Hi guys, been window cleaning for a few years with a pal but set up on my own now. Like an idiot haven't changed my resin in my di unit. its now reading between 7-10 ppm, after filtration and after going through the old resin, have ordered some resin but wondered if any one knew what is an acceptable level to still be able to clean windows before the water is not pure enough. hope this makes sense. Thanks in advance