The determination of moisture in the oil is an essential part of a comprehensive transformer maintenance program. Excessive moisture contents causes accelerated ageing of the insulation materials and reduce their dielectric strength. In extreme cases this can result in arcing and shot circuits within the windings. Accurate moisture measurements can also warn about leaks in the oil system, as water is absorbed from the surrounding air.

The water solubility of transformer oils versus temperature. The marginal show the range of variation of water solubility found in actual mineral oils.

Moisture levels in oil are affected by temperature cycling of a transformer. The water solubility of the oil is temperature dependant water solubility increases as temperature rises