Milwaukee CD611 EC Tester

Milwaukee EC Level Tester Features

PS: If you are growing organically you won't really need it in principle.

The Milwaukee EC Conductivity Tester is designed for hydroponics and others to help ensure your plants are receiving the right amount of nutrients. 

Measures conductivity and temperature with automatic temperature compensation
Ideal range for hydroponics (0 to 20,000 µS/cm)
Units in µS/cm easily convertible to ppm
Factory calibrated with the ability to recalibrate if necessary
350 hours of continuous use (3 x 1.5 V batteries included)