Interesting Chlorine Usage

[chem geek]

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

[chemistrydropout]

Towards the end of the summer I was measuring a CYA of 20-25(yes my kit goes that low but it is on the bottom end of the measurement so I'm not sure how accurate that was but adding stabilizer by the calculator I was close because I targeted 30 and didn't miss it). My water temps during max usage were getting into the 90's and the pool gets sun all day long in the hot and humid south. I quit working with the how many ppm Fc target in the afternoon thing "after" adding chemicals and went to trying to achieve a residual "before" adding chlorine at dark that was on target of 3 or 4, with 4 being my goal. This seemed to work well and no algae or icky water all summer. Some days I could do well and then other times when the bather load was varied it was a little harder. It seems to me some people are "ickier" than others . The shorter days and cooler water temps really make a big difference when combined with no swimmer loads. Some days in the summer I was adding 2 gals of 6% to get the residual I was looking for and that was with a solid reading of 25 on my cya. Later in the summer I let cya drift below 25 probably close to or below 20 and still had basically the same chlorine demand day to day. Thanks for the responses!!!!