Much of the air and moisture in a system are removed before reaching 3000 microns. For wet systems, you should open the gas ballast during this initial pull down. Doing so helps the pump run smoothly and keeps the oil in good condition for when you need it most – towards the end of the evacuation. At about 3000 microns, when the sound of the pump quiets down, the gas ballast should be closed so that a deep vacuum can be achieved.
