A wood drying installation for a wood drying kiln including a pair of electrodes adapted to be inserted into a wood stack contained in the kiln, a resistance connected in a series circuit with the electrodes, an AC voltage source connected to apply an AC voltage across the series circuit, a phase detecting circuit connected to the series circuit operable to generate a signal representing the phase angle between AC voltages applied to different parts of the series circuit, and a processor to receive the signal. The system is operable to determine a moisture value corresponding to a capacitive component of the reactive impedance between the electrodes in accordance with a predetermined arithmetic algorithm relating the value to the phase angle.