The ASTM D971 shows how to measure the interfacial tension of insulating oils against water. It is conducted using a tensiometer and the ring method.
ASTM D971: Application and Use
The ASTM D971 defines how to measure the interfacial tension between mineral oil and water. The test is used especially for insulating oils. Insulating oils are liquids used in electrical equipment to both insulate components and help manage heat. In addition, they protect the equipment by reducing moisture, oxidation, and general wear over time. Measuring the interfacial tension gives indication about polar containment and oxidation processes within the oil. It is mainly used to check the level of deterioration of such oils.
ASTM D971: Experimentation
Apparatus
For this measurement, the following apparatus is used:
- tensiometer capable of measuring tensions between 0 and 100 mN/m
- ring (minimum of 4cm circumference; made of platinum or platinum-iridium)
- sample container, with 45 mm minimum diameter
All Force Tensiometers of the DCAT series from DataPhysics Instruments comply with this standard.
Procedure
A sample of the oil, at least 25 mL, is obtained using the method described in ASTM D923. The density of the oil sample should be measured. Then, distilled water is poured into the sample vessel. Afterwards, the ring is immersed in the water. Next, the surface tension between the water and air should be measured to ensure the water is clean. The surface tension should be between 71 and 73 mN/m. To conduct the actual measurement, the oil is carefully poured onto the water. Then, the interfacial tension between the water and the oil is measured. Make sure that the measurement does not take longer than 60 seconds from the moment the oil is poured until the final measurement.
The ASTM D971 is in use since 1999, and was last updated in 2020.
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