Using Oil Color as a Field Test

Tags: onsite oil analysis, oil analysis
Oil Change Color Gauge
Without gauge compare sample to a previous sample and/or new oil sample.

An oil’s color comes from the light transmitting through it. Different colors are formed depending on the concentration and type of light-absorbing groups suspended in the oil.

These "chromophoric" compounds are commonly called color bodies. With new oil the higher the viscosity the more likely naturally occurring color bodies will exist.

Also, naphthenic base oils and oils high in sulfur and aromatics are typically darker in color.

The degradation and contamination of used oils can have a marked affect on the resulting clarity and color. Coke and carbon insolubles from thermal failure (high localized temperatures) can sharply darken an oil.

The mixture of incompatible oils can cause darkening from additive floc and other reaction products that form. Contaminants such as soot, process chemicals, detergents, and entrained air can lead to a change in the color and brightness of the oil. Photo catalytic reactions (UV) from exposure to sunlight (bottle oilers, etc.) can sharply discolor oil.

How Oxidation Darkens Oil

Oxidation is yet another common cause of color bodies forming and an over-all darkening to occur. The color change is more acute in oils high in sulfur and aromatics.

There is a synergy between aromatics and sulfur compounds that appears to degrade the oil and form the color bodies during thermal oxidation reactions (see diagram).

Causes of Oil Darkening

In many cases abnormal discoloration is a reliable field indicator of an oil that is distressed. In one study, 90 percent of lubricants that were abnormally dark were also found by laboratory analysis to be non-compliant.

There are other cases, however, where a dark oil may not be a real concern. The combination of experience with the specific lubricant and routine analysis is necessary in making these determinations.