Hi,
What's the highest tds reading you have used without any spotting?
Nick
What I did is chose our lounge window which is large and gets the early morning sun as a 'test site.'
I filled a backpack with pure water and thoroughly washed and rinsed the window. The next day I checked the window - no spots. I added a little tap water to the backpack and checked the tds before washing and rinsing the window.
I found with our water at 3ppm I couldn't see any spotting on the dried window. I could, however, see very faint spots in the early morning sun with the water at 5ppm. . I doubt a customer would notice them, but to be sure, I never let my pure water tds rise above 1ppm.
There is no way I could get away with pure at 10ppm because the spots were clearly visible, and I guarantee a customer would notice them immediately.
This took time to accomplish. After each tds increase, I checked the window when dry and then cleaned it with pure water, so I started with a total clean piece of glass. This meant that the previous results couldn't interfere with or contaminate the next test.
I did this as some windies were reporting that they could clean with pure at a higher tds than 10ppm. I now know I can't.