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Used oil analysis laboratories have long relied on viscosity measurement as a key parameter in determining the condition of in-service engine oil.
However, the methods used to determine viscosity often vary in laboratories due to the use of a wide array of instruments and measurement techniques. There is limited standardization between commercial oil analysis laboratories on how this key parameter is measured.
In contrast, the lube oil manufacturing industry relies on a standardized test methodology for determining the viscosity of engine oil throughout the formulary design, manufacturing and distribution processes.
This methodology has predominately been ASTM D445 - one of the oldest and most widely used test methods in all of ASTM D02. For more than 70 years, users have relied on D445 methodology and instrumentation to provide accurate results for their fluids’ viscosity.
Lube manufacturers have discovered that kinematic methods offer several advantages, including precision. Coupled with a high-precision thermostatically controlled bath and a timing device with an accuracy better than +/-0.001 second, kinematic viscometers are typically ten times more precise than other viscosity measuring devices such as dynamic rotational and differential pressure instruments.
A second advantage is intrinsic to the measurement method. Kinematic viscosity relies on gravity and the density of the fluid to provide the motivational force for movement of the liquid in the viscometer.
Consequently, kinematic viscometers test the liquid under a low and relatively narrow band of shear rates. Shear rate is an important instrument parameter when comparing results between two or more laboratories using different instruments. Correlation errors can result when comparing measurements of non-Newtonian fluids measured with other techniques where shear rates may be orders of magnitude higher than kinematic viscometers and perhaps even variable across the instrument’s measuring range.
While lubricant base oils are typically regarded as being Newtonian fluids, when formulated into today’s engine oils with additives and VI improvers, they can become non-Newtonian fluids. In-service oils with soot and other contaminates provide less predictable viscosities under variable or undefined shear rates.
Kinematic methods also offer an economic advantage. Within the last several years, there have been new advancements in the manufacture of automated kinematic viscometers that meet D445 test requirements.
These new instruments have a lower cost than earlier models, require less bench space and can provide measurement cycle times of three to five minutes. These attributes lead to a reduced cost per test while providing higher accuracy and better correlation of a standardized test.
In the past, operating cost pressures drove many laboratories to seek the quickest test cycle time and the lowest cost of instrument ownership and individual test measurement. The labor intensive manual kinematic viscometer procedure or the high capital expense associated with automated kinematic viscometers forced labs to look for alternative instruments or even modified D445 instruments.
Additionally, little guidance was provided by traditional kinematic viscosity methods such as D445 on how to properly test in-service lubricants and limited industry support for developing improved standardized methods for these fluids.
Fortunately, these issues are being resolved due to the establishment of the ASTM Committee D02.96 to standardize in-service lubricant testing and condition monitoring services.
This committee’s charter includes, in part, evaluating viscosity testing and making recommendations for revisions to existing test methods to cover the scope of testing in-service lubricants. Some of their work has culminated in the development of new kinematic viscosity test methods (Houillon viscosity) and the recent decision to add precision for in-service engine lubricants to D445.
At the June D02 meeting in Toronto, Ontario, Subcommittee 7 Section A held a meeting in which new provisions to the D445 method were discussed and approved, including more detailed provisions in Section 6.1.2 for the use of automated instruments.
Because the current D445 precision statement was derived using materials with older formulations, the subcommittee is planning a new round-robin study of precision and bias for manual and automated instruments measuring newer petroleum-based products including used engine oils.