Understanding Oil Analysis: Kinematic Viscosity Testing

In our recent post on oil analysis, we discussed how viscosity is considered one of the most important properties of a lubricant and how subtle changes in lubricant thickness can signal substantial maintenance issues.

This week, we’ll take a closer look at how an effective used oil analysis program monitors viscosity and the potential trouble signs to be aware of when analyzing results.

Kinematic viscosity testing is traditionally conducted by measuring the time needed for a fixed volume of lubricant to flow under gravity through a viscometer when heated at a closely controlled temperature – typically 40°C (104°F) or 100°C (212°F).

At these temperatures, a lubricant’s speed of descent is measured in centistokes (cSt) in line with international standards.  The lower the viscosity, the lower the measurement in cSt. For example, water has a viscosity of one cSt, compared to Mobil SHC 634, which has a typical viscosity of 460 cSt.  These viscosity measurements are then used to classify fresh oils into different viscosity grades, per ISO standards.  For example, an ISO VG 32 oil has a typical fresh oil viscosity around 32 cSt, while an ISO VG 680 fluid has a typical fresh oil viscosity around 680 cSt.

As extreme operating temperatures can affect viscosity to a level that lubricants are no longer able to provide adequate equipment projection, monitoring viscosity changes through a used oil analysis program can help maintenance personnel minimize equipment downtime, reduce costs and enhance safety.

Change in viscosity from typical fresh oil levels may indicate that corrective action should be taken.  The amount of change that is acceptable prior to corrective action depends on the product formulation, the application, history of the equipment, among other factors.  Many lubricant manufacturers set limits within their used oil analysis programs regarding the amount of viscosity change that should trigger corrective action.

A decrease in viscosity may indicate that oil films are not sufficient to protect equipment from wear damage.  Reduced viscosity can be caused by a number of different factors, such as:

  •  Mixing with a lighter grade oil;
  •  Shear down of multi-graded oils;
  •  Fuel dilution as a result of incomplete combustion;

An increase in viscosity may result in reduced performance from the oil, for example poor cold start pumpability or poor demulsibility.  Increased viscosity can be caused by a number of factors, including:

  •  Oxidation or nitration;
  •  Mixing with heavier grade oil;
  •  Soot loading (in mobile equipment); and
  •  Oil Contamination

Clearly, viscosity is one of the most important properties of a lubricant, but it’s just one area that is monitored in a comprehensive oil analysis program. Next week we’ll continue our discussion on the other key areas of oil analysis by examining additive elements, wear metals and contaminants testing.