What Is TDS in Window Cleaning?
TDS is one of the most important numbers in pure water window cleaning. It tells you how much dissolved mineral content is in your water and whether your system is producing water clean enough for a spot-free finish.
Quick Answer
TDS stands for Total Dissolved Solids. In window cleaning, it is used to measure the dissolved minerals in water. The lower the TDS, the purer the water. Most professional window cleaners aim for 0 TDS for spot-free results.
What does TDS mean?
TDS means Total Dissolved Solids. It is a measurement of dissolved minerals and particles in water.
Tap water naturally contains minerals such as calcium, magnesium and other dissolved solids. These are not normally a problem for drinking water, but they are a problem for window cleaning because they can leave marks behind when the water dries.
When ordinary tap water dries on glass, the water evaporates but the dissolved minerals remain. This is what creates spotting, streaking and white marks.
Why does TDS matter for window cleaning?
Pure water window cleaning relies on the water drying naturally on the glass. For that to work properly, the water needs to contain little or no dissolved solids.
If the TDS is too high, minerals are left behind after the water dries. If the TDS is low enough, the water can dry without leaving visible marks.
Simple way to think about it
High TDS water can leave spots. Low TDS water is cleaner. 0 TDS water is the target for professional pure water window cleaning.
What TDS should window cleaning water be?
Most professional window cleaners aim for 0 TDS at the point of use.
Some cleaners may still get acceptable results at very low readings, depending on the job and glass condition, but 0 TDS is the cleanest and safest standard to work to. It also removes doubt when diagnosing spotting issues.
| TDS Reading | What It Means | Window Cleaning Use |
|---|---|---|
| 0 ppm | Pure water | Ideal for spot-free window cleaning |
| 1–5 ppm | Very low mineral content | May be usable, but monitor closely |
| 6–10 ppm | Rising mineral content | Resin or system may need attention |
| 10+ ppm | Too much dissolved content | Risk of spotting increases |
How do you measure TDS?
TDS is measured using a TDS meter. This is a small handheld tester that gives a reading in ppm, which means parts per million.
To test your water, fill a clean container with water from your system output and place the TDS meter probe into the water. Wait for the reading to settle, then record the number.
Where should you test?
- Incoming tap water before filtration
- After RO filtration, if fitted
- After DI resin
- At the final output before filling or using the system
Testing at different points helps you understand what part of the system is doing the work and where a problem may be developing.
What affects your incoming TDS?
Incoming TDS depends mainly on your local water supply. Some areas have soft water with low TDS, while others have hard water with much higher mineral content.
A window cleaner in a soft-water area may be able to run a DI-only system efficiently. A cleaner in a hard-water area may use resin much faster and benefit from an RO/DI setup.
This is why two window cleaners can use the same equipment but have very different running costs.
TDS and DI resin
DI resin removes dissolved ions from water. The higher your incoming TDS, the harder the resin has to work.
If your tap water has a high TDS and you run DI-only, your resin may exhaust quickly. When resin starts to exhaust, the TDS reading after the vessel begins to rise.
That rising reading is the warning sign that the resin needs replacing or the system needs checking.
TDS and RO systems
RO systems reduce the TDS before the water reaches the DI resin. The RO membrane does most of the work, then the DI resin polishes the water down to 0 TDS.
This is why RO/DI systems are often better for professional window cleaners using a lot of water. They can reduce resin consumption and make the system more economical over time.
Common causes of high TDS after filtration
- Exhausted DI resin
- RO membrane not performing correctly
- Pre-filters overdue for replacement
- Water bypassing the resin vessel
- Incorrect hose or fitting connection
- Contaminated storage tank
- Dirty test container giving a false reading
How often should you check TDS?
For professional window cleaners, TDS should be checked regularly. If you are working daily, checking at the start of the day or when filling the tank is good practice.
It is much better to catch a rising TDS reading before it causes spotting on a customer’s windows.
PWS tip
Keep a spare TDS meter, resin and key fittings in the van. A small fault with water quality can quickly turn into wasted time if you cannot diagnose it on site.
Does hot water change TDS?
Hot water does not replace RO or DI filtration. TDS is still controlled by your filtration system.
Hot water helps with cleaning performance. Pure water controls the spotting. Heated water can help remove dirt, grease, bird mess and winter grime more effectively.
This is where Pure Heat systems fit into a professional window cleaning setup. Pure water gives the spot-free rinse. Hot water helps shift the dirt.
Summary
TDS is one of the simplest but most important checks in pure water window cleaning. It tells you whether your water is clean enough to dry without leaving mineral marks behind.
For professional work, aim for 0 TDS, test regularly and treat rising readings as an early warning sign. Good water quality protects your results, your reputation and your time.
Related guides
Need help with pure water equipment?
Speak to Precious Washers for advice on TDS meters, DI resin, RO systems, Pure Heat hot water systems and complete window cleaning van setups.
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