HLDers or liquid helium plant leak detectors, also known by the name of Mass Spectrometer Leak Detectors, are attuned to ascertain the possibility of helium leakage. They are designed to verify the absence or presence of leaks against a standard value of measurement.
As far as choosing the tracer gas is concerned, then helium is undoubtedly the best option. This gas is inert, non-toxic, non-flammable, non-condensable and present in the ambient atmospheric air, in very small quantities. The minute atomic size of the helium gas also enables it to easily pass through leaks. This gas is relatively inexpensive, and easily available in cylinders of various sizes. The only other gas that has a molecular size smaller than helium is hydrogen, but it is not inert.