Understanding Soot Test Methods, Part Two
By Dave Wooton

This week's newsletter is a follow up from Dave Wooton of Wooton Consulting. In Part One, we examined the TGA-Soot method... this week we look at another method of testing for soot in used oil analysis.

Infrared Light And The 'Cell'
FT-IR soot method (Fourier-Transform Infrared Spectroscopy) is measured by putting the sample into a "cell". Infrared light is then passed through the sample in the cell. The soot content is measured by comparing the absorption (or lack of light transmission) through the "sample-cell" with respect to new oil.

In this method the content of soot is actually measured by the sample's turbidity (or light scattering) values caused by the soot. This analysis is based on a Rayleigh Scattering and Debye Equations. These equations are the relationship between a scattered light and the physical properties of the particle in the solution that caused the light scattering.

The amount of scattering is related to the concentration of soot, the size of the soot algometric-particle, wavelength of light being used and to a small part… the formulation itself. This method is not always calibrated for soot content, but often reported as abs/cm (absorbance/centimeter). Abs/cm is the amount of light that is absorbed or scattered per pathlength of the measuring cell.

Calibration Is Key
To calibrate the method, one would need to develop a calibration for each engine, running under each condition. This would be an impossible task. Thus, one often generates an average relationship that has an increased error to the actual value (there are marketed soot analyzers that do this method of analysis - some using infrared light, others with additional wavelengths). The method works very well when one is trending an engine, or a series of engines running under similar conditions.

However, this method can deteriorate if one is attempting to compare several different engines with different formulations and operating conditions. The advantage of this method is low cost and fast speed. It was developed for field-testing, where the TGA method was too costly and time-consuming.

FT-IR soot measurement has been shown to be as capable as an on-line measurement, where as TGA requires 30-45 minutes per analysis with an off-line only environment. The user of the data needs to be aware that with lower costs of analysis, increased errors are the trade-off. The FT-IR soot measurement has good repeatability, but not always good precision. Therefore, it should be reserved for trending or a monitoring experiment and not absolute soot concentration experiments.

Keep in mind that both of these soot measurements (TGA-Soot Method and FT-IR) have their respective values. It is the responsibility of the user to be aware of the advantages and disadvantages of both and use them appropriately. If one needs assistance or advice on the proper method for their application, a knowledgeable lubricant analyst should be consulted.

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