Hydrogen and low molecular weight hydrocarbons are formed in the decomposition of the much larger hydrocarbon molecules that make up an insulating oil. The numerical values of ratios of these decomposition products are used in the diagnosis of the type of electrical or thermal fault leading to their formation in malfunctioning electrical equipment. The reproducibility, repeatability and bias of several ratios of gases calculated from the results of the interlaboratory test of D 3612 A and B are given in Fig. X1.1. The values of Bg, Sg and sg are much the same for the two methods; typically -0.1 to +0.1, 0.1 to 0.15 and 0.01 to 0.04, respectively. (The database of ratios is not large enough to support an estimate at the 95 % confidence level.)