ASTM D7844 Standard Test Method for Condition Monitoring of Soot in In-Service Lubricants by Trend Analysis using Fourier Transform Infrared (FT-IR) Spectrometry
10. Procedure and Calculation
10.1 Collect a background spectrum according to the procedure specified in Standard Practice D7418.
10.2 Collect the spectrum of a reference oil sample according to the procedure specified in Standard Practice D7418.
10.3 Collect the spectrum of an in-service oil sample according to the procedure specified in Standard Practice D7418.
10.4 Data Processing - All data are processed to give direct trend analysis and differential trend analysis spectra normalized to 0.100 mm according to the procedure specified in Standard Practice D7418.
10.5 Calculation of Soot Value - Soot has no specific frequency of absorption in the infrared spectrum, but causes a baseline shift and tilt in the spectrum due to light scattering. As such, soot is calculated from the normalized oil sample spectrum by directly measuring the absorbance intensity at 2000 cm(-1) using no baseline. For differential trending, soot is directly measured from the difference spectrum also using the absorbance intensity at 2000 cm(-1). Fig. 1 illustrates the effect that soot (increasing from 1 to 5) has on the baseline of the spectra of diesel crankcase oils in-service and the measurement frequency used to monitor soot.
10.6 Reporting:
10.6.1 Procedure A (Direct Trend Analysis) - Values are reported in units of 100*absorbance/0.100 mm; e.g., #4 in Fig. 1, the soot value equals 78.
10.6.2 Procedure B (Differential Trend Analysis) - Values are reported in units of 100*absorbance/0.100 mm (or equivalently absorbance units per centimeter).
10.7 High Soot Values - In samples where the value for soot is >150 Abs./0.1 mm, which corresponds approximately to a soot content of >3%, then it is necessary to dilute the oil sample with the new oil or mineral spirits prior to analysis to obtain reliable results. The measured value obtained from the diluted oil sample spectrum is then multiplied by the dilution factor to give the soot value for the original sample. Tests in which oils having soot contents of 2.9% and 10.8% were diluted in odorless mineral spirits (OMS) at a level of 0.5 g oil/16 mL OMS and their FTIR spectra were recorded at 15-min intervals showed no settling of soot over a period of 450 min.
10.8 Sample Carryover - To ensure the minimum amount of sample-to-sample cross-contamination or carryover, either a minimum volume of the subsequent sample or a solvent rinse is used to flush out the previous sample. The efficacy of the flushing protocol may be assessed by consecutively analyzing an oil having a low (or zero) soot level (L1, e.g., a fresh oil) and an in-service oil sample having a high soot level (H1) followed by a second run of the oil sample having a low soot level (L2) and then calculating the percent carryover (PC) as follows: PC = [(L2 - L1)/H1] x 100, where L1, H1, and L2 are the values measured for soot (for calculating values for soot, see 10.5) for the samples run in the indicated sequence. The calculated PC should be <5%.
