The dew point is the temperature at which the water vapour present in a gas reaches saturation and begins to condense. The drier the gas, the lower that temperature: SF6 in good condition gives dew points below −36 °C, and values near 0 °C indicate serious contamination.
It matters because water lowers the dielectric strength of the gas and, combined with the products of arc decomposition, forms acids that attack the solid insulation and the metal surfaces inside the enclosure. Moisture creeps in slowly through seals, bushings and during filling operations themselves.
It is measured with chilled-mirror hygrometers or capacitive sensors built into SF6 analyzers, which give the reading in °C dew point or converted to ppmv. IEC 60480 sets the reuse limits for recovered gas; above them the gas must be dried before being returned to the equipment.