Offtopic: Lol I have the same device at work. We were testing a chlorinator. First We thought it was not functioning, then it turned out that if you go above the upper limit of chlorine, the reagent goes transparent again - well it never goes pink, to be precise. It took us a good day to figure out what is going on, because although the whole lab already smelled like chlorine, the reagent still remained transparent. Thankfully nobody was injured. Oh, fun times those were..Fwiw, i test free chlorine with a $500 hach colorimeter at work and even after rinsing with fresh water, the vials turn pink at the bottom after a good 6 months of use.
Usually some alcohol and a long q tip do the trick but until then the readings arent accurate.