ASTM D 971
ASTM D 971 is a standard issued by the American standardization organization ASTM International for testing the aging condition of transformer oils by measuring the interfacial tension (IFT) against water.
Why is the aging of transformer oils dangerous?
The coil windings of transformers, which play a central role in the construction of power supply networks, are surrounded by electrically insulating oils. The insulation, which is very good when fresh, deteriorates due to oxidation processes in which polar, more conductive impurities form. In addition, the affinity to water increases, which is then stored in the oil from humid ambient air and additionally increases the conductivity. This can result in overheating and, in extreme cases, even voltage breakdown. This process is a frequent cause of fire, which is why regular quality testing according to ASTM D 971 is required.
Why can aging be detected by interfacial tension?
The IFT between water and fresh oil is initially very high because no polar interactions can be formed at the interface between the polar liquid water and the non-polar oil. Polar contamination of the oil in the course of aging leads to such polar interactions and the IFT is reduced.
The oxidation products of the oil have long-chain, non-polar groups and a polar head group; i.e. they are structured like surfactants and show a corresponding IFT-reducing effect. The measurement of IFT according to ASTM D 971 therefore reliably reflects the aging condition of the oil, long before the loss of quality can become a safety risk.
Which measuring method is prescribed by ASTM D 971?
The standard ASTM D 971 prescribes the measurement with a tensiometer according to the Du Noüy ring method. A ring suspended from a force sensor is pulled from the heavy water phase into the lighter oil phase (or the sample vessel is moved downwards). Below the ring, a lamella is formed whose elongation produces a maximum force. This maximum corresponds to the interfacial tension.
The standard specifies that the lamella is to be stretched beyond the maximum force until it breaks off (ring tear-off method), which corresponds to the function of older mechanical tensiometers. Modern, processor-controlled measuring instruments can stretch and relax the lamella as often as required, so that the maximum force is determined several times in the course of a single measurement and thus even more reliably. However, some of these instruments can still carry out measurements until tear-off in order to comply with the ASTM D 971 standard.