Well one thing you can measure is the airflow rates in an HVAC (Heating Ventilation and Air Conditioning) System. That is done by putting a constant concentration of SF6 into the ventilation system and measuring the SF6 level in the air stream at some distance downstream from where you started. A downstream location sampling point at least 25 times the average duct diameter is the best place to start. But go back to all the other reasons to do tracer gas testing to see all the other possible reasons for testing, there is a method for each of the reasons to test your HVAC or exhaust system.