It is important to note that although the conductivity level gives an indication of the amount of nutrients in the nutrient solution it does not indicate which nutrients, or in what quantities they are present.

The conductivity reading may show that the levels in the solution as a whole are correct for the type of plant you are growing and its stage of growth. However, the individual amounts of specific nutrients may not be correct.

This is best illustrated with an example: when a plant is in the fruit/flowering stage it requires plenty of potassium and phosphorus, so these elements will be rapidly depleted in the solution. Over time, this results in the ratio and availability of nutrients in the solution going out of balance.

In addition, some elements are better conductors of electricity than others. For example, monopotassium phosphate conducts very little electricity so will give a low reading even in large quantities.

The only way to ensure that nutrients are available all of the time is by regular renewal of your nutrient solution. When a plant is in full growth or fruit/flower this may need doing as frequently as every three days, with good maintenance in-between.