Measuring the water content of synthetic lubricants with polymer-coated sensors

Water sensors based on platinum electrodes coated with films of perfluorinated sulfonate ionomer (PFSI) and cellulose triacetate (CTA) have been used to measure the water concentrations between 50 and 2500 μg/g in synthetic lubricants based on polyolesters (POEs). The response characteristics of the sensors, the final current (if) at a given water level and the time needed to reach this current, depended on fluid composition, fluid temperature, and the amount of stirring in the system. The water concentration was proportional to i0.5f for POEs in a temperature range from 21 to 75 °C. These sensors were capable of monitoring rapidly changing water levels in POEs due to water absorption or loss at these temperatures. The ability of this type of sensor to function reliably in operating air-conditioning compressors was also demonstrated.