A method of operating a sterilization system having a vacuum chamber for sterilizing instruments, the chamber connected to a reservoir of sterilant by a valve in a closed state, is disclosed. The method comprises placing the instruments in a non-sterile state in a sterilization pack, opening the chamber, placing the pack into the chamber, closing the chamber, withdrawing a first volume of air from the chamber, changing a volume of liquid water into vapor, opening the valve, introducing the sterilant into the chamber, withdrawing the sterilant from the chamber, introducing a second volume of air into the chamber, opening the chamber, removing the pack from the chamber, and removing the instruments in a sterile-state from the pack. The method may also include taking a baseline humidity measurement while the pressure within the chamber is a first pressure, lowering the pressure within the chamber to a conditioning pressure, maintaining the conditioning pressure for a dwell time, increasing the pressure within the chamber, acquiring a second humidity measurement from within the chamber, and comparing the baseline humidity measurement to the second humidity measurement.