# Quantifying the Effect of Catalysts on the Lifetime of Transformer Oil
## 4 Results  
### Changes in the UV-Vis Region of the Electromagnetic Spectrum  
It is generally known that thermo-oxidative aging of oil causes a gradual change in its coloration and increases the concentration of the dissolved decay products, such as peroxides, aldehydes, ketones, and organic acids [28]. These changes can be effectively analyzed via UV-Vis spectroscopy [22; 23; 24; 25; 28; 29]. Figure 2 shows an example of the absorbance UV-Vis spectra recorded between 350 and 550 nm for samples of mineral oil aged at 110 \({}^{\circ}\)C for various times; the spectra of mineral oil aged both with (henceforth labeled as \"C+\") and without catalysts (henceforth labeled as \"C-\") are shown. The intensity of the recorded UV-Vis spectra increases with increasing the concentration of the dissolved decay products, as well as products that are insoluble in the oil, which contribute to sludge formation and affect the color of the oil. It is also evident from Figure 2 that UV-Vis spectroscopy does not provide very high resolution and sensitivity during the initial stages of the thermo-oxidative aging process because new oils are almost transparent, and therefore, no absorbance occurring in the visible region can be detected [24; 30]. On the other hand, UV-Vis offers very precise results for higher aging levels, which are already accompanied by a change in the oil color. For a visual comparison of these changes, Figure 2 is also supplemented with a color scale in the form of a cut-out photograph of the real oil samples at a given stage of thermal aging at 110 \({}^{\circ}\)C. In particular, this color scale effectively demonstrates the crucial role of catalysts in the acceleration of mineral oil thermo-oxidative aging. The increasing concentration of dissolved decay products has an undeniable effect on the intensity of the change in oil color. However, it is also important to note that in cases where the color of the test oil is already very dark, it is necessary to dilute the test oil with new, nonaged oil. This fact may contribute to the overall error in measurement with UV-Vis spectroscopy. This necessity arose in the case of an aging temperature of 110 \({}^{\circ}\)C for C+ samples that were aged for at least 1848 h, which is also indicated in Figure 2.  
Changes in the measured UV-Vis spectra can be further quantified in two ways: (i) integrating the area under the absorbance curve of the oil and, in this way, determining the so-called \"relative content of dissolved decay\" [28], or (ii) analyzing the change in the intensity of one of the local maxima of the UV-Vis spectrum. In our study, the latter method provided better data suitable for further processing. More precisely, the evaluation of the spectral maximum at a wavelength of 453.13 nm (henceforth referred to as \"UV-Vis_Amax\") was proven to be advantageous as an observable parameter because it responded to the applied aging by a marked increase, as evident from Figure 3, and its change could be mathematically fitted very well, as explained below. Figure 2: UV-Vis spectra for mineral oil aged at 110 \({}^{\circ}\)C (**a**) without and (**b**) with catalysts. Figure 3: Evaluation of UV-Vis_\(\lambda\)max for oil samples aged at constant temperatures for various aging times (**a**) without (C\(-\)) and (**b**) with catalysts (C\(+\)). Experimentally observed trends in UV-Vis_\(\lambda\)max values were fitted to derive the endpoint criteria and to assess the remaining useful life of the tested mineral transformer oil. The goodness of fit between the observed data points and the values predicted by the mathematical model was assessed based on a calculation of the coefficient of determination (R\({}^{2}\)). A higher value of this coefficient represents a more accurate mathematical model [31]. In this respect, the mathematical fit using a cubic equation proved to be the most appropriate.