A really cheap way to measure the ppm of fluids for arduino.
Change the R1 Resistor and you can use it for a wide range of applications:
-Aquaponics
-Hydroponics
-Drinking Water Contamination
For Arduino Code go to:
www.MichaelRatcliffe.com
For more Information and an updated script see:
https://community.element14.com/challenges-projects/design-challenges/vertical-farming/b/blog/posts/automated-green-house-blog-10---three-dollar-ec-ppm-meter-for-mcu
or
http://www.element14.com/community/community/design-challenges/vertical-farming/blog/2015/09/13/automated-green-house-blogs-main-page