ASTM D7418 Standard Practice for Set-Up and Operation of Fourier Transform Infrared (FT-IR) Spectrometers for In-Service Oil Condition Monitoring
10. Reporting of Spectral Data
10.1 All spectra must be processed in units of absorbance as a function of wavenumber.
10.2 Pathlength Normalization - The reporting units of the various in-service oil condition monitoring parameter test methods are based on a pathlength of 0.100 mm (see the respective test methods). Accordingly, all data must be normalized to a pathlength of 0.100 mm to account for pathlength variations, either by multiplying all data points in the absorption spectra by a pathlength correction factor (spectral normalization) or by multiplying the results from the respective test methods by a pathlength correction factor (see 5.2.2). Various instrument manufacturers provide a normalization procedure in their software. The manual procedure is described in 10.2.1.
10.2.1 Manual Procedure for Pathlength Normalization - The pathlength of the cell is calculated according to Appendix X2, and the pathlength correction factor is calculated as follows:
Pathlength correction factor = 0.100 mm/Actual pathlength
To perform spectral normalization, all the data points of the absorption spectrum are multiplied by the pathlength correction factor. Most software packages for FT-IR instruments include spectral multiplication as one of their spectral data manipulation functions.
10.3 Reporting for Direct Trend Analysis - Absorption spectra over the range of 4000 to 550 cm(-1) are stored to disk and used for measuring various in-service oil condition monitoring parameters (see the respective test methods). If spectral normalization has not been performed, the pathlength correction factor (see 10.2) for each spectrum must be reported; most software packages for FT-IR instruments provide a spectrum title or comments box in which this value may be entered and stored with the spectral data.
10.4 Reporting for Differential Trend Analysis - The normalized absorption spectrum of the reference oil is subtracted from the normalized absorption spectrum of the in-service oil sample using a 1:1 subtraction factor to generate the differential spectrum of the in-service oil sample. Most software packages for FT-IR instruments include spectral subtraction as one of their spectral data manipulation functions. Differential spectra over the range of 4000 to 550 cm(-1) are stored to disk and used for measuring various in-service oil condition monitoring parameters (see the respective test methods). If spectral normalization has not been performed, the pathlength correction factor (see 10.2) for each spectrum must be reported; most software packages for FT-IR instruments provide a spectrum title or comments box in which this value may be entered and stored with the spectral data.
11. Instrument Performance Checks
11.1 Periodically, the performance of the FT-IR instrument should be monitored using the Level 0 procedure of Practice E1421. If significant change in performance is noted, then testing should be suspended until the cause of the performance change is diagnosed and corrected.
11.2 Alternative instrument performance tests conforming to the recommendations of Guide E1866 may be substituted for the Practice E1421 test.
12. Sample Spectral Noise Test
12.1 The sample spectral noise test is designed to ensure that the noise level in the sample spectrum, including the amplitude of the fringe pattern that may be superimposed on the spectrum, especially when using ZnSe cells or when ratioing against an empty cell background, is acceptable.
12.2 This test shall be carried out periodically on the instrument that is in operation. The signal-to-noise ratio, S/N, should be acceptable for the desired precision.
12.3 To calculate the sample spectrum noise level, fill the cell with a check fluid. Acceptable check fluids would be one that has minimal to no absorbance peaks within the calculation region of 2000 to 1900 cm(-1). Suggested fluids would include hexane, heptane or squalane.
12.4 Background and sample single beam spectra are both measured using the sample cell filled with this check fluid. Divide the sample spectrum by the background spectrum to generate a transmittance spectrum.
12.5 The noise level is calculated by measuring the standard deviation of the transmittance values over the frequency range of 2000 to 1900 cm(-1).
NOTE 5 - The noise level can be improved by increasing the number of scans, whereby the noise level is decreased by a factor of N(1/2) when the number of scans is increased by a factor of N. Various fringe reduction techniques outlined in Appendix X1 can be used to help reduce sample spectral "noise" caused by fringing.
12.6 Acceptable sample spectral noise levels shall be 0.0001 AU or 0.0230 %T.
13. Keywords
13.1 condition monitoring; differential trend analysis; Fourier transform infrared; FT-IR; hydrocarbon based lubricants; infrared; IR; lubricants; oils; petroleum lubricants
