What Is the Relationship Between the Dew Point Temperature and Moisture Content of Sf6 Gas? How to Convert Between the Two Units?

In the SF6 equipment inspection reports of substations, two units often appear simultaneously: dew point temperature (°C) and moisture content (μl/l or ppm). For the same instrument, these two values refer to the same thing but are expressed differently. Understanding the relationship between them is essential for accurately interpreting the test results.

The meaning of dew point temperature

The dew point temperature is the temperature at which the water vapor in a gas reaches saturation under a certain pressure. In other words, when SF6 gas is cooled to this temperature, moisture begins to condense into liquid droplets. The lower the dew point, the drier the gas, indicating a smaller amount of moisture content.

The meaning of micro-water content

The moisture content is expressed as a volume ratio concentration, with the unit being μl/l (parts per million, ppm). For example, 150 μl/l means there is 0.15 μl of water vapor in each liter of SF6 gas.

The conversion relationship between the two

Under a certain gas pressure, there is a definite correspondence between dew point temperature and micro water content, which can be converted by consulting tables or calculations. The HZSF-702 color display simultaneously shows both dew point temperature (°C) and micro water content (μl/l), eliminating the need for manual conversion by operators. The two parameters are displayed side by side for more intuitive comparison.

The expression of industry standards

The national power industry standard specifies the moisture content limit as μl/l: for SF6 circuit breakers, ≤150 μl/l after factory production and major overhaul; ≤150 μl/l at handover; ≤200 μl/l during operation; and ≤300 μl/l for individual testing of interrupting units when necessary. DL/T 506-2018, on the other hand, uses dew point temperature for expression: no higher than -40°C for newly commissioned units and no higher than -20°C during operation. The HZSF-702 display both units simultaneously, ensuring compliance with either standard in a straightforward manner.