The corrosive-sulphur tests that protect transformers from copper sulphide damage answer a single question: is this oil corrosive? That is enough for a pass/fail decision, but not to manage a problem over time. IEC 62697-1 fills the gap by measuring the concentration of the compound most often responsible — dibenzyl disulfide, usually written DBDS — so the question becomes how much, not just whether.
What it covers
IEC 62697-1 specifies a gas-chromatographic method for the quantitative determination of DBDS in unused and used insulating liquids across a defined working range. The sample is diluted, spiked with a known amount of an internal standard of similar chemistry, and injected into a gas chromatograph fitted with a suitable detector — electron-capture, atomic-emission, or mass-spectrometric. DBDS elutes, is identified by its retention and detector response, and is quantified against the internal standard. The result is reported as a DBDS concentration.
Part 1 of the series is specific to DBDS. Companion parts address total corrosive sulphur and elemental sulphur. Because the method is instrumental rather than visual, it is largely free of the subjective judgement that the empirical copper-and-paper tests demand, and it is not fooled in the same way by metal passivators or by oil ageing.
The standard's chemistry covers mineral insulating oils and natural ester liquids. DBDS has a dual history — it was used as a sulphur-based antioxidant and extreme-pressure additive in lubricants before being recognised as a problem in transformer oil, and it can also enter an oil through refining and blending. Either way it is unwanted in a modern insulating oil, which is why "not detectable" is the practical target for new oil.
Why it matters in practice
A quantitative figure changes what you can do with the result. DBDS reacts with copper as a transformer runs, so its concentration falls over time as the damage proceeds. A binary test cannot tell apart an oil that never contained DBDS from one in which the DBDS has already been consumed attacking the windings — yet those two oils carry very different residual risk. A concentration trend can.
It also clarifies near-threshold cases. An oil can return a corrosive result while showing little DBDS, pointing to other reactive species; conversely a detectable DBDS figure can accompany a clean empirical result near the boundary. Neither test alone gives the full picture, and reading them together is where the engineering judgement lives.
How we use it
We run IEC 62697-1 alongside the binary corrosive-sulphur test — on new-oil acceptance, where "not detectable" is expected, and on in-service oil where corrosivity is suspected or confirmed. The concentration supports mitigation decisions: whether to passivate, treat, or change the oil, and how to monitor afterwards. When a result triggers action, we set the number in the context of the recognised mitigation and risk-assessment guidance rather than treating it as a standalone verdict. Reporting the empirical result and the DBDS figure together is our standard practice, because each answers a question the other cannot.