In reply to Stephen A. Lawrence's message of Sat, 22 Jan 2011 23:05:10 -0500: Hi, [snip] >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.
True, but they are likely to have measured somewhere past the end of the tube, so I suspect there sill be at least some entrained air. >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. Also true, it may not be the best option for task at hand. >(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. True if it's pure steam. However if there is some entrained air (after exiting the outlet tube), then any droplets that may have been in the steam will evaporate and raise the RH of the air to 100%, i.e. until it is saturated, once again resulting in 100% RH. >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. What happens downstream is irrelevant unless you are taking measurements there and relying on them. Regards, Robin van Spaandonk http://rvanspaa.freehostia.com/Project.html
