SF₆ moisture: dew point, ppmv and pressure

Across the range

Moisture content for each dew or frost point at the sensor pressure entered above, and the dew or frost point the same gas would show at the target pressure. The table follows the calculator settings.

Method and scope

Water in SF₆ is present as a trace vapour. A hygrometer measures its dew or frost point: the temperature at which that vapour would start to condense at the pressure the sensor is sitting at. The moisture content follows from the partial pressure of water, and that is what lets a reading taken at one pressure be compared with a limit stated at another.

e(T) is the saturation vapour pressure of water, over ice or over liquid water, from the equations of Sonntag (1990) on the ITS-90 temperature scale. Both pressures are converted to absolute on input and worked in pascals throughout. The converted dew or frost point is found by solving the same equation numerically. The implementation was checked against the Hyland–Wexler formulation at 5 °C intervals: agreement is within 0.09 % over water from −40 to +50 °C and within 0.11 % over ice down to −80 °C. At −40 °C a 0.1 % difference in vapour pressure is about 0.01 °C of dew point.

What this assumes

• Ideal mixing. The enhancement factor, which raises the effective saturation pressure slightly in a compressed gas, is ignored. At filling pressures up to about 1 MPa this is an error of the order of 1 % in ppmv, well inside the uncertainty of a field hygrometer.
• Frost point and dew point are not the same reading. Below 0 °C water can condense as ice or as supercooled liquid, and the two give different numbers for the same gas. At −40 °C at atmospheric pressure, 127 ppmv shows as a frost point of −40 °C, while a dew point of −40 °C corresponds to 188 ppmv. Set the toggle to match the instrument, or the result is wrong by tens of percent.
• The pressure is the pressure at the sensor. Many portable hygrometers sample through a needle valve and measure at atmospheric pressure; in-line sensors measure at enclosure pressure. A dew point reported without the pressure it was measured at cannot be converted.
• The sample is representative. Moisture desorbs from enclosure walls and from the sampling hose. A reading taken before the hose has dried out, or straight after filling, reads high and drifts down. Temperature matters too: the same enclosure gives a higher reading on a warm afternoon than on a cold morning.
• Range. Dew and frost points from −80 to +20 °C. Pure SF₆ is assumed for the ppmw figure; for mixtures the volume figure still holds, but the mass figure needs the mixture molar mass.

Using it

• Atmospheric reading to enclosure conditions. A portable instrument reads a frost point of −40 °C at atmospheric pressure. At an enclosure filling pressure of 0.6 MPa gauge the same gas has a frost point of about −21.6 °C. Same water content, 127 ppmv; only the pressure has changed. This is the comparison to make before deciding whether condensation inside the enclosure is possible on a cold night.
• Dew point to ppmv. Where a limit is quoted in ppmv, enter the reading and its sensor pressure and read the volume figure directly. To convert between pressure units on their own, use the pressure unit converter.
• Comparing two instruments. Two hygrometers that disagree are often measuring at different pressures, or one reports frost point and the other dew point. Convert both to ppmv before deciding which one is wrong.

Provided for reference and cross-checking. Does not replace equipment manufacturers' data, nameplate values or operating instructions, or the applicable standards. Version 1.0. Source: D. Sonntag, saturation vapour pressure formulations over water and ice, 1990.