I'm a TDS Meter Believer

So, I've had some time to use my TDS meter. What I found was that the output of my RO/DI unit was not the pure water I was hoping for. Instead of having a low reading, close to 0, the output was measuring anywhere from 200 down to 40ppm. The longer the RO/DI unit sat idle, the higher the reading would be. If I ran it for a long time the reading would drop.

My meter has two inputs. A slide switch selects which input is displayed on the LCD; in or out. I originally hooked it up to measure my water supply into the RO unit, municipal supply and the output out of the DI that went into my storage container. My municipal input has a TDS reading of about 300 ppm. Since I can't control it and since it doesn't need to be monitored regularly I moved the TDS in probe to measure the water out of the RO filter which feeds the input of the DI filter.

What I found was that the RO output had a TDS reading of 6 ppm. Since the DI output was varying between 200 and 40 it was obvious that the DI resin was actually contaminating the water rather than filtering it. Time to change the resin. The resin indeed was very dark but I never noticed it because the location of the filter is pretty dark.

So I ordered more resin from the Pure Water Club on eBay.This where I bought the filter 15 months ago. The refill resin arrived quickly and I replaced the resin in the two canisters. Now, my RO/DI filter produces pure water with a TDS measurement of 0 ppm.

The moral of this long-winded story is if your RO/DI filter is more than 6 months old you really should get a TDS meter to verify your water quality is what you think it is.