Tracer gas testing can show airflow rates in ventilation systems. The advantages of tracer gas testing is that elbows, junctions and other turbulence-producing components of the system do not wreck your tests. Pitot tubes or hot wire anemometers must be done in the laminar flow zone of the ventilation systems, away from elbows, junctions and other turbulence-producing components. Most of the time HVAC systems do not have many laminar flow zones. If there are any they are usually unreachable except with some star-trek kind of elevation system. On the other hand the more complex an HVAC System is the more places that tracer gas will need to be placed to determine the system effectiveness.