The main difference between PPM and EC is that:
1. With the EC (electrical conductivity) you express (measures) the strength of the
nutrient solution in its total. Basically there is no way to know (or measure) content of single elements compounds (NPK, Ca, Mg, S, etc..)
2. With PPM either the amount of
single components or the total amount of "parts per million" (strenght) of a nutrient solution is measured or expressed.
Example of a formula:
N=180 PPM
P=50 PPM
K=250 PPM
Mg=50 PPM
Ca=180 PPM
1 PPM means 1 milligram (of the respective element) contained in 1 Liter (water) respectively 1 gram contained in 1000 liter.
Most importantly: PPM refers to the ACTUAL value in molecular weight (actual elemental content), while classical NPK formulas don't.