Factors Impacting Precision of Viscosity Measurements at Low Temperature and Low Shear Stress

There are two distinct sources of variation when making viscosity measurements of lubricating oils at low temperature and low shear stresses -- the variations attributable to the instrument and the variations from the materials being measured. Oil variation arises primarily from the ‘crystal forming’ characteristics of the fluids tested at low shear stress. In this study, the viscosities of several fluids were measured at low temperature and low shear stress using the Mini-Rotary Viscometer (MRV), which contains nine separate viscometer cells that are cooled and controlled in unison. The testing procedure was to measure the viscosities of nine samples of each oil in a single test. Data were analyzed to compare the variances and standard deviations of the normalized viscosities. The results showed that the precision of the test on PAO-type oils was better than that for fully-formulated commercial oils. Before the variation of the fluids could be evaluated, the variation of the MRV was thoroughly investigated and several improvements to the instrument were made. This study suggests that fill volume, resistance to movement, heat leakage and calibration variation all contribute to precision but when these problems are corrected, there still remains a small contribution of the material characteristics to the precision of the measurements.