What does practice show? Even in trend analyses over several tens of thousands of operating hours, the oxidation value shows little change, although viscosity, neutralisation number, and/ or oxidation inhibitors indicate increased lubricant ageing (Example T1). Therefore, the significance of this standard, particularly for industrial lubricants, is rather limited. In determining oxidation in A/ cm at a specific point on the graph, a “band” at a wavenumber of 1710 cm -1 , the length change is calculated in cm compared to the base spectrum. In contrast, the oxidation index is based on an area calculation. The “oxidation index” is given as a dimensionless number and essentially corresponds to the increase in the oxidation area in cm 2 . The following examples demonstrate how further development of this method can lead to significantly improved results. After a brief introduction to the adjusted procedure for determining the oxidation index, the advantages of the method will be illustrated using typical examples. The first images show a typical example of a steam turbine, comparing the evaluation of the original FT-IR spectrum (fresh and used oil in transmission representation), as shown in Image 1. Science and Research 33 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0005 Oxidation Index - Determining the Ageing of Industrial Lubricants Carsten Heine* Presented at GfT Conference 2024 Every lubricant ages over its service life. For mineral oil-based lubricants, the oxidation is usually determined in accordance with DIN 51453. Alongside other analysis values, the oxidation value allows conclusions about the remaining service life of the oil or the progress of so-called oil ageing. The current version of DIN 51453 dates back to 2004, a time when neither today’s base oils nor current additive systems were available on the market. Furthermore, this method was developed for determining the ageing of engine oils. Due to a lack of alternatives, however, this method is still widely used for determining oxidation in almost all other applications, particularly for gas engine oils, hydraulic oils, and turbine oils. Keywords DIN 51453, mineral oil-based, engine oils, hydraulic, compressor, gear, oil changes Abstract * Carsten Heine OELCHECK GmbH Kerschelweg 28, 83098 Brannenburg, Germany Image 1: Representation of fresh and used oil in transmission dern base oil components) but rather analyse an area of the spectrum adjacent to the wavenumber 1710 towards the lower wavenumbers. As a result, the oxidation number of 3 A/ cm according to DIN is transformed into an oxidation index of 66 in this case. Many of the newer lubricants contain synthetic ester-based base oils. Not only do the anti-wear additives perform better in these oils, but they are also more temperature stable and age less. These synthetic components Science and Research 34 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0005 Since the original calculation of oxidation is based on the absorption representation, the next two images are also shown in this format. In Image 2, the determination of oxidation according to DIN 51453 is shown at the band of the wavenumber at 1710. Here, only the height of the peak in the difference spectrum is calculated and given as the result. In Image 3, it becomes clear that to determine the oxidation index, we do not rely on a single wavenumber (which could easily be influenced by additives or mo- Image 2: Representation of the difference spectrum in absorption (evaluation according to DIN 51453) Image 3: Representation of the difference spectrum in absorption (evaluation of the oxidation index) are advantageous, especially for oils in systems where oil changes are time-consuming and costly, or that operate at high temperatures. However, in oil analyses, these oils may cause difficulties because they oscillate around a wavenumber of 1740 and can thus influence the evaluation at a single wavenumber (1710 cm -1 ). Similarly, lubricants containing viscosity index improvers (VI-improvers) or special additives, which already show a significant peak area at 1710 cm -1 in the fresh oil, can