Sulphur hexafluoride (SF6) gives gas-insulated switchgear (GIS) and SF6 circuit breakers their compact size: at a few bar it insulates far better than air and quenches arcs effectively. Two things undermine it. Moisture inside the enclosure reduces insulation performance and reacts with arc by-products to form corrosive compounds, including hydrofluoric acid. Leaks lower gas density, which reduces insulation and breaking capability, and release one of the most potent greenhouse gases known. Routine SF6 maintenance therefore centres on two measurements: dewpoint and leak rate.
Why Moisture Matters
Water enters SF6 compartments through seals, from desorption out of internal insulating materials, and during filling or topping up. In normal service the main risk is condensation: if the gas is wet enough for water to condense on insulator surfaces as temperature falls, surface flashover becomes more likely. In compartments that interrupt current, water also takes part in reactions with SF6 decomposition products that form corrosive and toxic species.
Requirements for new gas are set out in IEC 60376, and for gas taken out of equipment for re-use in IEC 60480. Equipment manufacturers also set moisture limits for their own compartments in service. Check the applicable figure for the equipment rather than relying on a single generic value.
Measuring Dewpoint
A dewpoint meter samples gas from the compartment's service valve and reports the temperature at which water vapour in it would condense, usually alongside a moisture concentration in ppm by volume. Because dewpoint depends on pressure, note whether the result is at atmospheric or at operating pressure, and convert if comparing against a limit stated the other way.
The MEWOI-KST-VI is a portable example. The supplier specifies a dewpoint range of −80°C to +20°C with ±1°C accuracy from −60°C to +20°C, a measurement in under three minutes at +20°C, gas consumption of about 2 litres per test, and a self-locking connector to keep the sampling connection leak-tight.
Good practice: purge the sampling line before reading, since moisture absorbed on hose walls can make the gas look wetter than it is; record ambient conditions; and allow the reading to stabilise.
Finding and Quantifying Leaks
A falling density monitor shows that a compartment is losing gas, but not where. Qualitative "sniffers" locate a leak by sounding an alarm near it. A quantitative leak detector also reports the local SF6 concentration in ppm, so leaks can be compared, ranked and re-checked after repair.
The MEWOI-LF301 displays concentration directly in ppm with 0.01 ppm resolution and a response time of 1 second or less. Its buzzer frequency rises with concentration, which helps narrow down the source along a flange or joint.
Typical leak points include flange seals, gas pipework fittings, density monitor connections, bushings and burst discs. Survey them systematically, scanning slowly downwind and close to the joint, because SF6 is heavier than air and disperses in moving air.
Putting the Two Together
- After filling or topping up: check dewpoint once the gas has had time to mix, and leak-check the joints that were disturbed.
- Routine maintenance: trend dewpoint for each compartment and investigate any compartment whose readings rise faster than its neighbours.
- Density alarm: locate and quantify the leak, repair it, and confirm with a second survey.
Frequently Asked Questions
Can a dewpoint meter detect SF6 decomposition products? No. Arc by-products such as SO₂ and HF need separate decomposition-product analysis.
Why report both dewpoint and ppmv? Dewpoint relates directly to condensation risk; ppmv is a concentration that is convenient for comparing gas quality. Instruments calculate one from the other using pressure.