The SHT15 is a digital humidity sensor that outputs a fully calibrated humidity reading. And… because what we are measuring is actually relative humidity, and relative humidity being relative to temperature, the SHT15 has a builtin digital thermometer. This makes things much easier to work with than sensors without a thermometer onboard. You can pick one up from sparkFun here.
Hooking it up
The SHT15 uses a two-wire connection for communication that is similar to, but not, I2C. So we wont be able to use the Arduino’s dedicated lines for this. The down side is it is a bit slower to get readings from, the plus side is that you can connect it to any 2 digital pins you want. We are using pins 2 and 3 on our arduino.
BEFORE YOU SOLDER IT UP… Note that this board can not be washed! So if you are using flux, or solder that you normally clean up, don’t (They actually recommend to use “no-clean” solder just so you don’t have to worry about it). And be extremely careful not to get it wet at all.
AFTER YOU SOLDER IT UP… To get a clean reading, the sensor needs to be stored at >75% humidity for at least 12 hours to allow the polymer to re-hydrate (just what the doc says). If you don’t, your SHT15 may read an offset that slowly disappears if exposed to ambient conditions. Alternatively the re-hydration process may be performed at ambient conditions (>40% Humidity) for 5 + days.
Im not exactly sure how you do that… But someone noted that they put it in a ziplock with a wet towel (not touching) for 12H.
For more detail: Sensing Humidity With The SHT15 + Arduino