Abstract: A sterilizer for sterilizing objects with biocidal gas which is designed to resist the deleterious effects of the biocidal gas. The invention provides a sterilizer having a sterilizing chamber for holding the objects to be sterilized, and a gas control system for manipulating the gaseous environment within the sterilizing chamber. The sterilizer has a vacuum control system, a vacuum line in fluid communication with the sterilizing chamber, a humidity sensor connected to that vacuum line at a point removed from or outside of the sterilizing chamber, and an isolation valve connected to the vacuum line at a point between the sterilizing chamber and the humidity sensor for temporarily isolating the humidity sensor from the sterilizing chamber. This arrangement assures that the humidity sensor received a sufficient sample for reading the adequacy of the humidification of the objects to be sterilized, while at the same time protecting it from the biocidal gas.