Method of measuring humidity using an electrochemical cell

In a method of measuring humidity by using an electrochemical cell which includes an oxygen-ion conductive solid electrolyte, a porous negative electrode and a porous positive electrode tightly placed on a surface of the electrolyte, the negative electrode being arranged to be partly exposed to a humidity laden gas so that an oxygen component in the gas is transferred from the negative electrode to the positive electrode by a pumping action according to a voltage applied across the pair of electrodes;

• the method comprising:
• applying a voltage across the pair of electrodes to obtain a characteristic curve between an intensity of the voltage applied and that of an electrical current, the characteristic curve continuously originating from a first flat portion in which a first diffusion limit current remains generally constant with a predetermined voltage range and always passes through a point of inflection with the increase of the applied voltage in which a second differential derivative of the characteristic curve falls on zero at a certain voltage, and ends up with a second flat portion in which a second diffusion limit current remains generally constant with a predetermined voltage range, the point of the inflection being uniquely determined regardless of a humidity level in the gas at constant temperature and oxygen concentration;
• reading the point of the inflection, and at the same time, finding the intensity of the first diffusion limit current and the second diffusion limit current;
• and
• determining the humidity level in the gas from the fact that the humidity corresponds to a differential value either between the first diffusion limit current and the point of inflection or between the second diffusion limit current and the point of inflection.