PUB - High Pressure Rheology – On the Statistics of Falling Body Viscometers

11/09/2018
 

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. With broader operation points of fluid power systems the viscosity of fluids at high-pressure needs to be known. To meet this demand and to fine-tune a system or design a new component in an efficient way, holistic fluid data is needed. Especially assuming viscosity to be constant over temperature and pressure hardly meets the necessary standards. With an increase in operation conditions, mostly pressure-levels, an expanded set of fluid data is required.

In this paper, a close look is taken on potential errors of falling body viscometers. Eccentricity, tumbling and other factors are all 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 as of yet. Therefore, using the viscometer at IFAS, a statistical falling time evaluation is carried out. The results are then compared to the analytical solution using water as a reference fluid. Based on the results, suggestions are made in order to minimize the measurement error caused through tumbling and other relevant factors.

 
  Falling time scatter for 426 repetitions using the falling body with a spherical end Falling time scatter for 426 repetitions using the falling body with a spherical end   Distribution of normalised falling times comparing falling bodies “Bullet” (red) with “Spike” (blue) Distribution of normalised falling times comparing falling bodies “Bullet” (red) with “Spike” (blue)