Helium has been the default tracer gas for high-sensitivity leak testing for decades, and for good reason: it is inert, non-toxic, has a very small molecule, and occurs in the atmosphere at only around 5 ppm, so anything detected outside a part is almost certainly a genuine leak.
What has changed is the economics. Helium is a finite resource, recovered as a by-product of natural gas extraction, and supply has been repeatedly disrupted over the past two decades. For manufacturers running high-volume production, cost per test and security of supply have become real engineering constraints rather than purchasing footnotes.
That has pushed forming gas — a standard, non-flammable mixture of 5% hydrogen and 95% nitrogen — from a niche alternative into a mainstream option. This article sets out where each method genuinely wins.
The Short Answer
If you need the absolute lowest detectable leak rates, or you are testing to a specification that names helium, use helium. If your specification sits within reach of forming gas and running cost matters, hydrogen will usually be the better commercial decision. The detail is below.
Side-by-Side Comparison
| Helium | Hydrogen (forming gas 5%/95%) | |
|---|---|---|
| Sensitivity | To 10⁻¹² mbar·l/s with vacuum mass spectrometry | Around 10⁻⁷ mbar·l/s — ample for most production specifications |
| Detection technology | Mass spectrometer (vacuum pumps, filament ion source) | Selective solid-state sensor — no vacuum system required |
| Gas cost & supply | Finite, price-volatile, subject to supply disruption | Readily synthesised, inexpensive, globally available |
| Atmospheric background | ~5 ppm — very low, excellent signal-to-noise | ~0.5 ppm — even lower background |
| Molecular behaviour | Small, fast-moving molecule | Smallest molecule; higher velocity, lower viscosity — moves through leak paths faster |
| Recovery after a gross leak | Mass spectrometers can be saturated and need clean-up time | Solid-state sensors clear rapidly, resisting saturation |
| Maintenance | Vacuum pumps and ion source are consumable items | Pump-less designs have no internal moving parts to replace |
| Safety | Fully inert | At 5% hydrogen, below the lower flammability limit — classed as non-flammable |
Is Hydrogen Safe?
This is the most common objection, and it rests on a misunderstanding. Forming gas is 5% hydrogen in nitrogen — deliberately formulated to sit below the lower flammability limit of hydrogen in air. It is classed as non-flammable and is used routinely as an industrial tracer gas. It is not the same thing as handling pure hydrogen.
When to Stay With Helium
- Very low leak rates. Below roughly 10⁻⁷ mbar·l/s, helium vacuum mass spectrometry is the only practical route.
- The specification names it. Customer drawings, regulatory requirements or standards such as ASTM F2391 may specify helium explicitly.
- Hermetically sealed parts. Bombing techniques for sealed items with no test port are helium-based.
- Existing validated process. If a helium method is already validated and running, revalidation cost may outweigh gas savings.
When Hydrogen Makes More Sense
- Your specification is within reach. If 10⁻⁶ to 10⁻⁷ mbar·l/s satisfies the requirement, forming gas does the job at a fraction of the running cost.
- High-volume sniffing. Per-test gas cost dominates at volume, and this is where the saving compounds.
- Messy production environments. Selective sensors reject solvents, cleaning agents and moisture well, reducing false alarms.
- Gross leaks are common. Rapid sensor recovery avoids the clean-up delays that stall a line.
- Field and utility work. Rugged, battery-powered hydrogen units suit outdoor and buried-service testing.
You May Not Have to Choose
Some instruments span both. Mass-spectrometer platforms can be configured to switch between refrigerants and helium, and some accumulation systems accept either helium or forming gas — allowing cost-effective hydrogen testing at sub-assembly stage and helium validation at end of line, on compatible hardware.
Making the Decision
Work backwards from the leak rate your product genuinely requires, not the most sensitive method available. Over-specifying sensitivity is one of the most common and expensive mistakes in leak testing: it inflates equipment cost, lengthens cycle times and increases false rejects, without improving product quality.
If you would like help establishing the right leak rate and method for your application, see our hydrogen leak detection and helium leak testing service pages, or browse the INFICON leak detector range. For lower-sensitivity requirements, pressure decay leak testing with air may be all you need.
