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A low
torque is applied to the rotor shaft to measure yield stress
and a high torque is applied to determine apparent viscosity
of the oil. |
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The
rotation is achieved by applied load acting through a string
wound around the rotor shaft. |
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Sooted oil MRV evaluation equipment at
SwRI |
The mini-rotary
viscometer (MRV) laboratory test method offered by Southwest Research Institute (SwRI) has predicted field failures
caused by lack of pumpability. These documented field failures have all
consisted of oils tested at -25°C and are believed to be the result of the oil
forming a gel structure that results in excessive yield stress or viscosity of
the engine oil, or both.
In the mini-rotary
viscometer laboratory test, an engine oil is slowly cooled through a
temperature range where wax crystallization is known to occur, followed
by a relatively rapid cooling to the final test temperature, as
determined by the SAE viscosity grade of the oil.
Excerpts taken from page 30, ASTM Guide 44 Analysis of Petroleum
Products and Lubricants edited by R.A. Kishore Nadkarni
MRV Test
Method/Summary
The mini-rotary viscometer test method covers the
measurement of yield stress and viscosity of engine oil after cooling at
controlled rates over a period exceeding 45 hours to a final test
temperature between -10 and -40°C. The viscosity measurements are made
at a shear stress of 525 Pa over a shear rate of 0.4 to 15 s-1. Taken
from ASTM D 4684.
MRV Test
Parameters
An engine oil sample is held at 80°C and then cooled
at a program-med cooling rate to a final test temperature. A low torque
is applied to the rotor shaft to measure the yield stress. A higher
torque is then applied to determine the apparent viscosity of the
sample. Taken from ASTM D 4684.
MRV Apparatus/Test Fixtures
A mini-rotary viscometer apparatus that consists of
one or more viscometric cells in a temperature-controlled aluminum block
is used. Each cell contains a calibrated rotor-stator set. Rotation of
the rotor is achieved by an applied load acting through a string wound
around the rotor shaft. Taken from ASTM D 4684.
Report
Test temperature and viscosity (cP) or yield stress,
but not both.
Pass / Fail
Criteria
Pass/fail criteria are established by SAE J300, API
CI-4, and ACEA European Oil Sequences.
*Association des Constructeurs Européens d'Automobiles
/ European Automobile Manufacturers Association. For more information about
mini-rotary viscometer (MRV) evaluation, or how you can contract with SwRI,
please contact
Becky
Grinfield at
bgrinfield@swri.org or (210) 522-3652.
mrv.swri.org
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