TDS stands for Total Dissolved Solids, a measure of the minerals and impurities dissolved in water, and it's the key number behind why pure water window cleaning leaves glass spot-free without needing to be dried or squeegeed. Ordinary tap water typically has a TDS reading in the hundreds of parts per million; pure water used for window cleaning is purified down to close to zero.
That single number explains the whole method, so here's how it actually works and why it produces a genuinely better result than traditional squeegee cleaning.
Why dissolved minerals cause spotting
When ordinary tap water dries on a window, the water itself evaporates but the dissolved minerals it was carrying, calcium, magnesium and other salts, don't evaporate with it. They're left behind as a thin, often cloudy or streaky residue, the familiar hard water spots you see on glass, taps or shower screens.
Squeegee cleaning traditionally avoided this by physically wiping the water off the glass before it had a chance to dry and leave that residue, which is why it worked, but it relied entirely on the cleaner's speed and skill to get every last streak before it dried.
How pure water solves the problem at the source
Pure water systems pass tap water through a purification process, typically deionisation, reverse osmosis, or a combination of both, that strips out the dissolved minerals before the water ever touches the glass. With a TDS reading close to zero, there's nothing left to leave a residue behind, so the water can simply be allowed to air-dry on the glass with no spotting at all.
This is why modern window cleaners can leave windows genuinely wet after cleaning and walk away, something that would have been unthinkable with a traditional squeegee approach, because the water itself is no longer the problem.
How TDS is actually measured and monitored
Professional window cleaners carry a TDS meter and check readings regularly, aiming to stay in the low single digits or as close to zero as the system allows. A rising TDS reading usually signals that purification resin or membranes need replacing or regenerating, and a conscientious cleaner monitors this proactively rather than waiting for customers to notice spotting on their windows.
This is also a useful thing to ask a prospective window cleaner about: if they can't tell you roughly what TDS reading they're working with, they may not be maintaining their system properly, and spotting on your windows down the line is the likely result.
Why this matters more in some areas than others
Water hardness varies significantly by region across the UK, and areas with genuinely hard water have a much higher baseline TDS in their tap water than areas with naturally soft water, which means the improvement pure water cleaning delivers is more dramatic in some places than others. In a hard water area, switching from an old squeegee-and-tap-water routine to a proper pure water system tends to produce an especially noticeable difference in finish quality and how long that finish lasts before dust and light rain marks the glass again.
This is one of the reasons pure water systems became the industry standard so widely: the underlying TDS science works consistently everywhere, but the practical benefit is genuinely bigger in hard water regions, which covers a large portion of the country.
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