The concept of tracer gas leak detection is to trace escaped gases that exist at a low concentration in the atmosphere. When higher (tracer gas) concentrations are introduced into a pipe system, the tracer gas can be identified above ground where a leak exists. The most common tracer gas methods include:
HELIUM: the most popular for production leak testing. Helium is inert and is a small molecule — the atmosphere has between 4-6 ppm of He in it.
Hydrogen: is a cheaper gas, but is also hazardous at concentrations above 4%. (Remember the Hindenburg
CO2: has a lower sensitivity to tests and is widely used in the pharmaceutical industry.