I know these things are frowned upon... but I ran out of 0870 and 0871 while Slamming (fortunately right at the end), have to wait a day or 2 for new to arrive. Previous owner used pool service and strips, so I had some. Put a strip in and FC reads deep purple (= +10). As long as I test purple I can hold off adding Clorox. Just hope the chemicals get here soon! I assume they are accurate enough to be within 2 or 3 ppm. I HATE going to the pool store for anything.