You can easily measure and maintain nutrient strength levels with the waterproof Bluelab Truncheon Meter.
Truncheon reads in EC (0.2-3.6), CF (2-36), TDS/ppm 500 (100-1800) and
ppm 700 (140-2520). Accurate within ± 4% of reading with a resolution of
0.1 EC, the Truncheon runs on three AA alkaline batteries and comes
with a five-year manufacturer's guarantee.