Once I’ve recovered to -15" Hg, I valve off the columns and recover them separately, while simultaneously heating them to 150F with hot water through their jackets. At -22" Hg recovery, I switch to high vacuum pump and pull to -29.5, before backfilling with N2. I then pull -29.5 HG a second time and again backfill with N2. The material will contain less than 1% LEL.