I have a reverse osmosis and deionization unit that filters my tap water from 450PPM TDS to 0 PPM TDS. I have had it for about 3 months and my TDS reader is now creeping up to 3 PPM.
I was considering waiting until the TDS reader gave 10PPM to determine the amount of chlorine it in, and then decide on that result whether I should purchase new filters at 10 PPM every time in the future.
If I knew at what PPM of chloride causes adverse affects on the homebrew, I would be able to better estimate when I need to buy replacement filters.
– Grafton C. Dec 29 '15 at 20:43TDS measures total dissolved solids, as stated in an answer below. To measure chlorine reduction, you'll need a spectrophotometer and free available chlorine reagent, and you'll most likely want to test in accordance with NSF Standard 42. Very cost-ineffective for home brewing.
Follow your filter manufacturer's recommended life span for filter replacement. Provided you're using a trustworthy brand of filter, there is a lot of good test data validating the recommended life span.