Comparison of water measurement results in polyol ester‐based lubricating fluids determined by the coulometric karl fischer method and a thin‐film polymer capacitive water sensor

Abstract

AbstractField samples of MIL‐PRF‐23699 and DOD‐L‐85734, both polyol ester‐based fluids, were evaluated for water concentration by the coulometric Karl Fischer (KF) method according to ASTM D 6304 and with a prototype thin‐film polymer capacitance‐based sensor at Naval Air Systems Command (NAVAIR). The NAVAIR KF results showed a consistent negative bias, 190 ppm on average, compared to the results obtained with the water sensor. Since the calibration of the water sensor was performed by the sensor manufacturer using a different KF instrument and reagent/co‐solvent system, the ∼190 ppm negative bias probably reflects the difference in KF results between sensor manufacturer and NAVAIR. A limited round robin test, involving several laboratories using different instruments, reagents, solvents, and techniques, showed that water concentrations determined by the KF method can vary significantly from laboratory to laboratory. A larger round robin test was conducted to assess more accurately the variability in water measurement results. The results of the two round robin tests show that the particular KF instrument, reagents/solvents, and procedures being utilised by the various participating laboratories lead to a KF measurement variability of 20–30% for polyol ester‐based lubricating fluids. The results show that it is possible to obtain ∼190 ppm difference, observed between the water sensor and KF measurements at NAVAIR, between two laboratories. The results of the round robin tests showed a maximum repeatability of 9%. The significant difference between the repeatability and reproducibility indicates that there is potential for improved reproducibility of KF measurements through standardisation of, for example, instrument type, reagent/solvent system, test method, and laboratory procedure. Further work is required to determine the optimum KF test parameters for polyol.