A method of reducing the power output of a thick film heater to a predetermined fraction of its maximum power is disclosed. The method comprises applying a series of regular periodic bursts of electrical energy to the heater wherein the bursts are sufficiently short that the face of the heater plate opposite the heating track does not reach an equilibrium temperature. Also disclosed is a method ofdetecting dry switch-on by applying a lower power at the start of heating and determining whether a threshold rate of temperature rise is exceeded. In a method of calibrating a temperature sensor, the resistance at boiling and at another temperature (the latter e.g. being measured during manufacture) are measured and interpolation or extrapolation used. Other methods for estimating volume, reducing power as a given temperature is approached and detecting scale build-up are also disclosed.