Every TDS meter arrives with a chart sorting readings into bands from very low to high, with a comforting color scale attached. The chart is not wrong so much as incomplete: total dissolved solids is a sum of everything in solution, and the same reading can describe excellent mineral water or water you should not drink.
What the meter measures before it reaches the chart
The probe measures electrical conductivity and multiplies it by a conversion factor to produce a solids figure. The factor is an assumption about the mix of salts present, and different meters use different factors, which is why two pens in the same glass can disagree noticeably. Nothing in that chain identifies a substance. The chart is a presentation layer on a single physical measurement.
Why the bands mislead people
Low readings sit in the green zone on most charts, but distilled water reads near zero and is not superior drinking water, just empty. High readings sit in the red zone, yet a healthy mineral spring reads high because of harmless calcium and bicarbonate. Meanwhile the contaminants that actually harm people, including lead, arsenic and per- and polyfluoroalkyl substances, are present at concentrations too small to move the reading at all.
Hardness is a different number entirely
People often reach for a solids reading when what they want is hardness. The USGS defines hardness as dissolved calcium and magnesium, reported as calcium carbonate: soft up to 60 mg/L, moderately hard 61 to 120, hard 121 to 180 and very hard above that. A conductivity meter cannot separate those two minerals from the sodium, chloride and bicarbonate also in solution, so it cannot place you in a band. A titration kit or a laboratory hardness figure does that properly.
The one chart worth keeping
Make your own. Record the feed reading and the product reading from your treatment system each month in a notebook or a spreadsheet, along with the date and the temperature. The rejection you get today is the baseline; when the gap between the two narrows, your membrane or media is finished. That personal chart answers a question you can act on, which the generic band chart on the box never does.
Questions people ask about tds water tester chart
What is a good TDS reading for drinking water?
There is no single good number. Palatable water covers a wide range, and the reading says nothing about whether a harmful contaminant is present.
Is a zero reading the goal?
No. Near-zero water is simply demineralized, which is what a distiller or a reverse osmosis membrane produces, and many people find it tastes flat.
Why does my reading go up after the water softener?
Ion exchange swaps calcium and magnesium for sodium or potassium, and the total dissolved load does not drop. Softening changes what is dissolved, not how much.