How pure is your brew water base? Is your tap water soft or rich in minerals? How much total mineral content does your RO system remove? Does your water comply with competition regulations? Test total dissolved solids in water quickly with 1ppm resolution up to 990 ppm, and 10ppm resolution to 9990 ppm. Accuracy: +/- 2%. 

This product is recommended by The Coffee Chronicler 

Water TDS Meter