Finlay, you did some homework and presented the results... and I tend to agree with your results even though you're a chemist! :-) You're most welcome to join in the discussion... Yes, I have worked with RF and microwave technologies up to 20Ghz, and there are companies that make humidity measuring devices in grain silos that operate on the dielectric constant of water vs water vapor. Since some of your results are at odds with others, then it would really help to provide the links to your sources...
-Mark _____ From: Finlay MacNab [mailto:[email protected]] Sent: Wednesday, June 22, 2011 8:34 PM To: [email protected] Subject: RE: [Vo]:E-Cat vs. Water Heater for coffee/tea... If the relative humidity sensor measures capacitance then the dielectric constant of steam and the dielectric constant of steam plus water would be very different and yield very different readings. A quick google search for capacitance measurement of steam quality yields several papers and a multitude of patents on the subject so it would seem that a measurement of steam quality from capacitance values is possible. A quick literature search for the dielectric constant of steam results in an avalanche of data about the dielectric constant of steam at various temperatures and pressures. There is even data at 100.1C and 1 atm. Then again I am just a lowly chemist so what do I know? > Date: Wed, 22 Jun 2011 23:02:51 -0400 > To: [email protected]; [email protected] > From: [email protected] > Subject: Re: [Vo]:E-Cat vs. Water Heater for coffee/tea... > > At 06:45 PM 6/22/2011, Jed Rothwell wrote: > >Joshua Cude wrote: > > > >>If the meter is giving mass per unit volume of the output, you need > >>to know the *volume* of the output to get the mass of the steam. > > > >Ah. Here is what you overlooked. It also says that it gives mass of > >water per unit of mass. That is "degree of humidity (g/kg), partial > >pressure in water vapour in mbar/hPa." > > > >and: "Ethalpy kcal/kg" (Interesting that they use kcal.) > > > >See: > > > >http://www.testo.com/online/embedded/Sites/INT/SharedDocuments/ProductBrochures/0563_6501_en_01.pdf > > > >It also measures "Absolute humidity g/m^3" which is what you had in mind. > > > >- Jed > > > You do understand, Jed, that this is the Testo meter used by Essen > and Kullander, not the other meter used by Galantini, right? > > The absolute humidity is calculated from relative humidity and > temperature and pressure. These are displayed values, not measured > values. Basically, the device does some math for you, based on > certain assumptions. Unfortunately, the assumptions are the very issue here! > > Among other things you must notice are the specifications of the instrument: > > Notice that calibration certificates are not available for over 95% > RH, nor at any temperature over about 80 C. > > One page 6, the list of humidity probes begins. The "robust" probe, > part number 0628 0021, is rated to 180 C. The measurement range > extends from 0 to 100% RH. However, the accuracy is not rated above > 98%. Basically, the accuracy is 2%, from 2 to 98% RH. > > Technically, wet steam would have a "relative humidity" above 100%, > that is, the water content of the mixture would exceed that of pure > water vapor, defined, as I'm correct, as 100% humidity at the boiling > point. If I've got this right! > > But the meter has no capacity to measure that excess water, it would > simply peg at 100%, it seems. > > I see no sign, anywhere, of any expert opinion that RH meters have > any application to the measurement of steam quality. Instead, as Cude > and others have pointed out, there are complex methods described to > measure steam quality. If you could do it with an RH meter -- and the > devices described are merely fancy RH meters that will do some math > for you based on temperature and pressure and RH data -- all these > other methods would be unnecessary, and I'd expect the meter > manufacturers to advertise the function, and even provide a direct > calculation mode. > > > > >
