Method of calibrating a heater system

A method of calibrating a heater system for heating a surface of a structure, the heater system including a heater element, a temperature sensor proximate the heater element for outputting a signal indicative of the temperature of the heater element, and a controller for controlling the supply of power to the heater element in dependence on the signal to maintain the temperature of the heater element at a first substantially constant temperature. The method comprises: immersing the structure in a fluid for maintaining the surface of the structure at a second substantially constant temperature; supplying an amount of power to the heater element; receiving the signal from the temperature sensor and determining a temperature at the temperature sensor; and determining a setpoint temperature for controlling the heater element in dependence on the signal and the second substantially constant temperature.