just a quick word from me to share my design and code for a setup I built over Christmas to measure relative humidity in our apartment.
Code, images and circuit documentation can be found here: https://github.com/aweijnitz/Hygrometer
Sample data from logging the output (with sub sequent sub sampling) can be viewed here: http://wohnung.se/arduino/hygrometer/data.htmlBackground
It is common to find apartments without proper ventilation in Germany (where I live). This is not even legal in Sweden (where I come from). Without proper ventilation, you face a whole range of issues with the indoor climate, bad air and particles is just a start, condense and mold is at the other end of the spectrum. This triggered the idea to let my next project be a hygrometer. That and a wish to build a nice balcony greenhouse this year. :-)
Looking for input on approaches you find a whole lot of questions and discussions in this forum, but not so much solid information, so that's why I decided to post my project here. It far from perfect, but it is at least a working end-to-end circuit and sketch to look at and perhaps get inspired by.Notes
I believe my algorithm to measure the relative humidity has some issues, but I haven't gotten to a point where I can say what and why. Just notice a few odd things and "jumps" in my data. Ultimately it might be down to the crude approach of using a lookup table and interpolation to come up with the value. Input more than welcome!
Another thing to note is the (unusual?) way the EFS-10 humidity sensor works. You have to drive it with a square wave and then the impedance of the sensor will vary with the relative humidity. Not the most straight forward thing to use, at least if you are a inexperienced like me. Still haven't found any good sources for dealing with such sensors. If you have any pointers, it would be very welcome.
Many thanks to MarkT who provided the crucial pointers and code snippets to get me on the right track!