Convert leak rates between mbar·l/s, Pa·m³/s, atm·cc/s, sccm and Torr·l/s - with helium-to-air equivalent and leak tightness classification.
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Converting leak rates: one quantity, five units
The leak rate describes pV-throughput through a leak. Different industries state it differently: vacuum technology uses mbar·l/s, the SI world Pa·m³/s (factor 10 smaller), US specifications atm·cc/s, semiconductor and gas technology often sccm. Conversions: 1 Pa·m³/s = 10 mbar·l/s · 1 atm·cc/s = 1.013 mbar·l/s · 1 sccm = 1.69×10⁻² mbar·l/s.
Measured with helium – specified in air
Leak detectors measure helium, specifications often require air leak rates. The conversion depends on the flow regime in the leak: in molecular flow (small leaks below ~10⁻⁷ mbar·l/s) throughput scales with 1/√M – air flows slower than helium by √(4/29) = 0.374. In laminar-viscous flow (larger leaks) viscosity governs: Q(air) = Q(He)/1.08. Using the wrong factor puts you off by up to 3×.
Typical tightness classes
10⁻⁴ mbar·l/s: bubble-tight · 10⁻⁵: vapour-tight · 10⁻⁷: common limit for gas systems and valves · 10⁻⁹: hermetic, standard for high-purity metal seals (VCR) · 10⁻¹⁰ and below: UHV level.
Worked example
A detector shows 5×10⁻⁹ mbar·l/s helium at a VCR fitting. That is 5×10⁻¹⁰ Pa·m³/s or 3×10⁻⁷ sccm – and as air equivalent (molecular) 1.9×10⁻⁹ mbar·l/s. The joint comfortably meets a 10⁻⁸ specification.
