It is a widespread misconception that the flammability of Hydrogen would make it impossible to benefit from its advantageous properties in leak testing. In fact, Hydrogen is only flammable in the concentration range 4% - 75% in air or oxygen, and can only detonate in the range 18% - 60% in air or oxygen.

By using pre-diluted Hydrogen one can avoid the flammable concentration range altogether. Standard Hydrogen/Nitrogen mixtures are for example commonly used as shielding gases for welding purposes.
Hydrogen can therefore safely be employed for leak testing if used at the right concentration. A suitable concentration to use is the standard 5% Hydrogen / 95% Nitrogen mixture which is available in industrial grade from most gas suppliers. The price is only a fraction for example of the price for helium.

The 5% mixture is classified as non-flammable according to international standard ISO 10156. This standard not only describes how flammability limits of gas mixtures are to be determined, but also states that Hydrogen/Nitrogen mixtures containing less than 5.7% Hydrogen are non-flammable, irrespective of how this mixture, in turn, is mixed with air.