Specific weight of transformer oil is the ratio of its weight at 20ºC to the weight of the same volume of water at 4ºC.
Usually this parameters is not regulated and is within 0.856 to 0.886. If the oil is heated, its specific weight decreases, and vise versa when the oil is cold. When the insulation liquid cools or heats, its specific weight changes by 0.0007. This temperature coefficient is used when specific weight must be determined when oil temperature changes.
To measure the specific weight, an areometer is lowered into the glass cylinder with oil. The measurement is taken from the upper rim of the meniscus.
When the oil temperature changes by 100ºC, its volume changes by about 7%. In practice the temperature difference may be more than 100ºC, so the volume of the expansion tank should be 9-10% of oil volume.
There is also such parameter is specific gravity of transformer oil. It must be relatively low, since it allows the sediment to separate more easily and to settle better in the transformer, not hindering the circulation of the insulation fluid.