Method
TDS (Total Dissolved Solids)
Measure of dissolved minerals in water (ppm). Pure-water cleaning targets 0-5 ppm; tap water is 100-400 ppm.
TDS measures how many dissolved minerals — calcium, magnesium, sodium, silica — are in water. Tap water is usually 100-400 ppm. Pure-water window cleaning uses water filtered down to 0-5 ppm so that when it evaporates off the glass, nothing is left to spot or streak.
A home water softener doesn’t change TDS — softeners swap calcium for sodium, but the total dissolved solids stay the same. That’s why you can’t just run softened tap water through a squeegee and expect streak-free results; the minerals are still there.