I've been doing a good bit of reading this evening about metal interference with the CH test. My experience is somewhat similar to Swimgirl's post here. There are a lot of posts about the metal interference issue, but this is the one that seemed clearest to me on the "direction" of the error one would expect if CU is present.
Here's the history:
I have been using the Taylor kit and then the TFP kit for about 5 years now, meaning since I discovered this forum. I don't know what was done to my poor pool before I took custody.
I always had trouble with the CH test. I read about the "floaties" and the need to swirl and swirl and swirl and did the best I could, but my results were always more variable than I expected, and also higher than the local pool store was telling me. I didn't believe them, BTW.
I bought a stirrer from duraleigh about a year ago and that made things a little better, but the results were still more variable than I thought reasonable. Also, the results were consistently lower than what I was getting before using the stirrer, meaing 275-350 ppm as opposed to 350-400 ppm.
Recently I actually read the TFP kit instructions ( ) and ran across the comment about modifying the CH test when CU might be present. Yes, I read the instructions about a year after receiving the kit ...
When I use the modified procedure, two things happen. First, the results are now rock-solid reproducible. Second, the results are substantially lower, meaning dead on 225 ppm as opposed to 275 to 350 ppm before. Note that the lower reading is pretty close to what the pool store was telling me, FWIW.
So my interpretation is that I must have some metals in the water, and the 225 ppm reading is the correct one. The higher reading I was getting before was an incorrect one.
Does that make sense? That seems consistent with what chem geek says in this thread, but I was just hoping for a reality check before I spent $40 on calcium, and took a chance on maybe adding too much. Again, I found a lot information about possible problems with the test if metals are present, but it was a little difficult to tell whether using the usual testing procedure with metals in the water would tend to make you measure high, as opposed to just making the test results more noisy.
Thanks in advance ... Gary