A method for sterilizing a load in a chamber, said chamber coupled to a sterilant vapor concentration sensor, a pressure sensor, a source of gas, an evacuation pump, a sterilant source, a first valve having an open and a closed position for controlling the flow of sterilant from the sterilant source into the chamber, a second valve having an open and a closed position for controlling the flow of gas from the gas source into the chamber, the vapor sensor and pressure sensor providing data to a controller which controls the opening and closing of the first and second valves, the method comprising: (a) placing the load in the chamber; (b) operating the evacuation pump to decrease (104, 80) the pressure within the chamber; (c) opening the first valve such that sterilant flows (82, 91) from the sterilant source toward the chamber to raise the concentration to at least substantially a first predetermined target level and then closing the first valve after an initial predetermined period of time; (d) allowing (83, 93) the sterilant within the chamber to diffuse for a first diffusion period; (e) monitoring (84, 94) the concentration of sterilant in the chamber with the sterilant vapor concentration sensor and determining the length of time the first valve must be opened to raise the concentration to substantially a second predetermined target level; (f) opening the first valve to admit (85, 95) additional sterilant into the chamber and then closing the first valve at the end of the length of time determined in step (e); (g) allowing (86, 97) the sterilant within the chamber to diffuse for a second diffusion period; (h) after said second diffusion period, opening the second valve to admit (87, 98) a sufficient quantity of gas to increase the pressure within the chamber such that said gas and said sterilant diffuse, and further comprising the step wherein the controller (i) monitors the signals generated by the sterilant concentration sensor and uses these signals to provide a check to ensure that the concentration of sterilant in the chamber is at the first predetermined target level, (ii) if the concentration is not at the first predetermined target level, calculates how much sterilant must be added to reach the first predetermined target level and the amount of time the first valve must be in the open position to admit the quantity of sterilant that must be added to the chamber to reach the first predetermined target level, and (iii) opens the first valve for that amount of time and then closes the first valve. (a) placing the load in the chamber; (b) operating the evacuation pump to decrease (104, 80) the pressure within the chamber; (c) opening the first valve such that sterilant flows (82, 91) from the sterilant source toward the chamber to raise the concentration to at least substantially a first predetermined target level and then closing the first valve after an initial predetermined period of time; (d) allowing (83, 93) the sterilant within the chamber to diffuse for a first diffusion period; (e) monitoring (84, 94) the concentration of sterilant in the chamber with the sterilant vapor concentration sensor and determining the length of time the first valve must be opened to raise the concentration to substantially a second predetermined target level; (f) opening the first valve to admit (85, 95) additional sterilant into the chamber and then closing the first valve at the end of the length of time determined in step (e); (g) allowing (86, 97) the sterilant within the chamber to diffuse for a second diffusion period; (h) after said second diffusion period, opening the second valve to admit (87, 98) a sufficient quantity of gas to increase the pressure within the chamber such that said gas and said sterilant diffuse, and