influence the oxidation calculation. For modern lubricants, the oxidation value according to DIN 51453 is often inaccurate. For oils with ester components, determining oxidation according to DIN can be considerably challenging or even impossible. To provide a better indication of oil ageing for lubricants where DIN oxidation cannot be determined, viscosity increase, reduction in additive content, depletion of antioxidants, and the increase in AN (acid number), and in the case of engine oils, the decrease in BN (base number), have been considered. However, the new oxidation index provides a way to assess “oil ageing” more reliably and accurately via an oxidation value, which remains a focus for OEMs. The following section presents several typical examples from practice. The first example illustrates a case showing the significant consequences that can result if oxidation is not, or is unreliably, evaluated. Science and Research 35 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0005 Image 4: Example 1, Image 1 The FT-IR spectrum in Image 5 shows slight deviations in the region below (to the right of) the wavenumber 1710. These deviations are examined further. The FT-IR representation in absorption is used for calculation and visualisation. When oxidation is calculated according to DIN 51453, the result is 1.45 A/ cm, rounded by the software to 1. Using the new oxidation index method presented by OELCHECK, a result of 43 is obtained. As a dimensionless number, this requires interpretation. In the graph in Image 6, it is also evident that significant changes appear not only at the wavenumber 1710 but are more intense in the adjacent area, the basis of the new calculation method. Science and Research 36 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0005 This example concerns a steam turbine whose oil has been under our analytical supervision for an extended period. Images 4 and 5 show the current analysis, along with the trends from the last three analyses. In the data grid (Image 4), an increased copper content is noticeable, although it has remained stable across previous investigations. The sample from 2018 (second from the right) shows slightly notable values in the MPC test, foaming, and cleanliness class. Based on these values, temporary deep filtration and a partial replacement (about 10 % of the total volume) were performed, which returned the values to the previous trend level. It is notable that the oxidation value (according to DIN 51453) remains stable at a value of “1”. Image 6: Example 1, Image 3 Image 5: Example 1, Image 2 Example T1: Superheated steam turbine • DIN-Oxidation • 1.5 A/ cm • Oxidation-Index • 43 Example T1: Superheated steam turbine Example T1: Superheated steam turbine • DIN-Oxidation • 1.5 A/ cm • Oxidation-Index • 43 Example T1: Superheated steam turbine • DIN-Oxidation • 1.5 A/ cm • Oxidation-Index • 43 Example T1: Superheated steam turbine • DIN-Oxidation • 1.5 A/ cm • Oxidation-Index • 43 Example T1: Superheated steam turbine All FT-IR spectra of previous samples were recalculated using this method to examine changes over the investigation period. Image 7 clearly shows a surprising trend, indicating continuous oil changes over the entire monitored period. Example 2 In the second example (Images 8 and 9), the sample of an “initial analysis” of a gas turbine is shown. Other than the oil volume (approximately 20,000 litres), the only Science and Research 37 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0005 Image 7: Example 1, Image 4 Date Lab. No. Oxi DIN [A/ cm] Oxi-Index Ao ph [%] 06.02.2019 3926593 1 43 48.5 21.06.2018 3741784 1 35 56.4 27.12.2016 3274221 1 35 61.1 29.01.2016 3063875 1 27 64.9 24.02.2015 2833648 1 35 68.9 30.01.2013 2364102 1 34 67.6 31.01.2012 2141396 1 22 68.2 28.07.2010 1361871 1 18 81.5 Example T1: Superheated steam turbine Trend behaviour Date Lab. No. Oxi DIN [A/ cm] Oxi-Index Ao ph [%] 06.02.2019 3926593 1 43 48.5 21.06.2018 3741784 1 35 56.4 27.12.2016 3274221 1 35 61.1 29.01.2016 3063875 1 27 64.9 24.02.2015 2833648 1 35 68.9 30.01.2013 2364102 1 34 67.6 31.01.2012 2141396 1 22 68.2 28.07.2010 