tks sullyman and others, I use the tap water filter. the catridges change color when they are spent. But i noticed that the green cartridges dont last nearly as long as the orange/rust colored ones (change color when spent). Supposedly they are the same material so given the inconsistancy, I was thinking a tds meter might help take the guess work out. So it sounds like a hand held one is the way to go. do they all need to be calibrated or only certain models/styles. Was hoping calibration was not needed, just one more item to maintain.