This is a terminology issue. There is no such thing as a TDS meter. There are electrical conductivity meters mislabeled as though they give TDS readings.
To measure total dissolved solids one evaporates a known amount of water and determines the mass of minerals left behind. The usual unit is milligrams per liter, parts per million or something comparable. The measure does not involve moles because the number of atoms of different chemical species is not known. This can't be done by putting a meter into water.
An electrical conductivity meter measures the amount of current transmitted through the water, which can be converted to the total number of charged particles in solution. Some common ions in ground and tap water have one positive charge, and others two. These ions can be chemical species with different masses. Unless you know exactly the minerals in your solution and their proportional amounts, you cannot derive a measure of total dissolved solids from an electrical conductivity measurement. Furthermore, non-charged particles can contribute to total dissolved solids, and they are not measured by conductivity meters at all.
One of these meters can be useful if you already know the electrical conductivity of the desired amount of powdered fertilizer added to the desired amount of water. To get different volumes with the same fertilizer concentration, you just add fertilizer to water until the electrical conductivity (perhaps mislabeled TDS) reads the same. But a conductivity meter cannot provide any useful information about total dissolved solids in a novel water solution.
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