An apparatus such as a domestic oven incorporates an induction motor which drives a fan. The temperature of the circulating gas and the slip of the rotor, coupled to the fan, are used to determine the humidity of gas within the oven. The temperature of the stator coil is also taken into account. The fan is driven with relatively high torque to maximize the signal to noise ratio. The fan may be provided as the main fan or as a secondary fan dedicated to humidity measurement. In either case the fan has a long operational lifetime.