2. Chemical evolution

2.1 Pre-emptying (primary vacuum)

This operation evacuates gases from the atmosphere at around 10 ² Pa. The chemical species encountered are those introduced into the chamber at the start of the process. In many cases, these are, of course, atmospheric air, i.e. nitrogen, oxygen, argon and water vapor. For some processes, it is wise to avoid the ingress of moisture into the chamber, and to break the vacuum with dry air, dry nitrogen, dry argon, etc., in order to improve descent time.