A TDS tester estimates total dissolved solids by measuring how well the water conducts electricity, since dissolved minerals carry current. It does not identify anything: the reading lumps harmless calcium together with anything conductive, and misses non-conductive contaminants entirely. Its genuine strength is comparison, especially checking a reverse osmosis system by reading the water before and after the membrane.
How the meter works and what the number means
The probe passes a small current between two electrodes and converts conductivity into an estimated dissolved-solids figure. Minerals such as calcium, magnesium, sodium and dissolved salts all conduct, so they all raise the number together. The meter cannot say which of them is present, and substances that conduct poorly or not at all, including many organic compounds and microorganisms, barely register. A reading is therefore a bulk property of the water, like weight is a bulk property of a suitcase: useful for comparisons, silent about contents.
TDS is not hardness and not safety
People often buy a TDS tester to check for hard water, but hardness is a specific thing: the USGS defines it as dissolved calcium and magnesium, with water classed as soft at 0 to 60 mg/L as calcium carbonate, moderately hard at 61 to 120, hard at 121 to 180 and very hard above that. A TDS reading includes those minerals plus everything else conductive, so it cannot tell you the hardness class; a titration kit or lab test does that. Nor is a low reading a safety verdict: lead, bacteria and PFAS matter at concentrations far below anything a conductivity probe can distinguish.
The job the meter is actually good at
A reverse osmosis membrane rejects most dissolved solids, so comparing the tap reading with the reading after the membrane shows at a glance whether the membrane is working and how it is aging. Take both readings when the system is new to establish the baseline, then recheck every month or two: treated-side numbers creeping toward the tap number mean the membrane or its prefilters are due. The same comparison logic verifies a distiller and spots a failed cartridge. If you are choosing a meter, our best TDS meter guide compares the current models; whichever you buy, keep calibration solution with it, because probes drift.
Questions people ask about tds tester
What is a good TDS reading for drinking water?
There is no single safe number, because the reading does not identify what is dissolved. Judge safety from a lab test or your utility report, and use TDS only for comparisons over time.
Can a TDS tester detect lead or bacteria?
No. Lead matters at concentrations a conductivity probe cannot distinguish, and bacteria barely conduct at all. Only a lab test can answer for either of them.
How do I check my reverse osmosis system with a TDS tester?
Read the tap water, then the RO water, when the system is new. Recheck periodically: if the treated reading climbs toward the tap reading, the membrane or prefilters need replacing.