# How to calculate dew point of air-water system

I want to calculate the maximum water holding capacity of air at a given temperature (as in $$\frac{max\ mass\ of\ water}{mass\ of\ dry\ air}$$ vs temperature) I looked up online and there are various models to do such calculations. However for most of them the temperature range is not more than 100 C. Are there any relations which cover a higher temperature range, upto 300-400 C? If not, is not possible to determine saturation point above the boiling point of water?

Edit: I am considering a constant atmospheric pressure. Thus, I want to check variation only with temperature.

• Above the boiling point, water becomes a genuine gas, much like air. Gases don't have saturation points. They just mix. – Ivan Neretin Apr 30 '19 at 14:30
• You can get above 100 C by increasing the pressure of course. – MaxW Apr 30 '19 at 15:03
• @IvanVeretin Water becomes a gas above the critical point, not the boiling point. – Chet Miller Apr 30 '19 at 17:19
• @Chet Miller No, liquid water starts becoming gas by boiling, when temperature reaches boiling point. The water phase diagram. The critical point just means above this temperatures water does not form separate phases, as their properties at given p,T just converged to the same values. – Poutnik May 1 '19 at 7:41
• The conceptual mistake is, that the holding water vapour capacity is property of air. It is not. It is property of space. The air is just an obstacle and bystander.( with slight difference due nonideality). – Poutnik May 1 '19 at 7:45

How to calculate dew point of air-water system? I want to calculate the maximum water holding capacity of air ... However for most of them the temperature range is not more than 100 C. Are there any relations which cover a higher temperature range, up to 300-400° C? If not, is it not possible to determine saturation point above the boiling point of water?

At very high temperatures I could only find the Mollier enthalpy-entropy chart (for steam) here's one for up to 1931.67 degrees rankine (800 degrees celsius) and another for up to 2200 degrees rankine (949.072 degrees celsius):

Source: "Mollier H-S Diagram"

Source: Wikipedia - Enthalpy–entropy chart

On the diagram, lines of constant pressure, constant temperature and volume are plotted, so in a two-phase region, the lines of constant pressure and temperature coincide. Thus, coordinates on the diagram represent entropy and heat.

The Mollier diagram coordinates are enthalpy $$h$$ and humidity ratio $$x$$. The enthalpy coordinate is skewed and the constant enthalpy lines are parallel and evenly spaced.