DGA monitor measures relative oil moisture (%RS) inside its cabinet. The temperature of the moisture sensor location does not represent the temperature inside the transformer tank or inside the oil circulation pipes. Additionally, oil sampling through the steel pipes changes the oil temperature. As relative moisture saturation is strongly temperature dependent, the %RS measured and output by DGA monitor is not equal to %RS inside the transformer tank.
However, DGA monitor does output both the measured %RS and the temperature where it was measured. To calculate the %RS in the true temperature inside the transformer, use the following equation.
A is a water solubility coefficient. Vaisala uses this standard value in all of its oil moisture transmitters. You can also use this equation with an oil-specific coefficient if it has been determined for your oil.