TESTING AND MONITORING
Kinematic viscosity – The key to lubricant analysis
Thomas Feischl, MEng, Director of Business Development, eralytics GmbH
When talking about lubricants, independent of the origin or the technical purpose of use, viscosity is considered the most important physical property. Viscosity is the measure of internal friction of a liquid or in other words, the resistance to flow at a specific temperature. For technical reasons, but also during the purchasing process viscosity classes play a significant role. Industrial lubricants for instance are identified by their ISO viscosity grades. An ISO VG 46 has a nominal kinematic viscosity of 46 mm²/s at 40°C with a typical tolerance of +-10%. When an engineer speaks of an “SN 100”, this means a mineral base oil with a kinematic viscosity of 100 mm²/s at 40°C. The specification of the kinematic viscosity is therefore expected by professionals in the industry.
“Differential pressure capillary method” – The new high potential method
Because viscosity is such a fundamental and widely used property, there are numerous different viscosity measurement devices available on the market. Most of the devices use the so-called glass-capillary method which is considered to be the classic approach according to the standard ASTM D445.
Using a similar principle, Z-shaped capillaries are also often used. These are also known as “Houillon”
viscometers according to ASTM D7279. The fundamental measure is actually the time it takes for a specified volume to flow out of the capillary pulled by the gravimetry. Therefore, kinematic viscosity is derived because the resulting viscosity is also influenced by the mass of the sample.
A different approach is using a rotational measurement principle leading at first to the dynamic viscosity. This measurement principle is named “the Stabinger principle” after his inventor and works according to the method ASTM D7042. Due to an attached density tube the sample density is also determined at the measurement temperature which is the linking factor between dynamic and kinematic viscosity.
Recently, a newly developed technique was introduced, that also contains a capillary, but the driving force is a small differential pressure which pushes the sample out. This innovative new method has been made possible because of the precise and high-resolution determination of the pressure drop over time and can best be summariwed as “differential pressure capillary method”. This physical principle results in both the kinematic and the dynamic viscosity. In extension to the viscosities an independent density cell is connected in parallel.
Continued on page 19 LUBE MAGAZINE NO.181 JUNE 2024 17
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