Dew point ⇔ moisture concentration (ppm) converter
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KEY POINTS
Dew point is the temperature at which the partial pressure of water vapor equals the saturation vapor pressure. At atmospheric pressure, a dew point of −40 °C corresponds to about 127 ppm, −70 °C to about 2.6 ppm and −100 °C to about 14 ppb. Bulk gases in semiconductor fabs are dried by purifiers to −100 °C or lower. Dew point depends on pressure, so compare specifications in ppmv.
Inputs
Below 0 °C the saturation vapor pressure over ice (frost point) is used. Some hygrometers show the dew point over supercooled water even below 0 °C, which differs by about 4 °C near −40 °C.
Dew point and moisture at a glance (atmospheric pressure)
| Dew point | Water vapor partial pressure (Pa) | By volume | By mass in nitrogen |
|---|---|---|---|
| -100 °C | 1.37×10⁻³ | 13.5 ppb | 8.67×10⁻³ ppmw |
| -90 °C | 9.53×10⁻³ | 94.0 ppb | 0.0605 ppmw |
| -80 °C | 0.0543 | 536.1 ppb | 0.345 ppmw |
| -70 °C | 0.261 | 2.57 ppm | 1.66 ppmw |
| -60 °C | 1.08 | 10.7 ppm | 6.86 ppmw |
| -50 °C | 3.94 | 38.9 ppm | 25 ppmw |
| -40 °C | 12.8 | 127 ppm | 81.6 ppmw |
| -20 °C | 103 | 1,020 ppm | 656 ppmw |
| 0 °C | 611 | 6,070 ppm | 3,900 ppmw |
| 10 °C | 1,230 | 12,200 ppm | 7,880 ppmw |
Why fabs control dew point
Bulk gases in semiconductor fabs (nitrogen, argon, oxygen, hydrogen) and specialty gases are supplied with moisture reduced to the ppb level. Moisture causes oxygen contamination and defects in epitaxial growth, and with corrosive gases such as hydrogen chloride and chlorine it corrodes stainless steel piping, producing particles and metal contamination. Gas purifier outlets therefore typically aim for a dew point of −100 °C (about 14 ppb) or lower.
Dew point depends on pressure: for the same amount of moisture, the dew point measured in a high-pressure line is higher than the value converted to atmospheric pressure. Check whether a specification gives the dew point at atmospheric pressure or at line pressure, and compare values as moisture concentration (ppmv) to avoid mix-ups.
FAQ
Why does dew point depend on pressure?
Compressing a gas raises the partial pressure of its water vapor, so the same moisture content condenses at a higher temperature. A ppmv value does not change with pressure.
What dew point do fab gases need?
Bulk nitrogen and other process gases are typically purified to a dew point of −100 °C or lower, which is about 14 ppb at atmospheric pressure.
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Last updated: October 23, 2026