Even a perfect seal lets gas through the material itself. This is permeation, and it depends on solubility and diffusion.
The simple model
Permeation rate is proportional to the pressure difference and the exposed area, and inversely proportional to the path length (the seal thickness in the direction of flow). In an O-ring the exposed area is the cross-section edge and the path length is the squeezed width of the contact.
Increase the contact width, reduce the exposed area, and the leak falls. A small cross-section O-ring leaks less gas than a large one in the same pressure.
Which materials permeate more
| Material | Gas permeability (relative) |
|---|---|
| Silicone | Very high |
| Natural rubber, EPDM | High |
| NBR, HNBR | Medium |
| FKM, FEPM | Low to medium |
| Butyl | Very low |
| FFKM | Low for most gases, but varies |
| PTFE, filled | Medium; improved with fillers |
| Metal | Negligible |
Helium and hydrogen, with small molecules, permeate fastest. CO2 dissolves heavily in many rubbers and can cause swell.
Effects on engineering
- Vacuum and leak-tight equipment: choose low-permeation rubber, small cross-section, double seals with a pumped interspace.
- High-pressure gas: dissolved gas can cause rapid gas decompression damage when pressure falls.
- Hydrogen: permeates quickly and can embrittle some metals; seals need dedicated qualification.
- Process purity: permeation lets outside gas enter. Use metal seals or double seals if needed.
Reducing permeation
- Smaller cross-section and greater squeeze.
- Low-permeability compound (butyl, FKM).
- Add a barrier layer such as PTFE or metal-coated seals.
- Use a double seal with purge.
- Lower the temperature where possible. Permeation rises steeply with temperature.
Measuring
Standards for gas transmission include ISO 2782 (rubber) and ASTM D1434 (plastics). For seals, a helium leak test at assembly-level conditions is more realistic.
