Apparent Viscosity and Yield Stress (MRV)

Description

A critical test (MRV) for motor oils, determining the oil pump's ability to pump thickened oil in severe frosts without air binding.

A cold engine start is a challenge, but a true disaster strikes when the oil is so thick that the oil pump pumps air instead of fluid (air-binding). Apparent viscosity and yield stress analysis on a Mini-Rotary Viscometer (MRV) at the RENA laboratory guarantees that your motor oil will seamlessly reach all friction nodes even at -40 °C. This test unmistakably identifies defective batches of oil in which polymeric thickeners or paraffins form a rigid gel structure during prolonged cooling. The test according to the international ASTM D4684 (MRV) standard simulates the real conditions of a car cooling down outside during a freezing night. An oil sample is placed in the cell of the CMRV-5000 instrument and undergoes an extremely slow and strictly controlled cooling cycle lasting over 45 hours. The temperature gradually drops to the target sub-zero mark (e.g., -35 °C for 0W oils). Such slow cooling gives paraffin crystals time to form a dangerous spatial lattice (gel). After the cooling cycle is completed, a small static load (yield stress) is automatically applied to the viscometer rotor. If the oil has turned into a solid gel, the rotor will not move. If the rotor begins to rotate, the instrument applies a larger load and measures the apparent viscosity (pumping resistance) during rotation. The microprocessor calculates the value in millipascal-seconds (mPa·s).