THz-TDS in the range of 0.5-2.0 THz was evaluated for discriminating among water contamination levels (0%, 0.1%, and 0.2%) in diesel engine oil (SAE 15W-40). Absorption coefficient demonstrated potential to discriminate among the three water contamination levels with significant differences among all three levels across the 1.111-1.332 and 1.669-1.934 THz ranges. At each of these frequency ranges each water contamination level was significantly different from the other two. The 0% water had the lowest absorption coefficient while 0.2% water had the highest absorption coefficient. Refractive index demonstrated greater potential to discriminate among water contamination levels with significant differences among all three water levels across the 0.5-1.5 THz range. The refractive index of 0% water was the lowest and 0.2% water was the highest across the THz range. Linear regression analysis of refractive index as a predictor of water contamination level yielded a highly significant equation (p < 0.0001, R2 = 0.99, RMSE = 0.01) when using the refractive indices at 0.5 THz. The refractive indices of these oil samples were promising for discrimination of water contamination. THz spectroscopy should be evaluated for discriminating other engine oil contaminants.
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