What is TDS, and why do window cleaners measure it?

Start with the meter in the water. NGA's paper on cleaning architectural glass says “Water quality can be monitored with a handheld TDS or conductivity meter,” and it gives one equivalence to read the result by: a reading of 40 microsiemens per centimeter represents a TDS level of 20 PPM. That single line is where the number on the screen gets its meaning for glass.

Work backward from the reading. A conductivity meter responds to how well the water carries electricity, and the U.S. Geological Survey explains why that says something about what is in it: “Even a small amount of ions in a water solution makes it able to conduct electricity.” Water stops being an excellent insulator, USGS adds, once it starts dissolving substances around it.

Pure water is the opposite case. USGS calls it an excellent insulator and counts distilled and deionized water as pure, “although even water of this purity can contain ions.” So a conductivity reading responds to dissolved ions, and NGA's equivalence is what turns it into the dissolved-solids figure the glass specification is written in.

That conversion is NGA's. USGS, whose scientists almost always measure conductivity when they collect water samples, using a probe it calls a conductivity cell, does not state one. The term itself, with a short example, is in TDS, defined.

The reason window cleaners care is one sentence in NGA's description of the pure water technique. Rinse water in that technique is generally allowed to evaporate from freshly cleaned surfaces, and so, NGA writes, “the pure water used in both the wash and rinse must have a total dissolved solids content (TDS) of 20 parts per million (PPM) or less to prevent spotting and streaking of cleansed surfaces.” It adds: “The use of tap water is not acceptable.”

Tap water fails because of what it carries. USGS writes that water from a kitchen faucet, a pool, the ground or the sky contains significant amounts of dissolved substances, minerals and chemicals, and that the most common dissolved substances are salts. When a rinse dries on glass, the water leaves and that material stays. The mechanism in full is in why pure water dries without leaving spots.

Getting under 20 PPM takes treatment, and NGA names ion exchange and/or reverse osmosis equipment for it. The meter is how NGA says the result can be monitored rather than assumed.

The pole that delivers that water is covered in what a water-fed pole is, and the technique's place beside the squeegee method is set out under pure water and the water-fed pole on the window cleaning page.

Look up TDS and a second number turns up. EPA's secondary drinking water standard for total dissolved solids is 500 mg/L, listed against effects people notice: hardness, deposits, colored water, staining and a salty taste. EPA explains what such standards are for: “Secondary standards are set to give public water systems some guidance on removing these chemicals to levels that are below what most people will find to be noticeable.”

That is a guideline for what comes out of a tap, written for public water systems. NGA's 20 PPM is a specification for water that is left to dry on glass. The two figures come from different bodies for different jobs, and neither one governs the other.

Hard water is a narrower idea again. USGS defines hardness as the amount of dissolved calcium and magnesium in water, and says it is not a health concern but can be a nuisance, causing mineral buildup in plumbing, fixtures and water heaters. A TDS reading is the figure NGA writes its glass specification in; hardness names two minerals in particular.