My problem is my water is very hard to begin with. Coming out of the RO only, my TDS is nowhere near zero. I THINK it's somewhere in the 40-50 range. The problem is, I don't have a true base line TDS measurement, so I was thinking about testing with current carbon block, swapping in a new and testing again, bypassing the DI stage for the test. My TDS always comes out 0 after the DI, but obviously that tells me nothing about my carbon block.