Purity is not a property any handheld device measures. The instruments sold under that name measure something specific and narrow, most often electrical conductivity, and then convert it into a number that looks like a verdict. Knowing what each one actually reports is what turns a gadget into a useful tool rather than a source of false confidence.
Conductivity meters, sold as purity testers
The common handheld passes a small current between two electrodes and reports total dissolved solids derived from the water's conductivity. That tells you how much dissolved ionic material is present in total, and nothing whatever about what it is. Harmless dissolved limestone and something genuinely unwanted can give the same reading. More importantly, many contaminants that matter, including lead and most organic chemicals, are dangerous at concentrations far too small to move the number at all.
The demonstration to be sceptical of
A familiar sales demonstration puts two electrodes into a glass of tap water, passes a current, and produces a cloud of brown precipitate, which is presented as proof of contamination. What it actually shows is electrolysis: the iron electrode corrodes and its products colour the water, and the effect is stronger in water with more dissolved mineral because it conducts better. Distilled water produces almost nothing because it conducts almost nothing. It is a conductivity demonstration dressed up as a purity test.
What the meters are genuinely good for
Comparisons. Feed water against reverse osmosis product water, sampled before the tank, tells you directly whether the membrane still rejects what it used to, and that is the most useful maintenance check an RO owner can make. Tracking one tap over months flags a change worth investigating. A pH meter used on an acidic well tells you whether a neutralizer is still lifting the water. All of these read differences rather than verdicts, and all of them need calibration against a standard solution to stay honest.
The questions that need a laboratory
Bacteria, nitrate, arsenic, lead, metals and organic chemicals cannot be measured meaningfully by any consumer device. For those the equipment is a sample bottle and a courier, with an accredited laboratory stating a method and a detection limit. The USGS classifies hardness specifically as dissolved calcium and magnesium, from soft at 0 to 60 mg/L as calcium carbonate upwards, and even that is better measured with a hardness test than inferred from conductivity. Our softener installation guide covers acting on a measured figure.
Questions people ask about water purity tester
Can a handheld tester tell me if my water is safe?
No. It measures dissolved content in total, and many harmful contaminants matter far below the level that would change the reading.
What about the demonstration that turns water brown?
That is electrolysis corroding an iron electrode. It shows the water conducts electricity, which dissolved minerals do, not that it is contaminated.
Is a meter useful at all?
Yes, for comparisons: feed against reverse osmosis product water, or one tap tracked over time. It needs regular calibration to stay accurate.
What needs a laboratory instead?
Bacteria, nitrate, arsenic, lead, metals and organic chemicals, none of which any consumer device measures meaningfully.