Gas ratios have been in use since 1970 when Dornenburg utilised them to differentiate
between fault types. The use of ratios had the advantage that oil volume did
not affect the ratio and hence the diagnosis. Dornenburg first used pairs of gases to
form ratios, to differentiate between electrical and thermal faults. In his first ratios,
an electrical fault was indicated if the ratio of ethylene to acetylene exceeded unity,
and the ratio of methane to hydrogen indicated a thermal fault if greater than 0.1 or
a corona discharge if less than 0.1.