Stephen A. Lawrence wrote:
> > If it's pure steam (no entrained air) at 100 C, then the RH must be > 100%, a priori, since the vapor pressure of water at 100 C is 1 > atmosphere. I think it is mixed with air, otherwise the water pump would have to work harder and harder as a vacuum is created in the water supply tank. Harry > And at 101 C, pure steam will have a RH very close to 100%, > since it's only 1 degree above its dew point. So it's not clear to me > what a RH meter is going to tell you that you don't already know, > particularly given that the meter is +/- 3.5 % at 99% RH. (That assumes > they're in Bologna, which is only 54 meters above sea level, according > to some website or other. If they were up in the mountains, the RH of > 101 C steam would be lower, since the boiling point of water would be > lower, and we might expect 101 C steam to be drier, in general.) > Furthermore, I'm not sure it's correct to say <100% RH implies there > can't be entrained droplets. The drops don't form in the steam, they're > blown/broken/pulverized off the surface of the water and carried off > with the steam. They can be carried off with a stream of air at < 100% > RH just as easily as they can be carried off by a stream of pure steam. > If it's <100% RH at the point where you measure it, then some way > downstream the steam may be expected to cool as the droplets evaporate, > which means the temperature reading your probe is reporting won't mean > what you think it does. >
