Total dissolved solids meters are cheap enough that most people buy on price, and the differences that matter are not the ones on the front of the packaging. What decides whether a meter is still telling the truth in a year is whether you can calibrate it, whether it compensates for temperature, and whether the probe survives being kept in a drawer.
Calibration is the feature that matters
Meters drift, and one that cannot be recalibrated becomes a device that produces confident numbers with no relationship to reality. Look for a model with a documented calibration procedure and a standard solution available for it, and calibrate on the manufacturer's schedule and after any drop or heat exposure. A sealed unit with no calibration route is the classic false economy here: it works well for a season and then quietly stops being a measurement.
Temperature compensation
Conductivity changes with temperature, so the same water measured cold and warm gives different raw readings. Automatic temperature compensation corrects for that and is worth having, because household comparisons often involve water at different temperatures: a cold mains sample against water that has been standing in a storage tank, for instance. Without it, you need to compare samples at the same temperature to get anything meaningful, which is easy to forget and easy to get wrong.
Using it properly
Rinse the probe in the water you are about to test rather than testing straight from a dry pocket, since residue shifts the result. Let the reading settle rather than taking the first figure. Do not submerge past the marked depth, which lets water into the body. Dry the probe and cap it after use. And record what you measured and where, since the value of a meter is in comparisons over time, not in a single number written on a scrap of paper.
What the number is for
Comparisons, principally: feed water against reverse osmosis product water sampled before the tank, which is the best household use there is, and one tap tracked over months. What it cannot tell you is safety, since many contaminants matter far below the level that would move the reading, or hardness, which the USGS defines specifically as dissolved calcium and magnesium and which needs a hardness test. Our TDS meter guide compares models with straightforward calibration.
Questions people ask about tds reader
What is the most important feature in a TDS meter?
A documented calibration procedure with a standard solution available, since meters drift and an uncalibrated one is not a measurement.
Do I need temperature compensation?
It helps, because conductivity changes with temperature. Without it, compare samples at the same temperature.
What is the best household use for one?
Comparing feed water with reverse osmosis product water sampled before the tank, which shows whether the membrane still rejects.
Can it tell me if my water is hard?
Only very roughly. Hardness is calcium and magnesium specifically, while the meter counts everything dissolved.