The breakdown voltage of an oil sample increases significantly with temperature. For example, a natural ester sample with a breakdown voltage of around 35 kV at 30℃ could easily have a breakdown voltage of nearly 60 kV at 70℃. For this reason, all oil test standards specify that the temperature of the sample must be recorded in the test report. Here, from the chart included earlier in this technical guide, is the section that refers to temperature.
Note The trending of test results to identify changes in breakdown voltage is only valid if the sample and ambient temperatures for all results have been taken into account. Some breakdown testers measure oil temperature automatically. This helps to ensure that the sample temperature has been measured and avoids the possibility of introducing contamination by placing a thermometer into the oil sample.