An improved system and method of determining a low water and/or water-out condition in a humidifier chamber of a respiratory or surgical humidifier system can use a specific frequency band to detect changes in a temperature of a heater plate. The temperature changes can correlate to the specific heat capacity value of the humidifier chamber. The low water and/or water-out detection process can be performed without having to determine the gases flow rate and/or can be run continuously. A heater plate assembly of the system can include a compliant insulation sheet to improve thermal coupling between the heating element and the top heating plate of the heater plate assembly, thereby improving the low water and/or water-out detection process.