We have had our 15,000 gallon in-ground pool for over 10 years and I've always done the pool maintenance myself. It's fairly easy to keep it under control, but I've always wondered why I never get a low chlorine reading. I typically put 2 or 3 tablets in the skimmer basket and I try to shock the pool once or twice per month. Even if the pool goes without chlorine for a week (meaning the tablets in the skimmer basket dissolved), I've never seen a reading below 5ppm. Ditto for the free chlorine reading - I've never seen the level adjust after I've let the sample sit for a few minutes.

Could it be that the normal range of Chlorine for our pool is just outside of the measurement range of our test system and that it's fluctuating at a slightly higher level?