6. Conclusion: Vacuum quality. Clean vacuum

How do you define vacuum quality?

Should we focus on the container or the content?

In a tank initially filled with nitrogen at atmospheric pressure, immersed in a bath of liquid helium, the pressure is less than 10 ^-12 Pa. We don't know how to measure it, yet we haven't created a vacuum in the commonly understood sense.

The vacuum quality you require depends on the operation you want to carry out. He must therefore draw up precise specifications, in the knowledge that quality limits are not very well defined.

The only reasonable definition of clean vacuum is given by G. Rommel: "The composition of the residual gas, controlled by a partial pressure analyzer and given as the ratio of partial pressures to total pressure, is as follows: