## 3 Results and Discussion
### Absorption Coefficient  
Figure 3 shows the absorption coefficients of the four levels of glycol contamination in the SAE 5W-30 engine oil. The mean absorption coefficient for all glycol levels increased with frequency, starting at a low of 0.148 cm\({}^{-1}\) for 0 ppm at 0.25 THz and reaching a high of 2.743 cm\({}^{-1}\) for 500 ppm at 2.5 THz. At each frequency, the absorption coefficient increased with an increase in glycol contamination,  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \multirow{2}{*}{**THz**} & \multicolumn{4}{c}{**Refractive Index \({}^{*}\)**} & \multicolumn{4}{c}{**Absorption Coefficient (cm\({}^{-1}\)) \({}^{*}\)**} \\ \cline{2-10}  & **0 ppm** & **150 ppm** & **300 ppm** & **500 ppm** & **0 ppm** & **150 ppm** & **300 ppm** & **500 ppm** \\ \hline \multirow{2}{*}{0.25} & 1.4662 \({}^{\text{b}}\) & 1.4663 \({}^{\text{b}}\) & 1.4669 \({}^{\text{a}}\) & 1.4671 \({}^{\text{a}}\) & 0.148 \({}^{\text{a}}\) &  
with the exception that below 0.35 THz 150 ppm, there was a slightly higher absorption coefficient than 300 ppm. Differences among the curves of the absorption coefficients increased with frequency, with the difference between 0 ppm and 500 ppm increasing from 0.023 cm\({}^{-1}\) at 0.25 THz to 0.534 cm\({}^{-1}\) at 2.5 THz. While the absorption coefficient was measured at all 615 frequencies, Table 1 detailed data for absorption coefficients at 0.25 THz intervals. The increase in absorption coefficient with an increase in frequency was similar to previous research with other oils [20; 22; 25]. The increase in absorption coefficient with greater contamination levels was similar to the effect of water contamination [26] and gasoline fuel contamination [27], and was expected, as all these species were or contained polar molecules with strong THz absorption.  
The 95% confidence intervals increased gradually with frequency, but the difference among means increased at a greater rate. While there was overlap among the 95% confidence intervals at 0.25 THz, the overlap decreased as frequency increased. Based on ANOVA, there were significant differences (\(p<0.05\)) among contamination levels of glycol across the 0.355-2.5 THz range, with highly significant differences (\(p<0.01\)) across the 0.476-2.5 THz range. Based on LSD results, the only statistically significant difference among means from 0.355-0.428 THz was that 500 ppm had a significantly higher absorption coefficient than 0 ppm. Commencing with 0.432 THz, 300 ppm was also significantly different from 0 ppm. Beginning at 0.458 THz, 150 ppm was also significantly different from 500 ppm. Starting at 0.604 THz, 150 ppm was also significantly different from 0 ppm. Beginning with 0.754 THz, 500 ppm was significantly different from each of the other contamination levels. From 1.01-2.471 THz, each contamination level was significantly different from each other. Thus, the range of 1.01-2.471 THz was found to be able to discriminate all levels of glycol contamination. Although [PERSON] et al. [8] stated 500 ppm was not detrimental in engine oil, this study clearly showed that THz absorption coefficient was a decent discriminator for glycol contamination, by discriminating between each of the contamination levels. The sensitivity of THz at 150 ppm was better than that [PERSON] and [PERSON] [16] found with a mid-infrared spectroscopy model developed from glycol-contaminated oil samples of 14-9990 ppm and then validated with glycol contamination differences as low as 800 ppm. ASTM standard practice specifies 1000 ppm as the sensitivity of Fourier-transform infrared spectroscopy (FTIR) [6].  
Figure 3: Mean absorption coefficient of each of four glycol contamination levels of 5W-30 gasoline/diesel engine oil with 95% confidence intervals (for clarity when hidden, 95% confidence intervals are the same above and below the mean).