Viscosity : In transformers, for an oil circulation which provide sufficient cooling, kinematic viscosity should be low.

Water Amount Included : The perforation resistance of an oil decreases too much depending on the moisture of the oil. Only dried oil may be frozen in high voltage transformers. Water or moisture may exist in oil as solution or chemical. This has to be removed with a treatment process.

Air Amount Included : Gases may exist in transformer oils as solutions. The air existing in transformer oil as solution do not decrease the conductivity of the oil as fast as water. However, oxygen existing in the air accelerates the becoming old of the oil chemically. If the air in the transformer is circulating within the windings, this situation constitutes great risk for the transformer. Therefore, this air and other gases should be taken out during first oil filing processes. Vacuuming process accelerates the decomposition of gas solutions from oil.

Perforation Resistance : The most practical measurement method for determining the condition of a transformer oil is perforation resistance.

Perforation Tension is the tension which perforates the oil layer between two spherical electrodes of 2,5mm spaces and of 36mm radius from the sample oil. Transformer oil loses its durability depending on the water amount included and the pollution level of fused solid isolation substances. It is possible to reach a general idea concerning the condition of the transformer oil with this method but in some cases this method may be insufficient. For instance; when perforation resistance of an oil is found to be high, this means that the oil is old and lost its chemical characteristic.

Dielectric Loss Factor (tand):Loss factor measurement is the most effective in evaluating the fluid and electrical quality of the oil. The changes in the values of this greatness measured in the old oil also give the change level of the oil and solid isolation substances. By determining this value, a clearer information concerning the condition of old oil may be obtained.

Neutralisation and soaping Number : This value shows the amount of free acids in the oil. Neutralisation number's highness is a proof of oil oldness.

Mud Forming : Solid isolation substances which dissolved and became old depending on facility conditions combine with the carbonated oil molecules and sink at the bottom.