The working principle is a capacitive measurement operating on absorption of water in oil. The physical measured value is % Humidity.
New oil has the ability to hold a certain amount of dissolved water. The maximum water amount oil can hold is called “saturation point”. Above the “saturation point” free water will fall out which can cause corrosion inside of the engine.
The “saturation point” is influenced by temperature and other different factors like the composition of oil mineral or synthetic, formulation of additives and will change during the lifetime of the oil. The Water in Oil Sensor (WiO) is not measuring free water or emulsion, which is detectable by regular Water in Oil test kits, it measures the absolute water content in oil.