Understanding Soot Test Methods, Part One
By Dave Wooton

This week's newsletter comes from a great friend, Dave Wooton of Wooton Consulting. Thanks for the great input Dave and we look forward into hearing more from you, next week.

Where Does Soot Originate?
Soot always has been a monitoring property of used oil for diesel applications. The source of soot for diesel applications is the combustion process itself. The higher the soot generation, the more possibility that some of this material will get into the lubricant. This lubricant is designed with detergent and dispersants to suspend the soot and not allow it to collect around the engine on important moving parts or in oil journals. However, as long as it is entering into the lubricant; it is a very important parameter to measure in our used oil analysis programs.

Now Comes EGR Engines
With the advent of recent EGR engines, this importance is ever increasing. In the '80's soot levels were typically 1-3%. The EGR engines have demonstrated soot levels that can easily reach 6-9% and in some cases greater then 10%. Soot has been shown to cause wear and filter plugging.

If soot is present in the lubricant, it can replace the lubricant to prevent hydrodynamic boundary lubrication or block the lubricant from getting to the lubrication-site. Soot can also "use-up" the dispersant or detergent, not allowing these important additives to control the production of sludge by-products. Soot has also been shown to have some absorption properties for polar additives, like ZDDP and friction modifiers. Potential problems like these are all good reasons to put soot as an important condemnation criterion.

What Test Methods Are Used To Measure Soot?
With this importance in mind, explanation of the two major test methods for measuring soot is a must. In most used oil programs soot is measured by techniques called either TGA-Soot or FT-IR Soot. These two methods were developed in the early '80's for different applications. TGA-Soot was developed for precise laboratory determinations, while FT-IR soot was developed for field test applications and trending.

The TGA-Soot method (Thermal Gravimetric Analysis) operates by putting the sample on a very sensitive balance in a nitrogen atmosphere. The sample is first heated up until all the organic material (the base oil, and additives) has distilled off. The atmosphere around the sample is then changed to oxygen with the sample continuing to be heated and the soot is burned off, leaving only inorganic oxides remaining.

The weight changes on the balance during these operations allowing the analyst to determine the concentration of soot. This method relies upon the ability for the organic material to evaporate or decompose and leave the balance before the oxygen atmosphere is introduced. For the most part this is a very precise method.

For those testing labs that are heavily involved in this method, it is an ASTM-TMC (ASTM-Test Monitoring Center) controlled test. It does have one small interference one should be aware… TBN. The TBN of the lubricant comes from carbonate. At the temperature that the soot is burned, carbonate decomposes, evolving CO2. Thus, a weight loss is measured that is related to the TBN of the lubricant. This is why many have noticed a soot result for the new oil of high TBN oil. With 14 TBN oil, this accounts for around 0.1%. This isn't large, but could be a concern for some applications.

Next week Dave will tell us more about the second method of testing for soot properties... FT-IR, and which method you should monitor for your applications.

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