On Wed, Jun 22, 2011 at 10:34 PM, Finlay MacNab <finlaymac...@hotmail.com>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.
