I had a "DOH!" moment last night. When I first did the CH test with my HtH kit last summer, I read the instructions that you should count drops until the solution turned blue. I tested my CH, and came up with a number in the 300-330 range. When I got my TF-100, I looked at the instructions for it. Yep, add drops until the color changes to blue. I tested again, and came up with 290. OK. I tested my fill water, and was getting readings around 300 for it also. I assumed that this was within my margin of error. OK, so far so good.
Last night, on a whim, I decided to run the full battery of tests again. When I did the CH test, I came up with 180. This didn't make any sense. Where did all that calcium go? We had several inches of rain, but nowhere near enough to drop the CH by more than half. I ran the test again, and got the same result. A third time, and yet again got 180.
The color wasn't exactly blue - more of a light purple - but it certainly stopped changing color. This time, I kept adding drops after the color change stopped. Sure enough, around 23 drops the color started changing again and finally reached a consistent blue at 290. I was so focused on getting the test to turn blue and then to stop changing that I completely missed the purple stage where it stopped changing.
Since I have a vinyl pool, it doesn't really matter much, but still... (Hangs head in shame and slinks toward the door. )