I only check my TDS about once a week and a few weeks ago i checked the TDS of my water that i was using in the van and it read 0.37 its usually under 0.03
I had been clean windows with that for 2 days and so far not one person has rang up.
I will see the results in a few weeks when i go round and do them again.
I thought as the reading was high i would give my van a wash with it and to my surprise there were no marks left on the windows, I was expecting some spotting but nothing