The overnight free chlorine test at high levels of chlorine (16ppm-19ppm) is starting to baffle me. For the last 3-4 days, in my unprofessional estimation, my pool has looked great. However, I still can't get the overnight free chlorine test to show absolutely no loss in chlorine. Last night at 9:00 pm I got two back-to-back readings of 18 & 19. I used the EXACT amount of pool water on both tests, so I'm pretty sure that's not the problem. This morning at 6:00 am I got back-to-back readings of 17 & 17.5. Is it possible that there is some margin of error for these tests...like possibly the size of the drop that comes out of the solution...or something else? I kept my pool covered all winter and have been shocking for a week. And again, the pool likes great and is sparkling clear. My CC's are consistently at .5. What do you think? Should I continue to shock? Is my overnight FC loss such that I'm probably OK?