A deep fat fryer, comprising: a vat for holding a volume of cooking liquid, the vat in thermal communication with a heat source and configured to heat the cooking liquid disposed within the vat to a normal cooking temperature within a cooking temperature range; and a level detection system comprising a control system and a detector, the detector being disposed within the vat to monitor a level of the cooking liquid within the vat, the detector comprising: a temperature sensor; a heat producing element proximate to the temperature sensor; and a shell disposed around the temperature sensor and heat producing element, the shell being disposed within the vat to provide a barrier between the cooking liquid disposed within the vat and each of the temperature sensor and heat producing element, wherein the heat producing element is configured to transfer heat generated therein to the shell, and the temperature sensor is configured to measure a surface temperature of the heat producing element; the control system being in communication with the temperature sensor and the heat producing element; characterised in that the control system is configured to: energise the heat producing element to generate a flux of heat to heat the heat producing element to a hot setpoint temperature; and then de-energise the heat producing element; and then determine the presence or absence of cooking liquid proximate to the detector by comparing a rate of change of the measured surface temperature of the de-energised heat producing element with a reference value range; and the heat producing element and the shell are configured such that heat loss from the shell and the heat producing element is larger when the shell is not in contact with the cooking liquid than when the shell is in contact with the cooking liquid at the normal cooking temperature, and the hot setpoint temperature is close to or within the cooking temperature range, and the control system is further configured to determine a presence of cooking liquid proximate the detector when the rate of change is lower than the reference value range, and an absence of cooking liquid proximate the detector when the rate of change is within the reference value range. a vat for holding a volume of cooking liquid, the vat in thermal communication with a heat source and configured to heat the cooking liquid disposed within the vat to a normal cooking temperature within a cooking temperature range; and a level detection system comprising a control system and a detector, the detector being disposed within the vat to monitor a level of the cooking liquid within the vat, the detector comprising: a temperature sensor; a heat producing element proximate to the temperature sensor; and a shell disposed around the temperature sensor and heat producing element, the shell being disposed within the vat to provide a barrier between the cooking liquid disposed within the vat and each of the temperature sensor and heat producing element, wherein the heat producing element is configured to transfer heat generated therein to the shell, and the temperature sensor is configured to measure a surface temperature of the heat producing element; a temperature sensor; a heat producing element proximate to the temperature sensor; and a shell disposed around the temperature sensor and heat producing element, the shell being disposed within the vat to provide a barrier between the cooking liquid disposed within the vat and each of the temperature sensor and heat producing element, wherein the heat producing element is configured to transfer heat generated therein to the shell, and the temperature sensor is configured to measure a surface temperature of the heat producing element; the control system being in communication with the temperature sensor and the heat producing element; energise the heat producing element to generate a flux of heat to heat the heat producing element to a hot setpoint temperature; and then de-energise the heat producing element; and then determine the presence or absence of cooking liquid proximate to the detector by comparing a rate of change of the measured surface temperature of the de-energised heat producing element with a reference value range; and the heat producing element and the shell are configured such that heat loss from the shell and the heat producing element is larger when the shell is not in contact with the cooking liquid than when the shell is in contact with the cooking liquid at the normal cooking temperature, and the hot setpoint temperature is close to or within the cooking temperature range, and the control system is further configured to determine a presence of cooking liquid proximate the detector when the rate of change is lower than the reference value range, and an absence of cooking liquid proximate the detector when the rate of change is within the reference value range.