Fertilizers designed for hydroponics require a precise application of fertilizers. The volume of minerals in a feeding solution must be carefully measured and then regularly monitored throughout the grow cycle to maintain ranges that are safe for plants.
The volume of minerals dissolved in water is measured using an EC metre. EC stands for Electric Conductivity. EC metres work by measuring resistance to an electrical charge between two filaments while submerged in a feeding solution. Dissolved solids affect resistance allowing the EC metre to approximate the volume of dissolved solids in water.
Some EC metres output their measurement as TDS or PPM instead of EC. TDS stands for Total Dissolved Solids and PPM stands for Parts Per Million. TDS or PPM are different standards of measurement, similar to inches compared to centimetres.
A conversion from EC to the other measurement standards is simple:
1 EC = 500 TDS
1 EC = 700 PPM or 500 PPM*
*PPM is not a consistent measurement standard because sometimes it is 500 EC and sometimes its 700 EC. Use EC or TDS for your measurement standard to avoid accidental calculation errors.