On Thu, Jun 23, 2011 at 4:10 AM, Mark Iverson <[email protected]> wrote:
> ** > > Yes, agreed that at the most fundamental level it is making an electrical > measurement, that being capacitance. However, since relative humidity is a > moving target depending on the temperature, RH is usually calculated from > absolute humidity and temperature. > > I'm not making this up... this is from direct experience... a few years > ago we were using a temp/humidity sensor in the lab and I wrote the code to > query it and get its data. I believe it too was a polymer/capacitive sensor > and what it measured was absolute humidity (which doesn't change with > temperature), and the user manual provided an equation to convert that into > RH given the temperature, which it also measured. > If that device used a capacitive probe, then I doubt it measured absolute humidity, independent of what it reported to the user. Because the same absolute humidity at different temperatures would result in different wetness of the polymer, and therefore different capacitance measurements. Therefore, it certainly cannot deduce the absolute humidity from the capacity measurement alone. The wetness of the polymer will have a much simpler relationship with the RH than with the absolute humidity. There may still be a temperature dependence, but it will be weaker, and probably it is calibrated at different temperatures. And that's probably why it's only valid within a range of temperatures. If the device is calibrated at different temperatures, and uses the temperature to calculate the humidity, then it is entirely equivalent whether it calculates relative or absolute humidity. But if it deduces humidity from the capacitance measurement alone, then it can only be the relative humidity. The fact that you don't actually know the technology used in the probe in your lab, and that a capacitance measurement alone cannot give a unique absolute humidity, but that it could do reasonably well at giving relative humidity, suggests you were just guessing.
