The two measurements describe different things about the same gas. Purity describes what the gas is made of. Dew point describes how much water vapour it holds. Together they indicate whether the compartment has taken in air, whether it has taken in moisture, or both, and that distinction determines the handling.

Two measurements, one decision

Purity is expressed as the proportion of sulphur hexafluoride in the gas sample, with the remainder accounted for by air and other gases. In a sealed and correctly filled compartment the value sits at the designed level and changes only through ingress. Air entering the compartment through a leak reduces the purity figure directly.

Dew point is the temperature at which the water vapour in the gas would begin to condense. It is a temperature rather than a concentration, which makes it more directly relevant to the risk of condensation on an insulating surface, but less convenient for trending unless the pressure at which it was measured is recorded.

The decision the pair supports is whether the gas can be returned to service in the equipment. Standing alone, a satisfactory purity figure says nothing about moisture, and a satisfactory dew point says nothing about air ingress. Read together, they indicate the mechanism and the magnitude of any contamination.

Dew point and moisture equilibrium in gas

Moisture behaves differently in a gas compartment than in oil. There is no significant absorption, so water vapour remains in the gas phase and its concentration is uniform throughout the compartment once the gas has mixed. That makes the measurement representative of the whole volume in a way that an oil sample is not representative of a whole transformer.

The measurement is pressure-dependent. Dew point and water vapour concentration are related through the pressure at which the measurement is taken, so a reading taken at one pressure does not translate directly to another without conversion. Recording the measurement pressure, or recording the quantity in a form that is pressure-independent, is what allows results from different occasions to be compared.

Moisture enters a compartment through the same paths as air: seals, gaskets, valve stems and, less commonly, through the gas itself as supplied. Because the entry paths are shared, moisture and air ingress usually appear together, which is the practical reason the two measurements are interpreted as a pair.

Purity thresholds and what they indicate

Purity is usually determined from a physical property of the gas rather than from a chemical separation. Instruments that work on thermal conductivity or on a comparable physical principle give a rapid indication of the proportion of sulphur hexafluoride present, which is sufficient to detect air ingress.

The significance of a purity figure is not the absolute number but the deviation from the design value for the equipment. A compartment that is filled to a specified mixture has a nominal purity, and any reduction from that figure represents contamination. The size of the reduction, combined with the compartment volume, gives an indication of how much air has entered.

Where the purity is reduced, the leak that allowed the ingress becomes a separate finding. The gas measurement establishes that air has entered; locating the path requires leak detection, and the two activities belong together in the maintenance plan. Returning freshly processed gas to a compartment that still leaks simply restarts the process.

Interpreting results together before reclaiming

The third row is the one most often misread. Gas that has a moisture problem without an air problem points at a specific moisture source rather than a general leak, and processing the gas addresses the symptom while leaving the source in place.

Sampling practice and contamination risk

The sample path is part of the measurement. Gas has to travel from the compartment through a connector and a length of tubing to the instrument, and anything in that path can change the result. Residual air in the tubing, a connector that has been left open, or condensate inside a hose all introduce contamination that the instrument then reports as a property of the gas.

Purging the sampling line before the measurement is the standard practice, and the volume that has to be purged depends on the length and diameter of the tubing. Where the same sampling arrangement is used for both measurements, the purge has to be repeated when the instrument is changed, because the new instrument’s inlet path is also part of the system.

Where moisture is the measurement in question, the equipment itself deserves attention. A dew point instrument with a sensor that has been exposed to ambient air, or one that has been stored without its protective cap, will read high until it has been dried or regenerated. That behaviour is worth knowing before a marginal result prompts an unnecessary processing cycle.

Instrument calibration and verification

Dew point and purity instruments are both calibration-dependent, and the calibration requirement differs between them. A purity instrument working from a physical property can be verified against a sample of known composition or against a reference instrument. A dew point instrument can be verified against a reference gas of known moisture content or against a chilled-mirror reference.

The verification interval should reflect the consequence of an incorrect result. A reading used to decide whether a transmission switchgear compartment can be returned to service justifies a check before it is relied on, particularly when it contradicts a previous measurement or falls near a limit.

The practical rule is to verify the instrument before acting on an inconvenient result. A marginal value that would trigger a processing cycle at considerable cost is worth confirming with a second measurement or an instrument check, and that sequence is far cheaper than the processing it may avoid.

When to reclaim, purify or replace gas

Three outcomes are available, and the choice depends on how far the gas has departed from the condition required by the equipment. Within the limits, the gas returns to the compartment without treatment. Outside the limits but recoverable, the gas is processed to remove moisture and other impurities before being returned. Beyond what processing can restore, the gas leaves the site for reclamation or disposal.

The rules for re-use are set out in the standard that covers specifications for the re-use of sulphur hexafluoride recovered from electrical equipment, published as IEC 60480. The handling requirements that govern how gas is recovered, stored and transferred during the work are published as IEC 62271-4.

The regulatory framework sits alongside the technical one. The fluorinated greenhouse gas regulations, such as the framework published in the EU F-gas Regulation and the policy material maintained by the European Commission, place obligations on recovery, reporting and the competence of the personnel performing the work. Those obligations make the decision a documented one rather than an operational habit, which is the reason gas handling records are now part of the equipment file rather than a workshop note.

Recording results for the asset file

The record should identify the compartment, the equipment and the gas quantity involved, and should state the measured dew point with the pressure at which it was taken, the measured purity, the instrument used for each measurement and its verification status, and the ambient conditions during sampling.

The decision that followed belongs in the same record, together with the values that drove it. A line stating that the gas was processed and returned, with the dew point and purity figures that triggered the processing and the figures measured after treatment, turns an operational event into an asset history that supports the next maintenance interval.

Where the gas was recovered because of a fault, the record should also reference the fault, because the gas result and the fault history together indicate whether the compartment needs an internal inspection. The field experience behind current gas handling and re-use practice is coordinated through CIGRE study committees and the equipment research published by EPRI. The instrument range is grouped on the SF6 testing hub.