SF₆ liquefaction temperature

Saturation pressure across the range

The saturation vapour pressure at each temperature. A charge held at or above the value in this table for a given temperature will contain liquid at that temperature.

Method and scope

Sulphur hexafluoride is used well below its critical temperature, so at a high enough density it will condense. The boundary is the saturation vapour pressure curve: at any temperature, a charge cannot exceed the saturation pressure without part of it becoming liquid.

This is the vapour-pressure equation published by Funke, Kleinrahm and Wagner (2009), using the critical point T_c = 45.573 °C, p_c = 3755 kPa and the triple point at −49.595 °C, 227.5 kPa. The liquefaction temperature for a given pressure is obtained by solving the same equation numerically.

The implementation was checked against the published saturation table at 5 °C intervals from −50 to +45 °C; agreement is within 0.01 % at every point.

Why this matters in service

• Below the liquefaction point, pressure stops tracking density. Once liquid forms, the pressure is fixed by the saturation curve regardless of how much gas is in the enclosure. A density-compensated switch loses its meaning, and a gauge reading no longer tells you whether the charge is intact.
• Dielectric strength falls with the gas that has condensed out. The insulating capability depends on the density of the gas phase, which stops rising once saturation is reached.
• Cold climates set the filling pressure ceiling. This is why equipment for cold sites is filled to a lower pressure, or filled with an SF₆ mixture whose partial pressure keeps it clear of the curve.

What this assumes

• Pure SF₆. For SF₆/N₂ or SF₆/CF₄ mixtures it is the SF₆ partial pressure that must stay below the curve, not the total. A mixture tool is in preparation.
• Gas temperature, not air temperature. The relevant figure is the coldest temperature the gas itself reaches, which lags ambient and can differ from it in sun or after a load change.
• Equilibrium. The curve describes the equilibrium boundary; condensation in a real enclosure starts at the coldest local surface, which may be colder than the bulk gas.
• Range. Valid between the triple point (−49.6 °C) and the critical point (45.6 °C). There is no liquefaction above the critical temperature at any pressure.

Provided for reference and cross-checking. Does not replace equipment manufacturers' filling curves, nameplate data or operating instructions. Version 1.0. Source: R. Funke, R. Kleinrahm and W. Wagner, vapour-pressure equation for SF₆, 2009.