I've spent hours trying to understand conceptually how to find the density of moist, hot air at high temperatures, eg at 600° C. Any help would be great. This is not a homework problem, but I'm preparing for a problem i will likely encounter shortly at work, and thermo is outside my specific expertise. I'd like the density because i would like to find the Reynolds number of an air flow (i need Rey# to find air flow in a tube by two pressure measurements). I'm new to thermo and fluids, but am learning as fast as possible. 1. Generally, density of moist air is a simple equation Density = Pdry / (Rdry*T) + Pwet / (Rwet*T) Finding the water vapor pressure Pwet is proving tricky. Of course once I have Pwet I can find Pdry = P - Pwet, and evaluate the above equation. The system is near atmosphere, ~1000 hPa. There are equations by Herman Wobus, and also the simplified version Pwet=c0*10(c1*T/(c2+T) ), but both equations break down for high temps. How does one find the vapor pressure at high temperatures? 2. Complicating the above is that it's unclear how to find the Relative Humidity at these temps. Is it correct that the Dew Point is needed as the temp T inserted into the Wobus equation, which i tried using to find Pwet? My knowns are the amount of water removed from a mixture at a given temp, and hopefully also a given air flow. To find the Dew Point i need to find the RH, which i tried using Psychrometric tables; none go to temps that high, nor are they accurate enough at high slopes. Thoughts? Am I missing something fundamental here? I'm in a loop, with my goal being calculating an air flow of a known temp, with a known pressure drop. To find Density I need Dew Point, but to find Dew Point requires R.H., but to find RH I need gas flow, but to find the gas flow I need the Reynold's Number and thus Density.