>From this document http://www.macinstruments.com/pdf/handbook.pdf, from a >website trying to sell absolute humidity gauges, it would appear that a >relative humidity sensor can give accurate reading up to the boiling point of >water and that the measurement of humidity decreases in dry steam as the >temperature of the super heated vapor increases. The delta ohm probe in question is rated to 150C with an accuracy of +/- 3.5% above 95% RH from this spec sheet http://www.deltaohm.com/ver2010/uk/st_airQ.php?str=HD37AB1347. If the temperature of the vapor is above 100C and the pressure is 1 atm, then an an examination of the phase diagram of water suggests that no liquid water can be entrained in the vapor. Under these conditions the steam would be dry and the humidity sensor would read <= 100 +/- 3.5% according to the above information. The fact that the steam exiting the hose in the video is invisible is very strong qualitative evidence that the steam is relatively dry. Since the steam can only become wetter after it's exit from the chimney, it must be more dry when it is produced than when it exits the hose. If the temperature and pressure were measured accurately then the steam is likely to be significantly dry. In summary, it would appear that if the water is superheated then the steam is dry.
Date: Wed, 22 Jun 2011 23:52:51 -0500 Subject: Re: [Vo]:E-Cat vs. Water Heater for coffee/tea... From: [email protected] To: [email protected] On Wed, Jun 22, 2011 at 10:34 PM, Finlay MacNab <[email protected]> wrote: 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. >From what I found, it is not the dielectric constant of the fluid that >determines the capacitance, but the dielectric constant of a polymer which >absorbs more or less water depending on the humidity. I found variations of >this paragraph at several different sites: "Most humidity sensors use capacitive measurement to determine the amount of moisture in the air. This type of measurement relies on the ability of two electrical conductors to create an electrical field between them with a non-conductive polymer film laying between them. Moisture from the air collects on the film and will cause changes in the voltage levels between the two plates. " (www.tech-faq.com/humidity-sensors.html) In this case wet steam is likely to give a higher reading than dry steam, which would give exactly the wrong information. In any case, if the device does actually give different measures for different wetness of steam, it would have to be calibrated for the purpose. The manufacturer does not do this. They calibrate it to represent the humidity in air.
