Recently when testing a pool using a K-2006, the water sample would barely change colour when the DPD powder was added. But when the titration agent was added, the colour would get darker and darker with every drop until after about 5 drops, at which point it would lighten again as is normal. On a 10mL sample test, total drops was 10 before it went clear again.
In subsequent re-tests, I tried 10mL and 25mL samples, and I tried adding several extra dippers of DPD powder (to make sure there was enough in case of high chlorine), all with similar results. A test strip showed somewhere around 8-10ppm, but not dark enough that it looked >10.
What was happening?