What do you use to purge?

Now that I have a number of runs under my belt using fdd method of blasting it then letting the Pyrex sit out for a good amount of time til the erl turns cumbly. But that takes a lot of planning ahead so now I'd like to move on to vac purging. Which brought the question whats everyones else using? Ive noticed people are using 2 stage 3 cfm and 2.5 cfm pumps but what about chambers? Any input helps.