Quote Originally Posted by chemistrydropout View Post
I had resigned myself to the fact that my 20x40 Ig pool was going to eat a 1.46 gal jug of 6% bleach a day.
Assuming your pool was 30,000 gallons (probably on the high side, but conservative for these calculations), you were adding chlorine to replace a chlorine usage at a rate of around 3 ppm per day. Though that is not extraordinary, it is a little on the high side.

Out of curiosity, what CYA level do you have (or had during the summer)? Also, what FC level were you trying to maintain (lowest and highest values; assumed to differ by around 3 ppm)?

If you had 30-50 ppm CYA, then Ben's chart says to maintain 3-6 ppm FC so if I assume that was your situation, then the question becomes is it likely to start at 6 ppm and lose half of your chlorine over a day (24 hour) period. The answer is yes, if your pool is in direct sunlight for most of the day.

This chart shows the approximate half-life of chlorine in direct noontime sun at latitudes and times of year when the sun is directly overhead. The half-life of 6 ppm FC chlorine with 30-50 ppm CYA is around 6-7 hours so with long days of lots of sunlight it is possible that 12-14 hours of sun is equivalent to 6-7 hours of "noontime" sun. Also, you may have had other chlorine usage preventing algae, oxidizing organics and ammonia, etc. and that probably accounts for at least 0.5 ppm FC per day.

The lower temperature naturally inhibits algae growth and slows down all chemical processes including oxidation so the 0.5 ppm FC usage is probably close to 0. If the sun doesn't get to the peak of the sky anymore or if your days are cloudy (with rain, I assume you had cloudy days), then your chlorine usage will go way, way down. So this is not surprising and is pretty much what everyone sees going into the winter -- except for those lucky souls living in south Florida, Arizona and other "year-round" sunny areas.

I agree that an SWCG system is convenient. I haven't gotten one myself (yet), but I use 12.5% chlorinating liquid I get for $2.50/gallon (a little less economical than bleach, but much less to lug around) and have an opaque electric pool cover so my chlorine usage even during the summer is only around 0.5-0.7 ppm FC per day. So I only need to get a new case (4 gallons) of chlorine about once a month or so.

Richard