Transformer oil breaks down when something heats it or an arc passes through it, and that breakdown releases gases which stay dissolved in the oil itself. Dissolved gas analysis, known by its initials DGA, draws a sample and measures how much of each gas is present.
Its value lies in the fact that each gas betrays a different mechanism: hydrogen appears with partial discharge, methane and ethane with moderate overheating, ethylene with severe overheating and acetylene with arcing. Carbon monoxide and dioxide point to degradation of the paper insulation rather than the oil.
What matters is not so much the absolute concentration as the trend and the ratio between gases. The IEC 60599 and IEEE C57.104 guides set out the usual criteria — Rogers ratios, Duval triangle — for turning a list of ppm figures into a diagnosis. Today periodic laboratory sampling coexists with online monitors that watch hydrogen and moisture continuously.