Ryan, T. and Hinz, T., "Analysis of Water Content in Brake Fluid. Part I. Method Comparison: Karl Fisher Titration Versus Refractive Index," SAE Technical Paper 973023, 1997, doi:10.4271/973023.
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. They do not require use of reagents or volume-weight measurements. This study shows there is little change in absolute index for a wide range of brake fluids. A very significant change in refractive index occurs as water is added to samples. DOT 3, high boiling point DOT 3 and DOT 4 Fluids showed very little difference in absolute refractive index and identical changes in refractive index due to added water.