ASTM D923 standard practices for sampling electrical insulating liquids
GENERAL PRECAUTIONS
12. General Precautions
12.1 Take and handle samples or test specimens in such a manner that the tests made on them indicate the characteristics of the liquid. Some tests are greatly affected by minute traces of impurities, and it is imperative that utmost precautions be taken to prevent contamination when obtaining samples. Due to the hygroscopic tendency of insulating liquids, it is important to minimize exposure to the atmosphere of the sample being taken.
12.2 Take a sufficient quantity of liquid as a sample to cover the requirements of the respective tests to be made. Make reference to the procedures governing these tests to ascertain the quantity of liquid for each test specimen and the number of test specimens required.
12.3 When samples are to be taken the temperature of the liquid should be equal to or greater than the temperature of the surrounding air in order to minimize the possibility of condensed moisture from the air being absorbed by the liquid during the sampling process, particularly in a humid atmosphere.
12.4 When sampling large outdoor tanks, tank trucks, tank cars, and de-energized electrical equipment the temperature of the liquid to be sampled may be colder than the surrounding air. On such an occasion, determine and report the temperature of the liquid and air as well as the relative humidity with the results of tests. It is undesirable to collect samples that are exposed to the atmosphere when the relative humidity exceeds 50 % or under conditions of rain or snow.
12.5 Allow containers of new liquid to remain undisturbed for at least 8 h before samples or test specimens are taken. In some instances, such as in the case of tank cars, it is not practical to wait this prescribed length of time, and samples for routine tests may be taken after the liquid has remained undisturbed for as long a period as practicable. For referee tests, allow the full 8-h waiting period to elapse before taking samples or test specimens. Repeat samples or test specimens from tank cars may be taken without waiting an additional 8 h.
12.6 Unless otherwise specified, take samples of insulating fluids having a relative density (specific gravity) of less than 1 from the bottom of the liquid container. For drums, cans, small tanks, etc., design the sampling device so that the sample is obtained a distance of 3 mm from the bottom of the container, while for large tanks, tank trucks and tank cars, the distance is within 13 mm of the bottom.
12.7 Unless otherwise specified, take samples of insulating fluids having a relative density (specific gravity) of greater than 1 from the surface layer of the liquid.
12.8 When make-up liquid is added to any piece of electrical equipment or the liquid is filtered, allow sufficient time to lapse to allow for complete mixing before sampling in order that a representative sample is obtained.
12.9 If samples or test specimens must be taken when the liquid temperature is below 0°C (32°F), high water content may not be detected because of the formation of ice. Ice is a concern in both energized and de-energized electrical apparatus where insulating oil-filled compartments operate at temperatures below freezing, such as some tap changer compartments and circuit breaker tanks.
