This test is made to determine the temperature rise of the transformer under rated Load. Thermometers or thermocouples are installed to measure ambient temperatures, oil surface temperatures in the transformer tank and at some point (perhaps 2 or 3 inches) below the oil surface where maximum temperatures may be expected. Readings on these are taken before the test is started to obtain base temperatures on which to determine the rise at rated load These are also read periodically during the test, and continued until temperatures do not vary more than a few degrees over a paeriod of hours, and hence may be considered as stabilized.
Ref:
Electrical Transformers and Power Equipment by Anthony J. Pansini