Quote Originally Posted by mas985
I think Chemgeek showed that chlorine loss as a percentage of the residual ppm increase with higher residuals. If you have a residual of 9 ppm, then it may not be unreasonable to lose 6 ppm.
Though it is true that the amount of absolute chlorine loss from sunlight (UV) will be higher at higher levels of free chlorine (FC), I don't think that is the case here because with a CYA of 70 and an FC of 9 ppm, the half-life (i.e. losing 4.5 ppm) of chlorine due to sunlight is about 7 hours and that's with full-intensity sun which isn't the case (i.e. it's not "noon" all day long).

I agree with the other posts that there is something else eating up the chlorine and such chlorine demand will be mostly independent of the amount of chlorine you have in your pool (assuming you have enough to never get to 0). If it's algae, then doing the chlorine shock to try and kill it is the way to go. If the chlorine demand is still there, then it might be from junk getting into the pool (plus regular pool usage -- bather load) and causing excessive organic demand, but I don't know how to prevent that except with a pool cover.

Richard