Insulating gas comparison

Comparison at the operating pressure

The three columns that decide a gas selection are rarely the same three. Dielectric strength sets the pressure you need; global warming potential sets whether it is acceptable; boiling point sets whether it survives the coldest morning of the year.

What the numbers mean

Dielectric strength scales sub-linearly with pressure

Doubling the pressure does not double the withstand of a real enclosure. Surface roughness, particle contamination and the area effect all degrade the achievable field as pressure rises, so practical strength follows a power law with an exponent below one rather than the linear relation of an ideal uniform field:

The consequence matters for gas substitution: because the exponent is below one, replacing a gas of relative strength r at equal performance needs a pressure ratio of (1/r)^1/0.71, not 1/r. A gas at 40 % of the strength of SF₆ does not need 2.5 times the pressure — it needs about 3.6 times.

Why the pressure penalty usually decides it

• Enclosure design. The wall thickness and flange rating follow from the design pressure, so a three-to-four-fold pressure increase is a different piece of equipment, not the same one filled differently.
• Liquefaction. Raising the pressure moves the gas closer to its saturation curve. For the fluoronitrile mixtures this is the binding constraint in cold climates, and it is the reason they are used diluted in CO₂ rather than pure — see the SF₆ liquefaction tool for how that boundary behaves.
• Leakage. Leak rate scales with pressure, and for a high-GWP gas the emissions consequence of a leak scales with GWP as well.

Global warming potential

SF₆ has the highest GWP of any gas assessed by the IPCC and an atmospheric lifetime measured in thousands of years, which is what drives the search for alternatives regardless of their dielectric penalty. CO₂ and air carry essentially no incremental GWP in this application. Fluoronitrile blends sit in between: the blend GWP is low because the fluoronitrile is present at a few percent, while the pure compound is not itself a low-GWP substance.

Method and scope

Two separate things are calculated here, and they carry very different confidence.

• Relative strength — indicative ranking. The relative figures are round, approximate values consistent with the published literature ordering of these gases. They are adequate for comparing candidates and for understanding the pressure penalty of a substitution. They are not a design input.
• Indicative field strength — order of magnitude only. The absolute curve uses an empirical power law referenced to SF₆. It represents a practical design stress for a real enclosure, an order of magnitude below the ideal uniform-field breakdown of the same gas, and it is not tied to any particular geometry, surface finish or waveform.

What this is useful for

• Scoping a substitution. Establishing roughly what pressure an alternative gas would need before spending time on a detailed study.
• Understanding a specification. Seeing why an alternative-gas product is rated at the pressure it is.
• Framing the trade-off. Putting strength, GWP and low-temperature behaviour on one screen, which is where the actual decision gets made.

Provided for reference and comparison. Relative strengths are indicative literature-consistent values, not measured data; absolute field values are order-of-magnitude only and must not be used for insulation design. Does not replace type test data, standards or manufacturers' design values. Version 1.0.