Pure water

NGA's glass cleaning paper records that the International Window Cleaning Association recognizes a technique some professional window cleaning contractors use: pure water delivered to the glass with a specialized extension pole. Because that rinse water is generally allowed to evaporate off the glass rather than being wiped away, NGA sets the bar at 20 PPM or less and states plainly that tap water is not acceptable.

The number is a limit, not a promise of emptiness. The U.S. Geological Survey notes that even distilled or deionized water, the water it counts as pure, can still contain ions. In glass cleaning, then, pure describes water brought under NGA's stated figure, not water with every dissolved substance gone.

NGA's instruction is that effective water treatment, via ion exchange and/or reverse osmosis equipment, should be used with the pure water delivery and rinse methods at all times. The EPA, writing about drinking water, separately names distillation, reverse osmosis and electrodialysis as treatments effective at removing total dissolved solids, so reverse osmosis appears on both lists.

The treatment matters because whatever is dissolved in water stays behind when the water evaporates, the mechanism worked through in why pure water dries without leaving spots. One maker, Unger, describes its systems filtering tap water into deionized water through resin, or through reverse osmosis in a multi-stage process. How treated water is checked against the number belongs to the TDS entry, how it reaches upper glass to what a water-fed pole is, and why the glass is left wet afterward to why a window cleaner sprays from the ground and leaves the glass wet. The window cleaning page quotes the NGA passage in its section on pure water and the water-fed pole.