EC, which stands for electrical conductivity, is the most accurate way to measure the concentration of your nutrient solution. TDS meters use a calculation to determine the PPM of the solution. Those calculations are different from manufacturer to manufacturer which has caused a lot of confusion. To remedy that we are now selling the EC meter from Oakton. This meter is waterproof and it floats to protect the meter from accidental drops into the reservoir. Includes batteries and information on calibration and cleaning. Specifications: Range 0 to 19.90 mS Accuracy 1% full scale Automatic Temperature Compensation Auto shutoff after 8.5 minutes of nonuse