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The water content of hydraulic brake fluid affects boiling point, viscosity and the corrosive nature of the fluid. Brake overheating can boil fluids containing a significant amounts of water, causing loss of brake function. High water content can also corrode system components. A quick, easy method for determining water in brake fluid would enable service technicians to recommend brake fluid changes before either safety or corrosion concerns arise. This study compares the refractive indices and water content of a sample of new commercial DOT 3 and DOT 4 brake fluids. Measurements were made with 0 to 7% water added to the brake fluids. Added water content is compared to water content as determined by Karl Fisher titration. The 589nm refractive index of samples was measured at two temperatures to assess temperature sensitivity. Measuring refractive index to a high degree of accuracy is fairly easy. The measurements take less than 1 minute each.

The refractive index of ethylene and propylene glycol-based coolants is directly proportional to the concentration/freezing point of coolant in aqueous solution. A complicating factor in concentration measurement by refractive index is the temperature dependence of this physical property. The degree of change in refractive index due to changes in temperature is also concentration dependent. Accurate measurements require temperature equilibration between the refractometer, sample and environment. A refractometer which is able to sense temperature and automatically apply a correction factor greatly increases accuracy. Data showing the effect of temperature on a range of concentrations of commercial coolants aid pure ethylene and propylene glycol solutions is presented. The method and means of automatic temperature compensation necessary for obtaining the highest degree of accuracy are described.