A TDS meter is a small handheld device that measures Total Dissolved Solids in water and shows the result in parts per million. It works by reading the water's electrical conductivity, not by identifying specific contaminants. Plumbers use it to check how well a reverse osmosis system or water softener is performing.
A TDS meter is one of the fastest ways to see what is dissolved in your water. TDS stands for Total Dissolved Solids. That is the combined weight of the minerals, salts, and metals dissolved in the water. The meter gives you one number in parts per million (ppm). It is the tool a plumber pulls out to test how well a filter or softener is really working.
What a TDS meter actually measures
A TDS meter does not sniff out specific contaminants. It cannot tell lead from calcium. What it really measures is electrical conductivity. Pure water barely conducts electricity. Dissolved minerals and salts carry a charge, so more dissolved solids means more conductivity. The meter reads that conductivity and converts it to a ppm estimate. One microsiemens of conductivity equals about 0.5 ppm of dissolved solids. So the meter is a quick health check, not a lab test.
What the numbers mean
The EPA lists a secondary standard of 500 mg/L for TDS, which is about the same as 500 ppm. It is a taste and comfort guideline, not a health limit. Above that level, water can taste salty and leave hardness, deposits, and staining. Most tap water in the United States runs between 200 and 300 ppm. In the Phoenix area, hard water often pushes readings higher. A high number is a common reason people look into water treatment.
Checking an RO system with a TDS meter
The meter shines when you test a reverse osmosis membrane system. Dip it in the tap water first, then in the RO water. A healthy RO system cuts TDS by about 85% or more. Say your tap reads 400 ppm and the RO water reads 40 ppm. That is a 90% drop, which is a strong result. If the RO number climbs over time, the membrane or filters may be worn out. That is your cue to check filter replacement.