1361871 1 18 81.5 Example T1: Superheated steam turbine Trend behaviour Image 8: Example 2, Image 1 Example T2: Gasturbine Oi l: Turbine Oil ISO VG 46, 20.000 Litres Example T2: Gasturbine Oi l: Turbine Oil ISO VG 46, 20.000 Litres Date Lab. No. Oxi DIN [A/ cm] Oxi-Index Ao ph [%] 06.02.2019 3926593 1 43 48.5 21.06.2018 3741784 1 35 56.4 27.12.2016 3274221 1 35 61.1 29.01.2016 3063875 1 27 64.9 24.02.2015 2833648 1 35 68.9 30.01.2013 2364102 1 34 67.6 31.01.2012 2141396 1 22 68.2 28.07.2010 1361871 1 18 81.5 Example T1: Superheated steam turbine Trend behaviour Example T2: Gasturbine Oi l: Turbine Oil ISO VG 46, 20.000 Litres Example 3 In the third example, samples are shown from a steam turbine that, unlike the first two examples, is operated with a slightly EP-additive turbine oil. This case also serves to check whether the oxidation index provides similarly good results for more heavily formulated oils. The investigated sample comes from a steam turbine where there was a suspicion that the oil, which had been in use for around 30,000 hours, might already be showing signs of oxidation. The slightly more extensive standard analysis (Image 10) already provided initial indications of oil changes, with a significantly worsened water separation capability, deteriorating foam behaviour, and a reduction in amine antioxidants in the RULER test. The oxidation value (according to DIN 51453) was also somewhat elevated, with a value of “3”. However, with such small values, interpretation can be challenging. The FT-IR spectrum (Image 11) also clearly shows changes in the area to the right of the wavenumber 1710 A/ cm. In Image 12, it can be seen that, in this case, the wavenumber 1710 A/ cm lies directly at the centre of the IR changes, already indicating that it is pointing in the right direction. Science and Research 38 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0005 information available was that the turbine oil was ISO VG 46. No details regarding the oil service life or the total operating time of the system were provided. The oxidation value of “2” without any reference to oil service life is initially not very informative. However, the FT-IR spectrum (Image 9) reveals not only slight changes in the area around the wavenumber 1710 A/ cm, but also significant changes compared to fresh oil in relation to the phenolic antioxidants (wavenumber range at around 3,600 A/ cm). The RULER test was not requested for this sample, so no statement could be made in this regard. However, the calculation of the degradation of phenolic antioxidants via the FT-IR spectrum was very clear. Only about 16 % of the phenolic AO content remained when compared to the fresh oil. Additionally, the oxidation index calculated for this sample also produced a significant result. The oxidation index had risen to a value of 43. The recommendation in this case was to also examine the deposit formation tendency via the MPC test and to derive further measures from this if necessary. During the tank inspection, noticeable deposits were found on the tank walls at the usual filling level. Due to the unclear data situation and a scheduled shutdown, no further investigations were commissioned, and an oil change, along with a complete system cleaning, was carried out after another two months. Image 9: Example 2, Image 2 Example T2: Gasturbine Oi l: Turbine Oil ISO VG 46, 20.000 Litres Oxidation DIN [A/ cm] Oxidation-Index Ao ph [%] 2 43 16.3 (No Trend existing) Example T2: Gasturbine Oi l: Turbine Oil ISO VG 46, 20.000 Litres Oxidation DIN [A/ cm] Oxidation-Index Ao ph [%] 2 43 16.3 (No Trend existing) Example T2: Gasturbine Oi l: Turbine Oil ISO VG 46, 20.000 Litres Oxidation DIN [A/ cm] Oxidation-Index Ao ph [%] 2 43 16.3 (No Trend existing) Example T2: Gasturbine Oi l: Turbine Oil ISO VG 46, 20.000 Litres Oxidation DIN [A/ cm] Oxidation-Index Ao ph [%] 2 43 16.3 (No Trend existing) Science and Research 39 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0005 Image 10: Example 3, Image 1 Example: Steam Turbine with heavily oxidised oil Siemens SST 400, Control hydraulics, EP turbine oil ISO VG 46, 600 