SF6 calculator: gas mass and moisture (dew point ↔ ppmv)
Work out the kilograms of SF6 in a compartment with the van der Waals equation and convert moisture between dew point and ppmv, accounting for the measurement pressure.
Gas mass and moisture: dew point, ppmv and pressure
Work out the kilograms of SF6 in a compartment from its volume, pressure and temperature, and convert moisture between dew point and ppmv taking into account the pressure it was measured at.
Indicative result. The calculation runs in your browser and is not sent to any server. This website may contain errors or omissions: before making any decision about an installation, the information must be checked and validated by a qualified engineer. Legal notice and terms of use
What this calculator does
Two things that are routinely got wrong in the field: how many kilograms of SF6 there really are in a compartment, and what the dew point the analyser just reported actually means.
Gas mass
For the emissions record, for planning a recovery and for deciding whether a leak matters, what you need is mass, not pressure. It is computed from the compartment volume, the pressure and the temperature by solving the van der Waals equation for SF6 (a = 7.857 L²·bar/mol², b = 0.0879 L/mol, M = 146.06 g/mol).
This is not academic refinement. SF6 departs from ideal-gas behaviour far earlier than air: at 6 bar absolute and 20 °C its compressibility factor is around 0.94, so the ideal-gas formula leaves out roughly 6% of the mass. The calculator shows that deviation so the size of the error is visible.
Temperature matters as much as pressure: the same enclosure at −10 °C and at 30 °C reads very different pressures with exactly the same amount of gas. That is why density monitors are temperature compensated, and why a “pressure drop” in winter is not always a leak.
Moisture: dew point and ppmv
After decomposition products, water is the problem with SF6: it lowers surface dielectric strength and, in the presence of arcing, forms corrosive compounds.
The calculator converts both ways using the Buck equations, over ice below 0 °C, which is how dew-point hygrometers report:
- from measured dew point to ppmv;
- from ppmv to the dew point that would be read at another pressure.
That second conversion is what prevents the commonest test error. The water content in ppmv does not change with pressure — the water molecules are what they are — but the dew point does: compressing the gas raises the vapour partial pressure and the water condenses at a higher temperature. Measuring at atmospheric pressure and comparing against a limit stated at service pressure, or the other way round, passes wet gas as good.
Worked example
A 1 m³ compartment at 5 bar gauge and 20 °C, with a dew point of −40 °C measured at atmospheric pressure.
- Absolute pressure: 6.01 bar → density 38.4 kg/m³ → 38.4 kg of SF6. As an ideal gas it would have come out as 36.1 kg: 2.3 kg short.
- −40 °C at 1.013 bar is 127 ppmv.
- That same gas, at 6 bar absolute, has a dew point of −23 °C.
What this calculator does not do
It does not say whether the gas is fit for service: that also depends on purity (percentage of SF6 against air and CF4) and on decomposition products (SO2, H2S, HF), which are measured with the analyser and cannot be deduced from pressure. It does not replace the enclosure’s density monitor or a leak-rate calculation, which needs two measurements separated in time, each with its temperature. And it does not cover SF6/N2 mixtures or alternative gases, whose constants are different.
What measures it
Purity, moisture and decomposition products are measured with an SF6 gas analyser. Leak detection and the equipment to recover, store and refill the gas belong to the same switchgear testing range.
Frequently asked questions
- Why can't SF6 be treated as an ideal gas?
- Because it is a large, heavy molecule that departs appreciably from ideal behaviour already at service pressures. At 6 bar and 20 °C the compressibility factor is around 0.94: computing the mass as an ideal gas underestimates it by about 6%. On a 500-litre compartment that is nearly two kilograms missing from the emissions record.
- Why does the same gas give different dew points?
- Because the dew point depends on the partial pressure of the water vapour, and that partial pressure rises with total pressure. The same ppmv content that condenses at −40 °C at atmospheric pressure condenses above −25 °C at 6 bar absolute. That is why a dew-point figure without the pressure it was measured at means nothing.
- Is moisture measured at service pressure or at atmospheric pressure?
- It depends on the instrument and on the limit being applied. Many analysers measure at reduced pressure and then calculate; guides and limits are usually stated at a specific pressure. What matters is not to compare a measurement made at one pressure against a limit given at another — which is exactly the conversion this calculator does.