The concentration of each of these antioxidants in its
mineral insulating oil sample was in line with the actual
concentration levels recommended in the literature [(13-
20)]. They were 150 mg L−1 for DBDS, 30 mg L−1 for BTA
and methylated BTA and 0.30 % w/v for the other
antioxidants.
An antioxidant-containing-oil-sample was subjected to
artificial thermal aging (according to the standard method
ASTM D 2440 [6]) which involves maintaining it at 100 o
C while passing oxygen through it at a flow rate of 1.0 L
h−1 at for 24 h. The experimental setup shown in Figure 1
is identical to that used by Neto et al [1]. Identical samples
containing the same antioxidant were similarly treated for
aging periods of 48, 72, 96 or 120 hours.