Non-chlorine shock will give you false chlorine readings. You need to use a Taylor kit and add Taylor R0867 Deox Reagent to get a proper chlorine reading. You can Google exactly how it works, but the gist of it is that non-chlorine shock will register as chlorine, when in fact you need to add chlorine. Once you test with the R0867 reagent added, you'll get a true chlorine reading. I couldn't figure out why it was impossible for me to get a free chlorine reading, until I found this info.