I have been using my testing kit now for three seasons. I was slamming the pool last month and testing with new chemicals purchased mid season last year.
While testing with the DPD powder, my chlorine only cylinder and the Taylor Speed stir, I noticed that there is a black deposit on the cylinder and the magnet of the speed stir that will not come off. I wash and dry them both after each test.
I just ordered new DPD powder, R-00871 and a new cylinder, but I was curious if anyone had the same experience with the black residue? Bad chemicals or what??