Specific Gravity of Transformer Oil, Where to Draw Your Sample

Specific Gravity of Transformer Oil, Where to Draw Your Sample
Transformer fluids require sampling from different locations based on their specific gravity (SG).

One major element of a transformer maintenance program is periodic testing of its insulating fluid - and obtaining a good fluid sample starts with knowing where to draw the sample from.

Transformer fluids require sampling from different locations based on their specific gravity (SG). The specific gravity is of an insulating liquid is simply the ratio of the density of the fluid to the density of water.

SG = d of sample / d of water

Since water has a specific gravity of 1.0, free water in an insulating fluid will migrate to the top or bottom depending on the specific gravity of the fluid. The dielectric breakdown of most insulating fluid is inversely related to the water content, meaning the dielectric strength of the fluid is lowered as its water content increases.

Specific Gravity Rule of Thumb
If the specific gravity of an insulating fluid is greater than 1, sample from the top. If the insulating fluid specific gravity is less than 1, sample from the bottom. Flush 2-4 quarts of waste oil through the sample valve before collecting the sample.

For example, if the specific gravity of water is 1.0 and oil is 0.89, the oil will float on top of the water. Sampling from the bottom would provide the most water content in the fluid.

Location of transformer sampling valve.
Location of transformer sampling valve. Relative to specific gravity of the insulating fluid. Photo: TestGuy.

Of course there are exceptions to this rule as the sampling point can change throughout the life of the transformer. Mineral oil transformers with no drain valve are usually accessed and sampled by removing the top cover, for example.

Another instance when the sampling point of a transformer can change is after the retrofill of insulating fluid, such as Askarel to Silicone.


Specific Gravity of Transformer Insulating Fluids

Below is a table listing the specific gravity of common transformer insulating fluids technicians may encounter in the field:

Fluid Specific Gravity Sample Location
Askarel (PCB) 1.56-1.38 Top
Water 1.00 --
Silicone 0.98-0.834 Bottom
FR3 0.92 Bottom
BioTemp 0.91 Bottom
Mineral Oil 0.89 Bottom