An absolute high-pressure microviscometer based on refractive index

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Abstract
A new method of measuring refractive index in a lubricant point contact is described which allows the density, pressure and viscosity in the fluid to be determined. The viscosity measurements are absolute, only the elastic and optical constants of the glass are needed. Due to the very high pressures developed (1 GPa) in the 0.3 mm diameter contact very large pressure gradients are produced, hence viscosities four orders higher than conventional high-pressure viscometers normally reach, can be measured. Four lubricants tested all showed that the pressure viscosity coefficient dropped sharply above 10$^{3}$ Pa s (10$^{4}$ poise); the limit of the normal viscometer. One fluid - a polyphenylether - apparently vitrified at high pressures to a limiting viscosity of 10$^{6}$ Pa s. Ball bounce is shown to limit the range of fluids that can be tested with this apparatus.