9. Calibration and Standardization
9.1 Calculation of the Cell Pathlength - Use a cell with a known pathlength ofapproximately 0.025 to 0.1 mm. Calibrate the infrared cell pathlength using the interference fringe method:
9.1.1 Acquire the single beam background infrared spectrum. Using the empty infrared cell in the infrared spectrometer sample compartment, acquire the cell single beam infrared spectrum. Calculate the transmittance spectrum by dividing the cell single beam spectrum by the background single beam spectrum. Optionally, convert the transmittance spectrum to an absorbance spectrum by taking the negative logarithm (base 10) of the transmittance spectrum. The fringe calculation may be done on either the transmittance or absorbance. spectrum. The final spectrum is obtained by subtraction of the background spectrum from the cell spectrum.
NOTE 1 - This computation is generally an integral part of the infrared spectrometer software.
9.1.2 Choose 2 minima separated by about 20 measurable interference fringes as shown in Fig. 1. Count the number of interference fringes between the lower and the higher wave-numbers, referred to as λ1 and λ2.
NOTE 2 - The spectral range may be chosen freely in an area where the fringes are regular.
9.1.3 The cell pathlength is calculated by the formula:
where:
e = the pathlength in mm, and
n = the number of fringes between λ1 and λ2.
9.2 Instrument Performance Checks:
9.2.1 Periodically, the performance of the FT-IR instrument should be monitored using the Level 0 procedure of Practice E 1421. If significant change in performance is noted, then testing should be suspended until the cause of the performance change is diagnosed and corrected.
9.2.2 Alternative instrument performance tests conforming to the recommendations of Guide E 1866 may be substituted for the Practice E 1421 test.
10. Conditioning
10.1 Before using the infrared cell ensure that it is clean by washing through with a suitable solvent, for example, heptane. Dry the cell using dry air or nitrogen, if necessary. Calibrate this cell as described in Section 9.
11. Preparation of Sample of Used Oil
11.1 Refer to Practice D4057 (Manual Sampling) or Practice D4177 (Automatic Sampling) for proper sampling techniques.
11.2 When sampling used lubricants, the specimen shall be representative of the system sampled and shall be free of contamination from external sources. As used oil can change appreciably in storage, test samples as soon as possible after removal from the lubricating system and note the dates of sampling and testing.
11.3 If the sample of used oil contains visible sediment, heat to 60 +/- 5°C in the original container and agitate until all of the sediment is homogeneously suspended in the oil. If the original container is a can or if it is glass and more than three-fourths full, transfer the entire sample to a clear-glass bottle having a capacity at least one third greater than the volume of the sample. Transfer all traces of sediment from the original container to the bottle by vigorous agitation of portions of the sample in the original container.