EC or PPM?
The debate over EC and TDS has been an ongoing issue for a long time.
These two measurements are used to determine the strength of hydroponic solution. Although they are widely used they should only be used as a guideline and you should always follow mixing instructions on the label of your nutrient.
EC stands for Electrical Conductivity and is measured in mS/cm or millisiemens per centimeter. TDS stands for Total Dissolved Solids and is measured in PPM or parts per million. TDS is acquired by taking the EC value and performing a calculation to determine the TDS value. Because TDS is actually a calculation it is really only a guess at what the nutrient concentration is.
On top of that, there are three different conversion factors to determine TDS and different manufacturers use different conversion factors. In other words you could test the same solution with two different meters and get two completely different readings. However, the EC is read the same by all meters the only difference is the conversion factor.