Parts per million is a unit of concentration, not a thing a device measures on its own, so a handheld sold as a parts per million tester is measuring something specific and reporting it in those units. In household water that is almost always conductivity converted into total dissolved solids, and knowing that is the whole of understanding what the number can tell you.
What the handheld is actually doing
Passing a small current between two electrodes, measuring how readily the water conducts it, and converting that into an estimate of total dissolved ionic material using a fixed factor. So the reading describes how much is dissolved in total and never what. Two waters with identical readings can be entirely different: harmless dissolved limestone reads the same as something you would rather not drink, because both conduct.
The other things sold in these units
Chlorine test kits report in parts per million and measure a specific chemical through a colour reaction, which is a genuine measurement of one thing. Hardness kits do the same by titration. Dedicated meters exist for particular parameters. Those are not interchangeable with a conductivity handheld, and a household that buys a conductivity meter expecting a chlorine reading has bought the wrong instrument, which is a common outcome of shopping by the unit rather than by the parameter.
Where the conductivity reading is useful
Comparisons. Feed water against reverse osmosis product water sampled before the storage tank tells you directly whether the membrane still rejects what it used to, which is the best household use there is. One tap tracked over months flags a change worth investigating. And confirming a deionizing stage is working. All of those read a difference rather than a verdict, and all need the meter calibrated against a standard solution to stay honest.
What it cannot decide
Safety, since many contaminants matter far below the concentrations that move the reading. Hardness, which the USGS defines specifically as dissolved calcium and magnesium, classified from soft at 0 to 60 mg/L as calcium carbonate through to very hard above 180, and which needs its own test. Or whether a softener is working, since ion exchange swaps minerals rather than removing them and leaves the total much as it was. Our softener parts guide covers the equipment side.
Questions people ask about parts per million tester
What does a parts per million tester measure?
Usually conductivity, converted into total dissolved solids. It reports how much is dissolved, never what.
Can it measure chlorine?
No. Chlorine test kits use a colour reaction specific to that chemical and are a different instrument entirely.
What is it genuinely good for?
Comparing feed water with reverse osmosis product water before the tank, and tracking one tap over time.
Does it show whether my softener works?
No. Ion exchange swaps hardness minerals for sodium, so the total dissolved reading stays about the same.