On Tue, Jun 21, 2011 at 11:52 PM, Harry Veeder <hlvee...@yahoo.com> wrote:
> from http://en.wikipedia.org/wiki/Relative_humidity > -------------- > A common misconception > [...] > -------------- > > Reading this makes me think Galantini used the probe correctly. > > Harry > > That probe uses a capacitance measurement to determine the relative humidity. Typically, this measurement is made using a capacitor with a polymer dielectric which absorbs or releases water proportional to the relative environmental humidity, and thus changes the capacitance of the capacitor, which is measured by an onboard electronic circuit. ( www.sensorland.com/HowPage047.html) Such a device is calibrated in air, to represent the partial pressure of water vapor in air. It is not at all clear how that measurement can be used to determine the amount of mist (liquid droplets) entrained in water vapor. It seems likely that a mist-steam mixture would cause the polymer to be wetter than if the steam were dry. So a higher RH reading would indicate wetter, not drier, steam. Probably, inside that conduit, the polymer is saturated with water no matter what, and it reads close to 100% RH all the time. In any case, if the device is to be used to determine liquid content in steam, it would at least have to be calibrated for that purpose. There is no indication such a calibration was performed. It would be possible, just from the experiments performed, to determine if the RH probe were of any use. If the RH readings were *monitored* on a continuos basis, like the temperature, and *reported*, we could see if the reading ever actually changes. Presumably the steam must begin wet and then become drier as the power transfer increases. During this process, does the RH reading on that probe change? If it doesn't, whatever it is measuring is not relevant to the liquid content of the steam. There are two very simple ways to prove the steam is dry: (1) Measure the output flow rate (velocity); if it is steam, it should be 1700 times higher than the input flow rate; (2) Reduce the input flow rate so the steam temperature exceeds boiling by more than a few degrees -- say 120C or so. That these two methods are not used suggests the steam is not dry.
