Required sealing level
Tightness and mechanics- Notion of tightness and associated measures

Due to the discontinuous molecular structure of matter, a containment wall cannot, in theory, ensure the conservation of a fluid whose molecules are in motion for an infinite time: absolute tightness therefore does not exist. However, a very good seal is often necessary, which means more or less difficulty in achieving it. The level of tightness required varies greatly from one application to another, and it is possible to distinguish several levels or degrees:

• leak tightness that does not lead to any detectable leak with the measuring equipment used; in this case, the equipment is highly sensitive and the existing leak is extremely small (less than 10^-12 Pa.m³.s^-1);

• leakage-permeability, the value of which depends on the application of the material; it can vary over a very wide range from 10^-12 to 10^-1...