# Quantifying the Effect of Catalysts on the Lifetime of Transformer Oil
## 4 Results
### Changes in the IR Region of the Electromagnetic Spectrum  
When evaluating the aging process of inhibited mineral oils, one of the best-established and most frequently used methods is the analysis of changes in the IR spectral range from 3600 to 3700 cm\({}^{-1}\)[26]. In this region of the spectrum, a strong absorption band related to a low-temperature antioxidant with a maximum absorbance at 3650 cm\({}^{-1}\) can be found. This sharp peak occurs due to the stretch vibration of a nonhydrogen-bonded O-H [33]. It is generally known that the gradual depletion of the total amount of antioxidant during the aging of the mineral oil results in a gradual decrease in the intensity of the spectral band at 3650 cm\({}^{-1}\)[34; 35; 36]. This finding was the main reason why attention was focused only on this region of IR spectra in this study. As an example of the observed changes in the intensity of the spectral band 3650 cm\({}^{-1}\), the recorded FT-IR spectra for oil aged at 130 \({}^{\circ}\)C in the presence and absence of catalysts are shown in Figure 5. It can be seen that FT-IR has satisfactory sensitivity, even for a lower degree of aging of the oil, because it is analyzing a gradual decrease in the concentration of the antioxidant present and not the color of the oil, which changes mainly during the later stages of aging. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Temperature** \\ **(\({}^{\circ}\)C)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Catalysts** \\ **(Yes/No)** \\ \end{tabular} } & \multicolumn{3}{c}{**Fitting Parameters (\(\mathbf{y=\alpha^{3}+bx^{2}+cx\))**} & \begin{tabular}{c} **Endpoint** \\ **Criterion** \\ **(a.u.)** \\ \end{tabular} } & \
\begin{tabular}{c} **Time to** \\ **Criterion** \\ **(Hours)** \\ \end{tabular} &
\begin{tabular}{c} **Reduction** \\ **in Lifetime** \\ **(\%)** \\ \end{tabular} \\ \hline \multirow{2}{*}{110} & No & \(3.82\times 10^{-10}\) & \(-1.06\times 10^{-8}\) & \(1.22\times 10^{-4}\) & 0.988 & \multirow{2}{*}{1.06} & 2197 & \multirow{2}{*}{39.1} \\  & Yes & \(-2.17\times 10^{-11}\) & \(3.30\times 10^{-7}\) & \(-1.38\times 10^{-4}\) & 0.991 & & 1338 & \\ \hline \multirow{2}{*}{120} & No & \(-1.15\times 10^{-9}\) & \(3.94\times 10^{-6}\) & \(-1.22\times 10^{-3}\) & 0.996 & \multirow{2}{*}{2.15} & 1177 & \multirow{2}{*}{35.3} \\  & Yes & \(1.31\times 10^{-9}\) & \(4.31\times 10^{-6}\) & \(-1.22\times 10^{-3}\) & 0.995 & & 762 & \\ \hline \multirow{2}{*}{130} & No & \(5.90\times 10^{-9}\) & \(5.21\times 10^{-6}\) & \(-1.08\times 10^{-3}\) & 0.986 & \multirow{2}{*}{2.02} & 556 & \multirow{2}{*}{44.8} \\  & Yes & \(-3.96\times 10^{-9}\) & \(3.85\times 10^{-5}\) & \(-4.86\times 10^{-3}\) & 0.995 & & 307 & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: UV-Vis spectroscopy—summary of the fitting and calculated parameters of the mineral oil aged with (C+) and without (C–) catalysts. Figure 4: Aging temperature 110 \({}^{\circ}\)C—an example of the indirect derivation of the endpoint criteria for UV-Vis_\(\lambda\)max. In contrast, the change in the absolute values of absorbance at 3650 cm\({}^{-1}\) during aging is not very high and may affect the measurement accuracy, e.g., absorbance decreased only from 0.815 to 0.694 for C\(-\) and from 0.811 to 0.592 for C\(+\) during aging at 130 \({}^{\circ}\)C in times ranging from 24 to 600 h (see Figure 5). Therefore, in these cases, FT-IR measurements should be repeated several times for each level of aging, and only the calculated average of the recorded values should be further evaluated. Figure 5: FT-IR spectra between 3635 and 3675 cm\({}^{-1}\) for mineral oil aged at 130 \({}^{\circ}\)C (**a**) without and (**b**) with catalysts. Another approach is to use multivariate statistical methods for processing the measured spectra, such as principal component analysis (PCA) [37]. Such methods make it possible to quantify even very small changes in the measured spectra.