Calculating TDS from salt additions?

I just started getting into water chemistry and purchased a TDS meter. The TDS of my starting water (RO and deionized) is 2 ppm. I added 48.8 ppm Ca, 5.2 ppm Mg, 15.4 ppm Na, 57.4 ppm SO4, 63.9 ppm Cl, and 32.6 ppm HCO3. How can I calculate a TDS from this? If I add up those additions (and the initial 2 ppm) I should have 225 ppm, but when I tested the water it only read 172 ppm.

Am I predicting TDS incorrectly? Or is my weighing, or TDS meter, just inaccurate?

A tds meter reads conductivity and reports the concentration of sodium chloride in a solution with that same conductivity.

Ah, ok, so basically unless I'm only adding sodium chloride my TDS meter reading will never match up with my predicted concentrations. I assume other solids will change the conductivity, which will change the reading on the TDS meter, but the change won't correspond to the actual concentration of those solids then?

Not all TDS meters are calibrated or report in units of NaCl. But more importantly, they are not precision instruments. They are screening level instruments that can provide a good COMPARATIVE (aka: QUALATATIVE) assessment of water quality, not QUANTITATIVE. Don't worry too much about the disagreement with your mineral additions.

Thanks for the information! It will be nice to periodically check the water I get and make sure the quality is staying consistent, but I'm certainly glad I got a cheap TDS meter and didn't spring for a more expensive one!

GlowingApple said:

Ah, ok, so basically unless I'm only adding sodium chloride my TDS meter reading will never match up with my predicted concentrations. I assume other solids will change the conductivity, which will change the reading on the TDS meter, but the change won't correspond to the actual concentration of those solids then?

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The conductivity of a solution depends on the concentration of all the charged ions, their mass, their mobility and the temperature. Clearly if the ions are all sodium and chloride in 1:1 proportion you can calibrate to TDS setting the scale factor to the value which corresponds to the actual mg/L of NaCl that gives. Or if they are all K and Cl etc. but if a varying mix of Ca++, Mg++, Fe++, Fe+++, Cl-, HCO3-, CO3--, SO4--, HSO4- etc the TDS reading is pretty meaningless.

A good conductivity meter reads conductivity but will have a ppm display for which the operator can select the scale factor he wants. If the meter just reads TDS it had better tell you what that is TDS of. If nothing is stated assume that it is NaCl, not that it matters as you will be using it for relative readings only.