BS EN IEC 62961:2018 pdf download.Insulating liquids - Test methods for the determination of interfacial tension of insulating liquids - Determination with the ring method
To obtain exact values for surface or interfacial tension, it is necessary to measure the maximum force on pulling the ring out of the liquid. Because of the great risk of detachment in the case of the manual apparatus, extremely smooth manipulation is necessary since the value obtained immediately prior to detachment of the film is not identical to the maximum value. Automatic tensiometers can determine the maximum value electronically and reverse the platform movement promptly prior to detachment of the film. This makes it possible to obtain reliable, accurate time-consistent serial measurements without tearing the film.
7.1 General
The measurement shall be done in the temperature range between 18 °C and 25°C. The water and insulating liquid shall be at the same temperature. The density of the insulating liquid shall be determined at the temperature of measurement or can be calculated from a linear extrapolation of density from measurement at a standard temperature (e.g. 20 °C) to the temperature used for the IF T measurement. Round robin test results have shown that variations within this temperature range do not practically influence the test results.
The correction according to Zuidema and Waters [5] shall be used.
The correction of Zuidema and Waters [5] is the preferred correction, since it is the most widely used in existing standards (ASTM D971, EN 14210，EN 14370). Further corrections known are those of Harkins and Jordan [6], Fox and Chrisman [7], and Huh and Mason [8]. If another correction formula is used, this shall be noted in the test report.
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.BS EN IEC 62961 pdf download.