A method of sterilizing medical instruments using a vacuum chamber connectedtoreservoir by a valve in a closed state is disclosed that includes placinginstruments in anon-sterile state into a sterilization pack, opening a chamber, placing thepack into the chamber,closing the chamber, withdrawing a first volume of air from the chamber,changing a volume ofliquid water into vapor, opening the valve, introducing sterilant into thechamber, introducing asecond volume of air into the chamber, opening the chamber, removing the packfrom thechamber, and removing the instruments in a sterile-state from the pack. Theprocess may furtherinclude the steps of repeatedly determining pressure within the chamber,calculatingsecond-derivative values of pressure with respect to time, and determiningwhether any local maximaexist in a plot of second derivative values that correspond to pressures aboveand/or below thetriple-point pressure of water.
