It is essential to calibrate your EC (electrical conductivity) meter with CONDUCTIVITY 2.76 solution regularly. The measurement of available nutrient in the solution is measured as the electrical conductivity. The units used to report the readings are millisiemens per centimetre (mS/cm), Conductivity Factor (cF) or parts per million (ppm).
The conductivity increases with nutrient strength. Distilled or deionised water will give a near zero reading. As a general guide for hydroponics: 1mS/cm = 10cF = 700ppm. Nutrient requirements vary from plant to plant; therefore the ideal conductivity varies from plant to plant also.