On Wed, Jun 22, 2011 at 7:08 PM, Jed Rothwell <[email protected]> wrote:
> Joshua Cude <[email protected]> wrote: > > >> So, what specifically do you think that g/kg means in the context of a >> 2-phase mixture of steam and water? >> >> What do you use for the denominator to calculate the total mass of the >> steam? >> >> If it means the mass of water vapor per unit mass of water vapor, then it >> should be 1000 g / kg. How do you use that? >> > > If 10% by mass is liquid water then it would be 900 g/kg. That's the whole > point. > Again, how can a device that measures humidity as mass of water vapor per kg of air (or any other gas), give the mass of water vapor per kg of a water vapor and liquid mixture. The two are not the same thing at all. I don't know the exact construction of the probe, but it seems to me there are 2 possibilities. 1) Either that capacitor is exposed only to whatever gas is present, by somehow blocking liquid, in which case it is exposed to pure water vapor at atmospheric pressure (more or less) regardless of the steam quality. In that case, it will read the same thing no matter how wet the steam is. 2) Or the capacitor is exposed to the mixture of liquid (mist) and vapor. If liquid droplets contact the capacitor, you would expect the dielectric to be pretty well saturated with water, and as the steam becomes drier, the capacitor becomes less likely to be contacted by liquid and so the dielectric would be drier. Under the design conditions, a wetter dielectric indicates more water vapor per kg of air, but in this case a wetter dielectric would indicate less water vapor per kg of mixture. That means that not only is the device not calibrated for the purpose of measuring steam dryness, but its reading would have the opposite gradient. So, I do not believe that the measure of capacitance calibrated to indicate the mass of water vapor per kg of air has any relevance to the question of liquid content in steam. The manufacturers make no claim to this application, and people who are interested in steam quality do not seem to use such probes. It is entirely plausible that a few academics, who may never have actually had a need to study steam quality in any detail, and who may not have taken the time to do some homework, might be susceptible to the suggestion that such a probe can be used for that purpose. The evident reluctance to disclose the actual measurement values suggests they are not particularly confident of the claims. > How could it measure enthalpy or partial pressure of vapour if it doesn't > know how much vapour there is? > The partial pressure of the vapor is in fact the most direct measurement. That's what determines how wet the polymer is, and what capacitance gets measured. In the case of pure steam at atmospheric pressure, the partial pressure is ... 1 atmosphere. You don't need a meter to get that result. It doesn't measure enthalpy, it calculates it based on the temperature and RH in air. It most certainly does not give the enthalpy of wet steam.
