On Thu, Jun 23, 2011 at 1:30 AM, Harry Veeder <hlvee...@yahoo.com> wrote:
> > Joshua, and I think Abd, believe "...steam inside the conduit is always at > 100% RH. Regardless of what fraction of the water is converted to steam. At > 100C, the vapor pressure is 1 atm, and the steam pressure (also the partial > pressure of the water vapor) is also 1 atm. Ergo, 100% RH. > > > I think the RH of the steam is 0% when it is fully dry just like the RH of > fully dry air is 0%. > You can correct me if I am wrong, but here is my reasoning: > > Dry steam is water in the form of a gas and only a gas. (I prefer the word > gas over the word vapour because the meaning of water vapour is highly fluid > in common paralance. Pun intended) Water can only exist as a gas if > the atmospheric pressure drops considerably or if the temperature rises > considerably or through combination of the two. > > Therefore at room temperature and pressure water does not exist as a gas, > and the humidity of the air consists entirely of an extremely fine > suspension of liquid water drops. Air at room temperature and pressure is > free of water gas. > > When the relative humidity of the air (or some other gas) reaches 100% it > can't hold any more liquid water drops. > > On the other hand my assertions about water gas are hard to reconcile > with the phenomena of water evaporation so I may well be wrong! > You are indeed wrong. Time for a refresher. Look up vapor pressure in wikipedia for a start. Water evaporates into pure gas (not droplets) below its boiling point. Humidity measures the amount of water vapor (gas, not droplets) in the air. When the partial pressure of the water vapor in air equals the vapor pressure of water, evaporation stops, or at least it balances condensation. That represents 100% humidity. The ratio of the partial pressure of the water vapor to the vapor pressure of water is the relative humidity. The RH of steam at 100C and 1 atmosphere is therefore 100%.
