ASTM D3612 is a laboratory test that identifies and quantifies the gases dissolved in samples of insulating liquid drawn from transformers, voltage regulators, oil circuit breakers, reclosers, and other liquid-filled apparatus. A sample collected per ASTM D923 is analyzed using gas chromatography to measure the concentration in parts per million of hydrogen, methane, ethane, ethylene, acetylene, and carbon monoxide, and sometimes carbon dioxide. The test produces a snapshot of the gas mix present in the liquid at the moment of sampling, reported as individual ppm values for each gas.
Whenever internal arcing, partial discharge, overheating, or cellulose degradation occurs inside a liquid-filled tank, the energy released breaks down the liquid and insulation into smaller molecules, releasing characteristic gases that dissolve into the remaining liquid. Different fault types produce different gas signatures - for example, arcing favors acetylene and hydrogen, while sustained overheating favors methane and ethane - so a DGA result can reveal a developing fault long before it manifests as a failure. A rising trend over time is an earlier warning than a single snapshot, which is why NETA maintenance protocols call for periodic DGA sampling alongside the rest of the insulating-liquid screen and dissolved-gas analysis to detect slow changes before they become catastrophic.
Acceptance: baseline dissolved-gas analysis is taken before a new or reconditioned transformer is first energized, to establish normal reference levels for that unit so future samples can be compared against its own history rather than against fleet-wide tables. Maintenance: the same test is repeated on a periodic interval, typically annually or bi-annually depending on equipment age and condition class, and is always captured immediately after an event that could generate internal gas such as a through-fault, bushing work, or an unexpected pressure rise, so the evolution of fault gases can be tracked over days, weeks, or months and trended alongside winding temperature estimates and dissolved-gas interpretation methods.
A representative sample is drawn per ASTM D923, ordinarily from a live sampling valve after it has been flushed of standing liquid; sampling from the bottom drain is sometimes done deliberately, to check for settled sediment or free water, but that sample is not treated as representative of the bulk liquid composition for gas trending. The sample is placed in a clean, dry, sealed container or vial and is shipped and stored to keep moisture, oxygen, and light out and to avoid gas loss before it reaches the laboratory. There, a gas chromatograph separates the dissolved gases and measures the concentration of each component, often reporting the result alongside a fingerprinting or trending interpretation method such as Duval or Rogers triangle mapping so the technician can infer the likely fault type from the shape of the gas mix.
The dissolved gas concentrations in ppm for each component - hydrogen, methane, ethane, ethylene, acetylene, carbon monoxide, and carbon dioxide - captured alongside the companion insulating-liquid screen results drawn from the same sample: acid neutralization number, specific gravity, interfacial tension, color, visual condition, water content, power factor, and dielectric breakdown voltage. The sample date, equipment identifier, sampling location (main tank, tap-changer compartment, or bushing), sample temperature at draw, and any remarks about unusual conditions at the time of sampling are recorded so trends can be anchored to events in the equipment's operational history.
NETA ATS-2025 and MTS-2023 do not publish universal gas-concentration thresholds but instead point to recognized interpretation methods such as Duval or Rogers gas-ratio analysis, which infer the likely fault type from the pattern of gases rather than from any single value in isolation. A baseline DGA result from a new unit is treated as the normal reference; future samples are trended against that baseline and against each other to detect rising concentrations or changes in the gas mix. An accelerating trend or a result that plots into a fault-type zone on a diagnostic triangle warrants investigation and additional testing - typically a follow-up DGA sample taken within days to confirm the trend is real and not a sampling anomaly - before a decision is made to operate the unit under closer monitoring or to remove it from service for inspection. The interpretation method and alarm logic vary across utilities and equipment types; see the purchased standard and the equipment manufacturer's guidance for the complete diagnostic framework and any asset-specific acceptance criteria.
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