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A collaborative study on the precision and bias of the measurement of water in transformer oils indicated that an acceptable level of precision was achieved using the revised ASTM Test Method for Water in Insulating Liquids by Coulometric Karl Fischer Titration (D 1533-96) and a sample size of 7 mL. Analysis of the distribution of the laboratory results indicates the existence of a systematic bias in the accuracy of the measurement which is related to the instrument design, solvent composition, and calibration of the instrument. Additionally, evidence is presented that indicates that a portion of the water in the oils is not titrated by the coulometric method.
water, Karl Fischer method, systematic bias, transformer oil, coulometry, precision, D 1533-96
Research Chemist, NIST, Gaithersburg, MD