High Pressure Rheology – On the Statistics of Falling Body Viscometers

Viscosity measurements in fluid power are a standard tool to gain information on the fluid and its condition. The results are required for system design and simulation. Within fluid power systems, high pressures are common in piston-bushing contacts or lubricating gaps for example. A popular approach to high-pressure measurements is falling body viscometry. The terminal velocity of a falling body inside a fluid-filled tube is used to determine the fluids viscosity. However, falling body viscometry is prone to inaccuracies which raises questions regarding the reliability of the measurement principle and the measurement itself. Eccentricity, tumbling and material pairings above all other factors are responsible for deviations regarding terminal velocity of the falling body. Studies do indicate highly varying falling times at constant conditions, but a significant statistical analysis has not been done yet. Therefore, using the viscometer at IFAS, a statistical falling time evaluation is carried out and different falling body designs are compared to each other. Afterwards, the most advantageous geometry is identified.