TOPIC SERIES

Leak testing & acceptance

What helium testing and pressure-rise testing each measure, how to read them, and what belongs in an acceptance record.

2 articles in this series

When a chamber will not pump down, two causes must be separated first: leakage and outgassing. They are treated differently and judged by different criteria, and confusing them leads to the wrong conclusion. This series covers method selection, how to read the numbers, and which numbers are defensible in an acceptance document.

What this series covers

The pressure-rise method measures leakage plus outgassing together; it suits process checks and coarse localisation, not standalone acceptance.
Helium mass-spectrometer testing is performed in the 1×10⁻⁹ Pa·m³/s range, with the exact criterion and test points agreed on the project drawing.
Readings taken hot, under load, or after process has run are not comparable: leak testing is done unloaded.
A record without instrument model, tracer gas, duration and temperature cannot be reproduced or audited.

2026-11-01

Helium leak testing and pressure-rise testing differ by five orders of magnitude

A common question is whether pressure-rise testing is acceptable. It answers whether a chamber leaks; helium mass spectrometry answers how much and where. The gap is five orders of magnitude — a limit of the method, not of the instrument price.

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2026-10-27

Seven checks a vacuum chamber has to pass

A welded chamber is not a deliverable chamber. Seven checks are carried out, each signed off and filed, and the report is dispatched with the unit. Records remain available for customer audit.

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FAQ

Can pressure-rise testing replace helium leak testing?

Not as an acceptance criterion. It measures leakage and outgassing together and cannot separate them; only on a well-baked system where outgassing is negligible does the rise rate approach the true leak rate. Its value is that it is cheap, repeatable on site, and quickly locates gross leaks.

At which stage of fabrication should leak testing be done?

A coarse test after welding locates defects early; the final test follows assembly and precedes cleanliness processing. Testing only at the end means a failure requires disassembling already-clean parts, repeating both cleaning and prior labour.

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