ASTM E2412 Condition Monitoring of In-Service Lubricants by Trend Analysis
ASTM E2412 Standard Practice for Condition Monitoring of In-Service Lubricants by Trend Analysis Using Fourier Transform Infrared (FT-IR) Spectrometry
9. Procedures, Calculation, and Reporting
9.1 Sample Introduction - A representative sample is introduced into the infrared transmission cell, either manually or by an automatic pumping system. Autosamplers that hold a variety of oil sample container sizes are available from several manufacturers.

9.2 Sample Integrity Check - To ensure accurate and consistent results, the infrared spectrum of the sample should be checked to verify that the cell is completely filled and that air bubbles passing through the cell during data collection are not affecting the results. Multiple, automatic, computerized interpretation methods exist for this procedure. A sample integrity check based on measurement of the absorbance intensity over the wavenumber range from 3000 to 1090 cm(-1) is suitable for multiple lubricant types. The exact absorbance intensity will depend on the spectral resolution and the pathlength of the cell being used. The manufacturer's suggestions and recommendations should be considered.
9.2.1 Petroleum based lubricants have their maximum absorbance in the 3000 to 2800 cm(-1) range (or transmittance value close to 0 %T).

9.2.2 Ester based lubricants have their maximum absorbance in the 1390 to 1090 cm(-1) range (or transmittance value close to 0 %T).

9.3 Sample System Cleaning and Checks - To ensure the minimum amount of sample cross-contamination or sample carry-over, either a minimum volume of the next sample can be flushed, or a volatile solvent can be flushed through the cell and the cell dried. If the cell is dried, the amount of absorbance from either the previous sample or residual wash solvent in the sample cell can be checked. This check is performed by the same spectral analysis operation as described above. The maximum absorbance intensity should be below a preset threshold in the monitoring region (that is, CH stretch in petroleum based fluids). For most petroleum and synthetic lubricants and wash solvents, this intensity will be less than 0.2 absorbance units. The optimal threshold will depend upon the specific system configuration, in that some systems are designed to "push-out" the residual oil sample and wash solvent with the next sample. The manufacturer's suggestions and recommendations should be considered.

9.4 Data Processing - All spectra will be processed in units of absorbance as a function of wavenumber. Calculated data must be corrected to the reference pathlength of 0.100 mm prior to reporting to account for cell pathlength variation that will be seen in commercially available cells. Any other spectral data treatment should occur prior to calculating results from the spectrum.
9.4.1 Spectral data processing results can be trended directly from the in-service oil spectrum (direct trending). The only spectral data treatment is the correction of the spectrum or results to the 0.100 mm reference pathlength and the application of fringe reduction algorithms to the spectrum, if required.

9.4.2 Spectral data processing results can also be obtained by spectral subtraction processing, which requires a reference spectrum (spectral subtraction). Where spectral subtraction is used, processing of results is done from the difference spectrum that is generated by subtracting the appropriate new oil reference spectrum from the spectrum of the in-service oil sample. The in-service oil spectrum and new oil reference spectrum must both be corrected to the reference pathlength of 0.100 mm prior to subtraction and a 1:1 subtraction factor used. The subtracted spectral results can be trended over time and treated in a manner similar to those collected using the direct infrared trending method. The most commonly used reference is a sample of new oil. If possible, the new oil should be from the same lot and drum as the in-service oil. An alternate approach that might yield a more representative reference would be to take a sample of oil one hour after the oil has reached operating temperatures.

9.4.3 Post-analysis data treatment can use simple multipliers and other scaling techniques; for example, "value x 100" at the request of maintenance personnel for ease in evaluation and presentation (see Annex A1).

9.5 Spectral Analysis of Sample Data - Selected spectral regions containing information relevant to condition monitoring are measured and reported. The regions analyzed are specific to different lubricating fluid types. New oil sample parameters can be used as the point from which to trend when initially implementing an analysis process for a lubricant type. Statistical analysis shown in the annexes also provides examples. Details of the spectral analysis process can be found in the annexes to this Practice.