A humidifier 28 for CPAP treatment has a tub to hold water, a heater to heat the water, a temperature sensor to sense ambient temperature, and a timing means to at least one of monitor time, record time and set time during the treatment session. A controller 26 controls the heater element to provide a humidity profile which is determined and adjusted as a function of sensed temperature and the above temperature parameter(s) so as to ration and maintain efficient use of the water during treatment.
