I feel happy as long as my TDS is below 5ppm on the output side of my RO/DI unit. I double checked my inline meter with one of the handheld meters and it is within 1 both +/-. Rich, you don't even want to know the ppm of the water from HEB around here.. I was like holy crap, my tap water is almost as clean as their water as well as WalMarts.