About measuring EC
So what is being measured by EC? The concentration of dissolved salts. To indicate and measure the EC, the unit used is milliSiemens or microSiemens per linear centimeter (where 1mS = 1000µS), or in short mS/cm. Often a mistake is made by thinking that the only EC measured is of the initial fertilization solution. However, drain water should also be monitored for its EC levels, as it is a great indicator of how much nutrients the plants absorb. Also critical is to monitor the EC levels within the stone wool substrate.
When looking into the substrate, the EC levels in the substrate are fluctuating throughout the day. There are several aspects that can influence the EC levels, such as the pH, water content (WC) of the substrate, room temperature and humidity factors, and more. For instance, the higher the WC in the substrate, the lower the EC will be. That is why, for example, after the dark period (when no irrigation is provided), the EC levels in the substrate will be rather high, and will gradually decrease again when you realize drain. That can be further seen in the Figure below.