Research in 2005 at Lawrence Berkeley National Laboratory worked on using tracer gases for air flow testing in buildings. The LBNL study on focused on using readily available inexpensive hardware to inject and sample tracer gas—usually CO2—without the problems of poor mixing and sampling that have previously plagued this technique.
For duct air flows, in particular, problems with getting uniform mixing and sampling also made tracer gas diagnostics unreliable. That research focused primarily on improving the measurement of air flows in commercial ducts, but the techniques developed through this research can be applied just as usefully to measuring air handler flows in residential systems.
With these problems resolved, and with the cost of high-accuracy CO2 analyzers dropping, tracer gas techniques are quickly becoming both cost-effective and reliable outside of the research environment. ...