Interesting. I use a safety mesh cover and, like WaterMom, I rarely have any CYA in the spring, but also, if I open early enough and have no algae, don't have a high chlorine demand either. Since it's a mesh, like an open pool, it doesn't trap gases. Of course, prior to closing, I bring my pool up to shock level, add a full quart of Polyquat 60%, wait for it to drop the FC (48 hrs) then bring it to shock level again and close. At Richard's rate, of -2 to -3ppm/month, that would be a loss of about -21ppm of CYA over the winter, yet I seem to lose more.

Then again, I drain out about 1/3 of my water first, then replace it. So....say I had a CYA of 45ppm when I closed. I drain off 1/3 of my water, CYA drops by -21 to 24. By adding back the water in the spring, by dilution I drop the CYA to 16, which, when you test it by the R-0013 black dot tube is almost impossible to measure.....

Yeah, that DOES add up, doesn't it?