Oil-Gas Separation for Online DGA: A Guide to Sampling & Degassing

The Pre-Processing Bottleneck

The performance ceiling of online dissolved gas analysis (DGA) is often set not by the detection module but by oil-gas separation. Whatever the detection principle, the measurement is only as good as the degassing stage that brings dissolved gases out of the oil and into the gas phase quickly and stably. The sampling system carries three responsibilities: extracting the characteristic gases in accordance with gas-liquid equilibrium laws, protecting the transformer’s oil condition, and setting the response speed of the entire system.

Four Engineered Degassing Routes

Online DGA products fall into four main degassing families:

• Headspace (static and dynamic) — Static headspace draws an oil sample into a sealed chamber, heats it to a set temperature, and analyzes the upper gas phase after gas-liquid equilibrium. Dynamic headspace continuously displaces the gas phase with a circulation pump, cutting equilibration time to under 15 minutes in engineering practice. Headspace provides an ample gas sample, which benefits detection of gases with low solubility, but it is periodic and requires oil-sample isolation.
• Vacuum degassing — A vacuum pump or oil-column piston makes dissolved gases escape rapidly under low pressure, concentrating the sample for high sensitivity. The Vaisala OPT100 uses a patented vacuum degassing design that requires no vacuum pump and is claimed maintenance-free (vendor data).
• Membrane degassing — A semipermeable membrane — PTFE capillary, AF2400 (a highly gas-permeable amorphous fluoropolymer), or a super-oleophobic membrane — isolates the oil phase from the gas chamber; gas permeates through, driven by the partial-pressure difference. No carrier gas is needed, degassing is continuous, and oil consumption is zero. Equilibration time depends on membrane area and oil temperature.
• Vacuum-assisted membrane — Applying a vacuum to the gas chamber on the far side of the membrane enlarges the partial-pressure difference, substantially shortening equilibration time while keeping operation continuous. Commercial L-PAS online devices commonly use a front end combining membrane separation with vacuum assistance, sometimes with optional patented spray or thermostatic-vacuum configurations.

Degassing Techniques Compared

There is no universally best route: the choice trades equilibration speed, continuity, gas sample volume, and temperature sensitivity. Fast-acting, continuous, zero-consumption configurations such as vacuum-assisted membrane are the direction of most commercial L-PAS front ends.

Oil-Circuit Safety: The Transformer Comes First

The oil circuit is the interface between the degassing module and the transformer body. Its primary constraint is not detection performance but not endangering the transformer itself. Oil is drawn from a sampling valve or a dedicated oil sampling port and returned through the degassing module as a closed-loop bypass. Three requirements dominate: no oil consumption, no alteration of oil flow, no introduction of air or moisture. No stage may exhibit net oil loss or long-term stagnant dead zones.

• Bubble traps — Load changes, oil top-ups, and pumping can carry free gas bubbles; entering the degassing chamber, they cause measurement spikes and erroneous readings. A bubble trap or degassing buffer at the oil circuit inlet, together with venting, prevents this.
• Flow control — A constant-flow pump and throttling elements stabilize oil flow. Flow fluctuations alter the partial-pressure and thermal equilibrium across the membrane and appear directly as measurement noise, so closed-loop flow control is a basic requirement for membrane-type devices.
• Anti-clogging and anti-leakage — Sludge precipitated by oil aging can block capillary membranes or valves; filters belong in the oil circuit and the periodic maintenance schedule, and all fittings use sealed designs to prevent leaks that could cause abnormal oil levels and safety hazards.
• Pressure and temperature monitoring — Oil circuit pressure and oil temperature should be monitored in real time; when they exceed safe limits, oil flow should shut off automatically with an alarm, preventing the device from operating in a degraded state under extreme conditions.

IEC 60567 Consistency

IEC 60567:2023 treats online monitoring as an important complement to offline sampling and analysis, emphasizing that online results should be as consistent as possible with laboratory methods. Comparability over a time series depends on the oil circuit maintaining a stable systematic deviation; if the degassing stage drifts, trend judgment is distorted. The engineering acceptance criterion is simple: regardless of how the degassing and detection stages operate, the transformer side should perceive only a stable, controllable bypass oil flow.

PAS DGA for online DGA monitoring

PAS DGA products put the pre-processing stage where it belongs. The DGA-900 9-gas plus moisture L-PAS monitor pairs a vacuum-assisted membrane front end with a closed-loop oil circuit — zero oil consumption, zero flow disturbance, and bubble-trap protection at the inlet. For a broader view of the technology behind the front end, see our photoacoustic spectroscopy technology guide, and for deployment trade-offs read the online DGA monitor selection guide.

To discuss degassing and oil-circuit requirements for your fleet, contact PAS DGA.