Specific Gravity or relative density is a measure of the density of transformer fluids relative to water, being scaled at 1.000. The value from testing is normalized to 15C/15C.
With the introduction of different transformer insulating fluids such as Natural and Synthetic fluids, specific gravity testing can be key in a quick determination of fluid type involved for further testing and thus interpreting the subsequent test results properly.
- Mineral Oils have a range of 0.8200 to 0.9100
- Natural Esters are <0.9600 with a nominal value of 0.9200
- Silicone Fluids are 0.9570 to 0.9640
- Synthetic Esters are nominally 0.9600 to 0.9800
Besides the determination of the type of fluid, the Specific Gravity can also indicate contamination of a known fluid with another fluid. Elevated mineral oil specific gravity can indicate mixing with esters or silicone fluids. Lower mineral oil values can indicate contamination with other fluids or solvents such as diesel fluid or mineral spirits. This can also be relative to ester fluids that mix with mineral oil.
