Camille, in my mind, it is that discrepancy that makes TDS a terrible control factor.
Most professional growers of commercial greenhouse crops here use "ppm N" as their controlling factor, having selected a formula that provides the correct balance of nutrients. Fertilizer manufacturers provide the EC versus ppm N data, so an EC meter becomes the easy-to-use tool for controlling the concentration.
For example, poinsettia growers all target for 1/2 gram of nitrogen fed to the plants over the time period from the planting of rooted cuttings to harvest. They know the volume of the pot and the solution retention of their medium, so by knowing the time to harvest and frequency of watering in their controlled greenhouse environment, they know what concentration of fertilizer to apply, so that the seasonal sum is 0.5g N.
TDS meters are just cheap EC meters with a built-in standard conversion factor, and as my example shows, There really cannot be an accurate "standard".
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