Disclaimer: I'm not a physicist or chemist, just a software developer who struggled with all those water vapor related concepts for a few weeks himself.
Dew-point vs. boiling-point
I'm not sure if this holds true for all substances, but water does not only vaporize when it reaches it's boiling point. Instead, a few molecules will always leave the surface of liquid or even frozen water. Depending on the partial water vapor pressure of the surrounding gas mixture, those might be balanced by the water molecules that condense / sublime back into liquid or solid phase. The temperature at which those two processes balance is called dew-point.
The dew-point depends on
- the temperature (of the water in all it's states)
- the partial water vapor pressure
- the surface geometry of the liquid / solid water
- presence and abundance of particles suspended in the air mixture that promote condensation on their surface
- presence and concentration of salt or other impurities on the surface of the liquid / solid water
As user @pentane has stated in his answer, the boiling-point does depend on the total air pressure, the dew-point in contrast depends on the partial water vapor pressure instead.
Unlike the boiling-point,
The dew-point does NOT depend on the total air pressure in the system!
The Pressure Dew Point (PDP) formulas that user @tony-dinitto talks about in his answer let you calculate how the dew-point changes when you change the pressure in the system while holding the percentage of water vapor in the air mixture constant.
If you're like me dealing with a system, where the sensor measures temperature, relative humidity and total air pressure "at the same time" -
and you don't need to predict what would happen if you encapsulated this exact air mixture and compressed or expanded it, then Pressure Dew Point (PDP) formulas are of no use to you.
According to Wikipedia, the Arden Buck Equation is the most accurate formula to calculate the saturated water vapor pressure for a given temperature. Using this fact I came up with the following way to
calculate the dew-point:
- Calculate saturated water vapor pressure for current temperature using the Arden Buck Equation.
- Calculate the current water vapor pressure by multiplying the saturated water vapor by the relative humidity as a factor (divide by 100 if given as percent)
- Use the inverse of the Arden Buck Equation to calculate which temperature would have the current water vapor pressure as saturated water vapor pressure.
Since for me it was quite difficult to find the inverse of the Arden Buck Equation, I'll post what I came up with to make it easier for readers who try to follow my steps:
let (a, b, c, d) = (611.21, 18.678, 234.5, 257.14); // empirical constants of the Arden Buck Equation for temperatures > 0°C
let g = (actual_water_vapor_pressure_pa / a).ln();
-0.5 * c * (((b - g).powi(2) - (4.0 * d * g) / c).sqrt() + g - b)