All air must be evacuated from an autoclave before the relief valve is closed. It is the steam under pressure that does the sterilization and if the air is not released as the water in the autoclave starts to boil, correct sterilization will not take place. The evacuation takes around 5 to 7 minutes at which point, the relief valve, seen pointing up in the image, is lowered to a horizontal position, allowing the autoclave to come up to the correct pressure for sterilization.

Autoclave model here is:

All American
25X-1
120 volt
serial numbers:
0013689
0013905