The Seven Characteristic Gases, at a Glance
Dissolved gas analysis (DGA) works because transformer oil and paper insulation decompose under electrical, thermal, mechanical and chemical stress, and that decomposition produces gases that dissolve in the oil. The type of gas and its concentration are not random — they correspond to the fault type and the energy driving it.
Both IEC 60599 and IEEE C57.104 recognize seven characteristic gases as the core of DGA: hydrogen (H2), methane (CH4), ethane (C2H6), ethylene (C2H4), acetylene (C2H2), carbon monoxide (CO) and carbon dioxide (CO2). Under certain operating conditions, oxygen (O2), nitrogen (N2) and moisture are added to assess sealing and aging status.
Reading the Seven Characteristic Gases
Each gas is produced by a dominant decomposition pathway, so each carries a different diagnostic message:
- Hydrogen (H2) — the first gas generated in most faults, from partial discharge, low-energy discharge and oil overheating. It is your primary early-warning gas.
- Methane (CH4) and ethane (C2H6) — products of low-temperature thermal cracking of oil, indicating low- and medium-temperature thermal faults.
- Ethylene (C2H4) — the marker of high-temperature thermal cracking, above roughly 300 °C.
- Acetylene (C2H2) — produced only at high energy (arcing, high-energy discharge). Its very presence is a strong discharge signal; even ppm-level concentrations warrant attention.
- Carbon monoxide (CO) — from pyrolysis or oxidation of cellulose, meaning solid (paper) insulation is being consumed.
- Carbon dioxide (CO2) — reflects aging and overheating of solid insulation.
Acetylene deserves special emphasis: because it is generated in significant quantities only at relatively high energies, a rising C2H2 trend is one of the strongest single indicators of a developing discharge fault.
The Supporting Cast: Oxygen, Nitrogen and Moisture
The auxiliary gases are easy to overlook, but they answer different questions. Rising O2 and N2 point to air ingress or a sealing problem and tell you about the degassing state of the oil. Moisture reduces dielectric strength and accelerates aging of the oil-paper system, so it belongs in the same monitoring picture, not a separate one.
Because H2, O2 and N2 have no infrared absorption, they are typically measured by auxiliary thermal-conductivity-type channels rather than by optical spectroscopy — a detail that matters when you compare online monitors.
Gas-to-Source-to-Fault Reference Table
The correspondence between each gas, its principal source and its fault indication is summarized below — keep this table handy on the job:
| Gas | Principal source | Fault indication |
|---|---|---|
| H2 | Oil cracking (partial discharge, low-energy discharge, overheating) | Partial discharge, low-energy discharge, oil overheating |
| CH4 / C2H6 | Low-temperature thermal cracking of oil | Low- and medium-temperature thermal faults |
| C2H4 | High-temperature thermal cracking of oil | High-temperature thermal fault (above 300 °C) |
| C2H2 | Arcing discharge, high-energy discharge | Discharge (arcing); warning required even at ppm level |
| CO | Cellulose pyrolysis / oxidation | Overheating decomposition of solid insulation (paper) |
| CO2 | Cellulose aging / oxidation | Aging and overheating of solid insulation |
| O2 / N2 | Air ingress, sealing condition | Sealing and degassing status |
| H2O | Moisture absorption / aging of oil-paper | Moisture content; reduces insulation strength, accelerates aging |
The Single-Gas Spike Caveat
Here is the most important rule in DGA field practice: an increase in a single gas cannot, by itself, characterize a fault. A lone ethylene reading, for example, could come from a genuine hot spot or from a sampling artifact. Diagnosis requires a combined judgment built from three things:
- the relative proportions of multiple gases (ratio and graphical methods),
- the historical trend of each gas, and
- the gas generation rate — how fast concentrations are rising.
Treat any single-gas spike as a prompt to widen the picture, not as a conclusion. This is exactly why the industry’s diagnostic methods — IEC gas ratios, the Duval triangle and pentagon, and trend analysis — are built on combinations and trajectories rather than isolated numbers. See our diagnostic methods hub for how those tools fit together.
From Field Reading to Action
A practical workflow: record a baseline, sample at regular intervals, and compare each reading with both the previous value and the longer trend. Escalate when a characteristic gas rises above its threshold and the rate of rise accelerates. For critical assets, this is precisely the gap that online monitoring fills — continuous readings replace annual or quarterly snapshots, so a discharge fault that starts today is flagged today, not at the next scheduled sample.
For the underlying chemistry, see our guide to transformer oil decomposition chemistry, or start from the basics in what is dissolved gas analysis.
PAS DGA for Online Multi-Gas Monitoring
Field guides are most useful when the data arrives in time to act. The PAS DGA DGA-900 online monitor measures 9 gases plus moisture — all seven characteristic gases plus oxygen and nitrogen — using laser photoacoustic spectroscopy (L-PAS). It runs with no carrier gas and no consumables, and its acetylene detection limit of ≤0.1 ppm is low enough to catch the early rise of a discharge fault. Every reading lands in your SCADA or DGA system as a clean time series, ready for ratio, Duval and trend analysis.
Explore the full PAS DGA product range, or contact PAS DGA to discuss a monitoring plan for your fleet.