The effect of water content on the dielectric strength of polyalphaolefin (PAO) coolants

Many variables can affect the dielectric strength of coolants and insulating fluids including particulates, water content, temperature, viscosity, and pressure. The present paper evaluates the effects of dissolved water content, adjusted in a controlled manner, on the dielectric strength and the variability of breakdown voltages in synthetic polyalphaolefin (PAO) MIL-PRF-87252 coolants obtained from three different suppliers. Additionally, the effect on dielectric strength of using nitrogen gas versus air for the removal of water is evaluated. The results indicate that the dielectric strength of the fluids decreases by up to 80% over a range of 0 to 70 ppm water content. The variability in breakdown voltage generally increases with increasing water content for all of the samples tested. These results indicate the notable effects of water at concentrations below the MIL-PRF-87252 specified acceptable limit of 50 ppm. The effect on the dielectric strength of using two different sparging gases for the removal of water, dry air or dry nitrogen, was irrelevant when compared at similar water contents.