Standard reference solutions are used. The bottles are marked with the conductivity (EC) value in milliSiemens/cm and the corresponding ppm values for sodium chloride (NaCl) and potassium chloride (KCl) solutions, and sometimes for a “442” reference mixture. The conductivity of sodium chloride solutions is close to that of hydroponic mineral nutrients, so a “1000-ppm NaCl” standard is most frequently used when calibrating the meter for hydroponic solutions. You should follow the calibration instructions in the manual, which the manufacturer of your meter provided.
