Calculating relative moisture saturation inside transformer - OPT100

OPT100 User Guide

Document code
English (United States)
Document type
User guide

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.

% R S T 2 = % R S T 1 × 10 ( A ( T 2 T 1 ) ( T 1 + 273.15 ) × ( T 2 + 273.15 ) )

Relative moisture saturation output by DGA monitor
Temperature of the in-oil measurement chamber inside DGA monitor
Temperature at location where new relative moisture saturation is needed at

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.