A method of sterilizing medical instruments using a vacuum chamber connected to reservoir by a valve in a closed state is disclosed that includes placing instruments in a non-sterile state into a sterilization pack, opening a 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 sterilant into 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 process may further include the steps of repeatedly determining pressure within the chamber, calculating second-derivative values of pressure with respect to time, and determining whether any local maxima exist in a plot of second derivative values that correspond to pressures above and/or below the triple-point pressure of water.
