On 01/22/2011 10:13 PM, mix...@bigpond.com wrote: > In reply to Jeff Driscoll's message of Sat, 22 Jan 2011 21:24:36 -0500: > Hi, > [snip] > >> This probe does not measure the amount of liquid water droplets in the >> "steam" (ie. mass fraction of water vapor to to total water). It measures >> Relative Humidity (Relative Humidity measures how saturated the air is for a >> given temperature). >> What we want is a a device that measures "quality" of the steam. For >> reference, 100% quality = 100% vapor. >> > Consider that droplets can't form unless the RH is 100%. Anything less than > that > and no droplets form. > In short if they measure an RH < 100% then the steam must be dry. >
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. 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. > Regards, > > Robin van Spaandonk > > http://rvanspaa.freehostia.com/Project.html > > >
