UHP Valve Helium Leak Rate Standard
A helium leak test is the acceptance measurement that turns a clean valve into a qualified UHP valve. It is performed with a mass spectrometer leak detector, either by evacuating the valve and spraying helium outside (inboard leak) or by pressurising one side with helium and measuring at the seat (across-seat leak).
Inboard and across-seat are different tests
Inboard leakage measures the integrity of the body, diaphragm and stem boundary against atmosphere. Across-seat leakage measures how well the closed valve isolates the downstream side. Both belong in the specification; quoting only one leaves half the valve unverified.
Why helium
Helium is inert, has a small atomic radius and is present at only trace levels in air, so the detector sees a very low background. It is a conservative, repeatable tracer for systems that will carry hydrogen, silane or other small-molecule gases.
Specifying the acceptance criteria
State the test method, test pressure, direction, acceptable leak rate and whether the test is performed at room temperature and after cycling. Ask for a per-valve record rather than a batch certificate when the panels will be qualified by a fab.
Leak rate versus system purity
A tight valve is necessary but not sufficient. Moisture and particulate from assembly, packaging and installation still dominate real contamination, so leak testing belongs alongside electropolishing, cleanroom assembly and double-bag packaging.
Field re-check
After welding or re-making a face seal, re-check the affected joint. A valve that passed at the factory can leak after installation if a gasket is misaligned or a weld is porous.
Related reading
UHP Diaphragm Valves
Ultra high purity diaphragm valves for specialty gas delivery. 316L VIM/VAR bodies
How to Select a Specialty Gas Valve
A practical selection guide for specialty gas valves: gas compatibility, pressure, Cv
Cleanroom Assembly & Double-Bag Packaging
How UHP valves are cleaned, assembled and double-bagged so they arrive at a fab gas panel without
Specialty Gas Safety & Material Compatibility
Safety principles for specialty gas valves: gas compatibility, fail-safe state
How to Select a Specialty Gas Valve
A practical selection guide for specialty gas valves: gas compatibility, pressure, Cv
Diaphragm Valve vs Bellows Valve
Compare UHP diaphragm valves and bellows sealed valves for specialty gas: dead volume, temperature
316L VIM/VAR Stainless Steel for UHP Valves
What VIM/VAR refining changes in 316L stainless steel used for ultra high purity valve bodies
Frequently Asked Questions
Is a helium leak test enough to call a valve UHP?
No. UHP also covers material quality, surface finish, cleaning, assembly environment and packaging.
Should I request per-valve test records?
Yes, for fab and other qualified installations; a batch certificate is weaker evidence.