Technically they are entirely different. Total dissolved solids is a measurement of actual dissolved solids in solution. EC is only a representation of the resistance in solution. The two are theoretically not interchangeable and entirely different. Your confusion comes in from companies converting the resistance into a possible representation of total dissolved salts or "part per million". This conversion is based on particular parameters and works more as a representation than an actual account for the solute. Try aim to use EC rather than TDS/PPM. If you know what you put into your solution, you can use a bit of simple math and work out your concentration by resistance.
alemo's post in EC VS TDS was marked as the answer
