert E. Sheriff, MS, CIH, CSP, President
November 9, 2018
Sulfur Hexafluoride, also known as SF6, is by far the most common tracer gas used for ventilation studies. The primary reason is that it is easy to detect at very low concentrations—even in the parts per billion (ppb) or parts per trillion (ppt) range and has a very low toxicity. The best instrument to detect SF6 is an infrared spectrophotometer. Although the portable units are still a bit heavy, it allows direct reading measurements in real time.
An alternate to direct reading is to collect air samples in a gas bag (the best is a tedlar bag) for transport to a laboratory for analysis. The problem with this approach is that you are starting at an unknown point and finishing also at an unknown point. Taking multiple samples over various times makes this approach a little more reasonable but still not as effective as real time measurements.
Although not as sensitive as an IR Spec, a Flame-Ionization Detector (FID) is also useful. Its weakness is that in the presence of other VOCs, it can give uncertain readings; i.e. subject to readings involving other VOCs.
The most common use of SF6 tracer gas in ventilation application is as a tracer gas to measure Air Changes Per Hour (ACH) in a commercial building Generally it is for locations that primary use general ventilation rather then local exhaust ventilation to move air in – and out – of the building.
Another use of SF6 is as a leak detector in closed air systems—and even leaks in an open duct system.
It is important to note that a Photo-Ionization Detector (PID) cannot be used to detect SF6 since the ionization potential of SF6 is above the detection range of the standard detector and even the expanded range detector of the PID.
Very often tracer gas is the recommended method of measuring Air Changes Per Hour (ACH) over actual airflow measurements where the access points in the HVAC System are very numerous or difficult to access.
It is also useful where a building is classified as a “safe haven” in the event of a release of hazardous chemicals or gases into the atmosphere.
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