The new standard IEC 62961 for the transformer oil quality testing has just been released. Although the old ASTM D971 standard still exists, it is likely that the new standard will eventually replace the old one. Some of the issue related to the old standard are better addressed in the new one. In this blog post, the main differences between the old and the new standard are discussed.
1. The time point of the measurement
The most significant difference is the time point at which the measurement is done. According to the ASTM standard, the measurement should be performed within 60 s after the oil -water interface has been formed. At this time point, the equilibrium has not been reached as it generally takes much longer for the impurities to adsorb at the oil -water interface. This makes the method less accurate and more sensitive to the timing of the measurement. With the new standard, the time point is set to 180 ±30 s. Although the equilibrium is probably not achieved even in this time frame, it gives much more realistic value for interfacial tension without increasing the testing time too much. Also, with this measurement protocol, it is possible to distinguish between differently aged ester liquids, which is not possible with ASTM D971 standard.
2. Acceptable surface tension range for the water adjusted
The purity of the water is also highly important. The old ASTM standard instructs to check the purity of the water prior to each interfacial tension measurement. It also defines the acceptable surface tension range for water to be 71 – 73 mN/m. This is quite strict as the surface tension of water is very easily affected by even the smallest impurity. The new standard loosens the requirement a bit and the range is now set to 70 - 73 mN/m. Also, the measurement of surface tension is not required before each test but a daily check is enough.
3. Use of the automated instrument encouraged
As the measurement is done with the du Noüy ring method, the speed at which the ring is moved through the interface is meaningful and the motion should be as smooth as possible. The force determining the interfacial tension is measured just prior the detachment of the film and is not equal to maximum force. The automated instrument can detect that maximum electronically and reverse the platform movement to prevent the detachment of the film. This makes it possible to do several measurements in a row.
4. Instruction for the cleaning of the beaker and du Noüy ring clarified
As purity is highly important for successful measurements, the new standard gives a lot of emphasis on cleaning. Clear instructions for the cleaning of both the beaker and the du Noüy ring are given.
Please watch the short webinar below that discusses the changes more in detailed.
Interfacial tension of insulating liquids against water has been used for a long time as a criterion for aging evaluation. The latest revision for the ASTM D971 standard dates back to 2012. The new IEC standard is about the be released to improve the accuracy of the method.