A respiratory or surgical humidifier system (100, 101), comprising: a base unit comprising: a heater plate including one or more heating elements; and a hardware controller in electronic communication with the one or more heating elements of the heater plate and configured to energize the one or more heating elements of the heater plate; a temperature sensor coupled to or adjacent the heater plate, wherein the temperature sensor determines a temperature of the heater plate; and a humidifier chamber defining a volume and including a conductive base receivable onto the base unit such that the conductive base contacts the heater plate, the humidifier chamber configured to hold a level of water, wherein the hardware controller is configured to: apply a characteristic energization signal to the one or more heating elements of the heater plate, wherein the characteristic energization signal is superimposed onto a heater plate control signal; process a temperature signal from the temperature sensor corresponding to the characteristic energization signal, wherein the hardware controller passes the temperature signal through any one of: a bandpass filter, a high pass filter, a homodyne filter or an infinite impulse response filter, having a filter frequency corresponding to a frequency of the characteristic energization signal such that temperature signal corresponding to the frequency of the characteristic energization signal are passed; determine a value from which the specific heat capacity of the humidifier chamber including its contents can be inferred based on the processed temperature signal; determine a low water or water-out condition based at least in part on the determined value from which the specific heat capacity can be inferred; and output a low water or water-out warning in response to the low water or water-out condition. a base unit comprising: a heater plate including one or more heating elements; and a hardware controller in electronic communication with the one or more heating elements of the heater plate and configured to energize the one or more heating elements of the heater plate; a heater plate including one or more heating elements; and a hardware controller in electronic communication with the one or more heating elements of the heater plate and configured to energize the one or more heating elements of the heater plate; a temperature sensor coupled to or adjacent the heater plate, wherein the temperature sensor determines a temperature of the heater plate; and a humidifier chamber defining a volume and including a conductive base receivable onto the base unit such that the conductive base contacts the heater plate, the humidifier chamber configured to hold a level of water, wherein the hardware controller is configured to: apply a characteristic energization signal to the one or more heating elements of the heater plate, wherein the characteristic energization signal is superimposed onto a heater plate control signal; process a temperature signal from the temperature sensor corresponding to the characteristic energization signal, wherein the hardware controller passes the temperature signal through any one of: a bandpass filter, a high pass filter, a homodyne filter or an infinite impulse response filter, having a filter frequency corresponding to a frequency of the characteristic energization signal such that temperature signal corresponding to the frequency of the characteristic energization signal are passed; apply a characteristic energization signal to the one or more heating elements of the heater plate, wherein the characteristic energization signal is superimposed onto a heater plate control signal; process a temperature signal from the temperature sensor corresponding to the characteristic energization signal, wherein the hardware controller passes the temperature signal through any one of: a bandpass filter, a high pass filter, a homodyne filter or an infinite impulse response filter, having a filter frequency corresponding to a frequency of the characteristic energization signal such that temperature signal corresponding to the frequency of the characteristic energization signal are passed; determine a value from which the specific heat capacity of the humidifier chamber including its contents can be inferred based on the processed temperature signal; determine a low water or water-out condition based at least in part on the determined value from which the specific heat capacity can be inferred; and output a low water or water-out warning in response to the low water or water-out condition.