Lab Spotlight: Viscosity Testing

It’s the most important lube quality. You can’t afford not  to regularly check the viscosity of the oils in your systems.

As noted in previous articles in this publication, oil life can be compromised by chemistry changes in a fluid (i.e., oxidation) or the introduction of contaminants (i.e., particles and water). There are several causes for such problems—including introducing or mixing incorrect/incompatible products in a system. Thus, regular oil analysis of both industrial and engine oils is critical to the health of your equipment and processes. That includes testing regularly for viscosity.

Methodology
Viscosity, defined as a fluid’s resistance to flow, is the most important property of a lubricant. The most common viscosity test is that of Kinematic Viscosity, which involves the use of a curved capillary tube, shown in Fig. 1.

The time it takes for the fluid to flow a certain distance in the tube is measured and compared to a standard that is water with a defined viscosity of one centistoke. Viscosity measurements are conducted at 40 C for industrial oils and 100 C for engine oils. The ISO grade is used to classify industrial oils. This is a range of viscosities that are +/-10% of the midpoint (which is the ISO grade). Table I details the ISO Viscosity Classification System.

Viscosity grades for engine oils are classified as SAE viscosity—which is a range of kinematic viscosities in centistokes. This is shown in Table II.

Establishing limits
Oil-analysis laboratories have specific guidelines that let you know when oil condition deteriorates. It’s important to adhere to whatever limits have been established by your chosen lab(s). In the meantime, general guidelines for the viscosity of industrial oils are as follows, with proactive values reported as “CAUTION” (which indicates a problem exists) and “WARNING” (which indicates action must be taken):

CAUTION: 
+/- 10% over initial value
WARNING:
= /- 15% over initial value

Note: Some OEMs require
+/- 10% from ISO grade

Conclusion
Again, the information here is offered only in the context of general guidelines: There is much more involved in viscosity testing. Before you make any decision regarding an oil’s condition, be sure to consult with your own oil-analysis lab(s).

What’s Next?
The July/August “Lab Spotlight” will cover Elemental Analysis. LMT

(This article is based on a compilation of articles by contributing editor Ray Thibault that have previously appeared in this publication.)