Conductivity meter electrodes invariably become coated with inorganic or organic impurities. This causes a loss of accuracy. Conductivity (cF, mS) or TDS measurements are used to gauge the total concentration of all salts (i.e. both beneficial and toxic) present in the nutrient solution. They cannot provide any detail on the concentrations of individual nutrients. Therefore, since a ?satisfactory? reading does not imply a balanced nutrient mixture (or provide information on the level of nuisance salts), it is wise to dump every 7?14 days. For calibration of conductivity (EC) and TDS meters calibrates to 2.76mS/cm or cF 27.6 at 25øC (77øF). Contains potassium chloride.