Using TGA, FTIR and Fixed Infrared Analyzers to Quantify the Levels of Soot in In-Service Lubricants

Soot is an airborne contaminant and indicates the condition of the oil. Diesel engine maintenance programs carrying out regular soot checks can generate significant savings in cost by reducing used oil disposal and extending drain periods. As soot levels get higher and higher due to exhaust gas recirculation, measurement of soot in diesel engines has become an important aspect.

Thermal Gravimetric Analysis

Thermal gravimetric analysis (TGA) is a standard measurement technique used for soot analysis, but this method takes a significant amount of time and is labor intensive. A large number of oil testing laboratories are using more rapid infrared (IR) analysis. IR corresponds well with TGA, as illustrated in Table 1.

Table 1. TGA vs IR

Engine Hours TGA IR
0 0.13 0.1
50 1.76 1.7
100 3.13 3.2
150 4.48 4.5
175 5.16 5.0
200 5.80 5.8
225 6.38 6.1
0 0.09 0.0
24 0.83 0.8
48 1.40 1.4
72 1.98 1.9
96 2.52 2.5
120 3.05 3.0
144 3.58 3.5

FTIR vs. InfraCal Soot Meter

Although a lot of people think that FTIR spectrometers are used for performing IR measurements, fixed filter infrared analyzers used for customary tests are not only less costly, but also easier to use, making them more suitable for repetitive analyses.

InfraCal soot meter

Figure 1. InfraCal soot meter

The TGA/IR comparison shown in Table 1 was performed on the InfraCal soot meter, a single wavelength fixed filter analyzer (Figure 1). Comparative data for a fixed filter IR and FTIR analyzer is shown in Table 2. These comparisons reveal that both filter-based IR and FTIR analyzers can substitute TGA for determining soot levels.

Table 2. FTIR vs InfraCal soot meter

Sample FT-IR Soot Meter
A1 1.46 1.4
B1 1.59 1.6
B2 1.82 1.8
B3 1.94 1.9
B4 2.56 2.5
A2 2.74 2.6
B5 2.68 2.7
B6 2.81 2.8
B7 2.82 2.6

With complete access to a full spectrum, an FTIR spectrometer has the ability to determine several different oil parameters besides soot. For testing laboratories conducting over 1,000 soot tests a week or for remote and mobile laboratories, it would be beneficial to reserve the more costly system for unique tests and allow a non-technical operator to utilize the single wavelength analyzer for repeated soot measurements.

Advantages of InfraCal Infrared Analyzer

When compared to an FTIR, the InfraCal single wavelength infrared analyzer provides the following benefits for measuring soot in diesel engine oil:

  • Measures up to 12-15% soot without dilution: The InfraCal system can determine up to 12-15% soot without dilution, while an FTIR with a transmission sampling stage will calculate a maximum soot concentration of only 5%. In addition, samples beyond the saturation point need to be diluted to be within that measurement range
  • Rugged construction: Filter-based IR analyzers have no moving parts and come with a battery pack option, thus making them suitable for field-based operations such as high volumes of samples in a lab and heavy duty off road vehicles on mining and construction sites
  • Small foot print: Whether in a crowded testing lab or an on-site mobile lab, the InfraCal soot meter has a small foot print and measures only 6” x 6” in dimension
  • No cleaning solvents required and cells to clean: Samples can be used directly on a cubic zirconia ATR (attenuated total reflection) surface that is scratch-resistant and the same can be cleaned with a paper towel. In contrast, FTIR transmission cells need to be carefully cleaned with solvents
  • Compatible with ASTM D7686 test method: Standard test methods for field-based condition monitoring of soot in in-service lubricants utilizing a fixed-filter IR system

With the rising usage of biodiesel, there have been concerns over whether dilution of biodiesel fuel would affect soot measurements. Results obtained from a recent study for the ASTM D7686 Method showed that there is no interference from biodiesel or diesel dilution through IR analysis (Table 3).

Table 3. ASTM D7686 fuel dilution study

Intial Soot Concentration Biodiesel Concentration Diesel Concentration Soot concentration with dilution InfraCal Soot Meter
7.7% 0.0% 0.0% 7.7% 7.8%
7.7% 5.0% 0.0% 7.4% 7.3%
7.7% 15.0% 0.0% 6.6% 6.7%
7.7% 0.0% 5.0% 7.4% 7.3%
7.7% 0.0% 15.0% 6.6% 6.6%


When compared to the TGA method, infrared analysis provides an easier and faster means of measuring soot levels in used oil. Filter-based IR analyzers, such as the InfraCal soot meter, provide a cost-effective way to perform field capable spectroscopic assessments of the working condition of lubricating oils in different types of diesel engines. In addition, the analysis takes less than 30 seconds. Filter-based IR analyzers can be used by non-technical operators with little or no IR spectroscopy knowledge.

This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.

For more information on this source, please visit AMETEK Spectro Scientific.



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