This is a super sensitive, precise sensor for just 35 dollars (for example here at adafruit)
It tells you the temperature too.
I adapted this from my tutorial on my blog
I was trying to figure out how to use the Sensirion Parallax humitidy sensor to output humidity data using Arduino. Finally, I found a library If you want to measure humidity data using the SHT11.
Step 1: WHAT YOU NEED
1 SHT11 Humidity Sensor (There are two data sheets, there is the sensirion one, and the parallax one)
A few wires
Step 2: Wire the sensor
At first I found this hard to tell what needed to hook up to Arduino.
Using the datasheet from Parallax was eventually the most helpful for me. (see the first image here or the datasheet, under the "connections" section)
Because there are built in resistors, I was simply able to hook it up like this:
SHT-11 PIN 3 (SCK) ---> ARDUINO PIN 11 (SCK)
SHT-11 PIN 4(VSS) ----> ARDUINO's GROUND
SHT-11 PIN 1 (DATA) ----> ARDUINO PIN 9 (data)
SHT-11 PIN 8 (VDD) ----> ARDUINO +5V
Step 3: Program Arduino
Download the library from https://github.com/practicalarduino/SHT1x
Move Sht1x folder into Documents/Arduino/Libraries
Under File-->Examples, you should see SHT1x
Open it (called ReadSHT1xValues.pde)
Upload the sketch,
Step 4: See the values
Temperature: 25.4199981689C / 77.8459930419F. Humidity: 23.45%
Temperature: 25.4399948120C / 77.8639984130F. Humidity: 23.45%
Temperature: 25.4799957275C / 77.7919921875F. Humidity: 25.12%
Temperature: 25.4599990844C / 77.8999938964F. Humidity: 27.65%
Temperature: 25.3600006103C / 77.7739944458F. Humidity: 27.87%
Temperature: 25.5199966430C / 77.9719924926F. Humidity: 26.86%
and so on.
This sensor is supposedly very accurate and also, very inexpensive.
(More to come... HOW TO LOG THIS DATA and VISUALIZE IT in a graph...)