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: "Mollier enthalpy-entropy chart for steam, US units"
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.
See the Wikipedia webpage for more information, or assistance reading the diagram.
For restricted ranges (of which you complained, but are useful for comparison and to view the bottom of the chart) see Wikipedia's webpage for "Psychrometric charts":
Source: "Psychrometric Chart for Sea-level pressure using SI units"
Wikipedia's webpage for "dew point temperature" shows some calculations and means to make simple estimates, but it cuts you off at 100°C.