litres SST 400 Sample and Cap Infrared spectrum Example: Steam Turbine with heavily oxidised oil Siemens SST 400, Control hydraulics, EP turbine oil ISO VG 46, 600 litres SST 400 Sample and Cap Infrared spectrum Image 11: Example 3, Image 2 Example: Steam Turbine with heavily oxidised oil Siemens SST 400, Control hydraulics, EP turbine oil ISO VG 46, 600 litres Example: Steam Turbine with heavily oxidised oil Siemens SST 400, Control hydraulics, EP turbine oil ISO VG 46, 600 litres Example: Steam Turbine with heavily oxidised oil Siemens SST 400, Control hydraulics, EP turbine oil ISO VG 46, 600 litres Example: Steam Turbine with heavily oxidised oil Siemens SST 400, Control hydraulics, EP turbine oil ISO VG 46, 600 litres SST 400 Sample and Cap Infrared spectrum The value determined by the oxidation index, “66”, clearly shows that substantial oil ageing has already occurred. Science and Research 40 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0005 However, Image 13 also shows that there are significant changes in the area adjacent to the wavenumber 1710 A/ cm. Image 12: Example 3, Image 3 Oxidation DIN = 3 A/ cm Example: Steam Turbine with heavily oxidised oil Siemens SST 400, Copntrol hydraulics, EP additivated oil, ISO VG 46, 600 litres Oxidation DIN = 3 A/ cm Example: Steam Turbine with heavily oxidised oil Siemens SST 400, Copntrol hydraulics, EP additivated oil, ISO VG 46, 600 litres Image 13: Example 3, Image 4 Oxidation DIN = 3 A/ cm Oxidation index = 66 Example: Steam Turbine with heavily oxidised oil Siemens SST 400, Copntrol hydraulics, EP additivated oil, ISO VG 46, 600 litres Oxidation DIN = 3 A/ cm Oxidation index = 66 Example: Steam Turbine with heavily oxidised oil Siemens SST 400, Copntrol hydraulics, EP additivated oil, ISO VG 46, 600 litres Oxidation DIN = 3 A/ cm Example: Steam Turbine with heavily oxidised oil Siemens SST 400, Copntrol hydraulics, EP additivated oil, ISO VG 46, 600 litres Oxidation DIN = 3 A/ cm Example: Steam Turbine with heavily oxidised oil Siemens SST 400, Copntrol hydraulics, EP additivated oil, ISO VG 46, 600 litres Oxidation DIN = 3 A/ cm Oxidation index = 66 Example: Steam Turbine with heavily oxidised oil Siemens SST 400, Copntrol hydraulics, EP additivated oil, ISO VG 46, 600 litres Oxidation DIN = 3 A/ cm Oxidation index = 66 Example: Steam Turbine with heavily oxidised oil Siemens SST 400, Copntrol hydraulics, EP additivated oil, ISO VG 46, 600 litres Oxidation DIN = 3 A/ cm Oxidation index = 66 Example: Steam Turbine with heavily oxidised oil Siemens SST 400, Copntrol hydraulics, EP additivated oil, ISO VG 46, 600 litres Summary More than 15,000 samples of turbine oils, hydraulic oils, compressor oils, and gear oils have now been investigated and evaluated using this method. The experiences gathered from these samples clearly show that oxidative changes can be detected and assessed at a much earlier stage. The oxidation index is primarily an early indicator of oxidative changes in industrial oils. The large number of samples has enabled us to establish initial limit values for different applications. These are currently being continuously reviewed, refined, and adjusted. This alternative method of evaluating the FT-IR spectrum regarding oxidation provides meaningful values for assessing oxidation in modern lubricants. By offering more understandable and stable trend values, this method significantly improves operational safety for plant operators. Through trend progression, a much better prediction of the remaining oil life in relation to oxidation can be made. In conjunction with the classical values of oil analysis, further meaningful tests (e.g., MPC, RULER) can be recommended, if necessary, to assess oil changes accurately and identify causes, as well as find solutions. This method has also attracted interest in the DIN working group for used lubricants. Currently, a task force from this working group is evaluating the incorporation of this method into a DIN standard. References: DIN 51453: 2004-10 Science and Research 41 Tribologie + Schmierungstechnik · volume 72 · issue 1/ 2025 DOI 10.24053/ TuS-2025-0005