BS EN IEC 62961:2018 pdf free download – Insulating liquids – Test methods for the determination of interfacial tension of insulating liquids – Determination with the ring method
7.3 Determination of the surface tension of water used for the test
Start the measurement according to the manufacturer instructions. Introduce water into a clean vessel to a level of 20 mm to 30 mm. Slowly raise the platform and the measurement vessel until the ring is immersed in the water phase. The ring shall be at least 3 mm below the water surface and centred in the vessel. Ensure that the ring is properly wetted and not repelled from the water surface. Depending on the test temperature, a value of 70 mN/m to 73 mN/m shall be obtained. If lower values are found, repeat cleaning of the sample vessel and the ring. Check the source of the sample water in respect to surface active contaminants. In exceptional cases, search for a fresh sample of water from another source (see also 6.3). If the water quality (according to 6.3) and the cleanliness of vessels (according to 6.1 ) and the ring (according to 6.2) are validated in the testing laboratory by an established and reproducible procedure, it is not necessary to carry out the determination of the surface tension of water (according to 7.3) prior to each sample, but once in a daily successive series of tests.
Some laboratories have evaluated other methods of testing the IFT of insulating liquids. The drop volume method is a particularly reasonable alternative to the force (ring) method as it overcomes some of those weaknesses. In the tests carried out within the round robin tests, fast and reproducible results at a well-defined interface age could be obtained. It allows multiple tests to be carried out with many fewer samples and without the need for refilling. The parts in contact with insulating liquid do not need flame cleaning. Syringes and needles are stable and robust.
The size of each droplet increases with specific volume flow until they break off from the dosing capillary. The detached droplets are detected by a suitable sensor. When the volume flow and the number of droplets are known, the break-off volume for each droplet can be calculated. This measurement of break-off volume enables calculation of the surface tension from the balance of forces at the moment of break-off. In the case of interfacial tension, the needle is dipped into the test liquid – either insulating liquid or water – depending on the instrument configuration. The droplet will detach at the moment when the buoyancy balances the wetting force at the capillary tip.