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Viscosity Lube Oil Testing

The viscosity of lube oil (VIS) is technically described as a fluid’s resistance to flow with respect to temperature. Two temperatures are routinely employed for most oil analysis, 40oC and 100oC. Occasionally a very low temperature VIS may be performed in problem solving mode, but not routinely.

Tools

VIS determinations in most labs consist of oil baths heated to one of the two referenced temperatures. A small sample of the oil is heated in a capillary tube until it reaches equilibrium with the bath temperature. The oil is then caused to flow vertically downward via gravity through a measuring volume area within the tube and timed as it flows through that area. The lubricant viscosity is calculated based on the time spent in the measuring area.


Off-Line, most labs use VIS baths that are usually semi-automated or fully automated.

On-Site testing may include a variety of flow resistance measurement techniques performed at non-standard temperatures and, while perhaps not as accurate as VIS baths and tubes, will normally yield sufficiently accurate results with good repeatability. Some of these techniques involve heating the oil to the desired standard temperature before measuring, thus achieving a higher degree of precision.

On-Line uses vibration or acoustic technology to provide an inferential VIS measurement at lubricant operating temperature that may be extrapolated to standard temperatures.


Reporting Units: The standard unit of reporting VIS @ 40°C or 100°C is the centistoke, usually abbreviated cs or cSt or cST. VIS determined at 40°C is reported as “ISO” (International Organization for Standardization) grades per table below. VIS at 40°C is usually reserved for industrial fluids (gears, hydraulics) where the test and operating temperatures are similar.

Viscosity table 1

VIS determined at 100°C is reported as “SAE” (Society of Automotive Engineers) grade per table below. VIS at 100°C is usually reserved for mobile applications such as engines, differentials and power shift transmissions, again reflective of operating temperatures.

Viscosity table 2

* Asterisks indicate a ‘w’ (Winter) designation for cold temperature specification purposes.VIS Application by Temperature:


VIS General Information:

While one frequently hears that VIS is the most important property of oils (with good reason), it can be a slightly misleading statement for in-service (used) oils as there are several reasons that a VIS might change from the lube’s fresh starting value. It is not unreasonable to have a VIS appear to be ‘normal’ when, in fact, there are two undesirable factors affecting it in two directions, e.g., a fuel diluted oil that is also loaded with fuel soot. The liquid fuel lowers the lube oil viscosity, while the soot raises it. The result may be an acceptable VIS, but two problems that need attention. Luckily those properties are routinely tested for, as well, and the riddle can be solved with proper diligence.

Here is a table of VIS effectors:

Viscosity table 3

Things to Note regarding the viscosity of lubricating oil:

  • The ISO scale has an unusual ‘feature’ (not a benefit)… there is a ‘gap’ between one grade to the next where a result meets neither grade’s specification range. Note that a result of 17.5 at 40oC meets neither the ISO 15 or ISO 22 specification. Yes, that’s rather silly, but there’s a ready workaround. Most people evaluating test data usually extend or extrapolate the bottom of the higher range and the top of lower one in equal proportion, closing the gap, thus resolving the matter in logical fashion
  • Solvents usually test like fuel. Avoid using solvents in sampling suction tubes prior to collecting a sample so as not to skew a fuel dilution determination. Use fresh, clean tubing each time

 

On & Off-line Lube Oil

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