Good point above ^^^^Nunas wrote:It is important to differentiate between overall TDS and hardness minerals.
As Nunas explained, especially if you are measuring softened or mineralized water you have to be aware that you can have high TDS but low hardness.
Below 50 ppm TDS on an inexpensive conductivity TDS meter is pretty sure to be below 50 ppm total hardness.
And even at a low TDS neighborhood, especially in bottled/formulated water, you might possibly have a dangerous level of corrosive chloride ion. (For example, 50 mg/L of CaCl2 would give you a TDS reading of about 60 ppm, a calcium hardness of 45 mg/L, and a chloride ion of 32 mg/L )
And a further wrinkle, especially with mineralized recipe water, you need to be aware that calcium carbonate limescale (as predicted by LSI) is related to calcium hardness and alkalinity. The scale prediction game changes significantly if most or all of your hardness is from magnesium sulfate.