Interesting Chlorine Usage

Since I'm new to all this pool business, I thought I would tell you guys/gals about my fall experiences with pool ownership. 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. I was thinking that a SWG was definitley going to be a necessity for my pool due to hauling truckloads of bleach. A SWG is still a good idea mind you. Well fall has fell and water temps are at 68 degrees and chlorine demand is down to a crawl. I shocked the pool on 10/12 because I was going on vacation and didn't want a mess upon returning home. Well here it is 10/20 and I still have chlorine values that are above the 5 ppm reading on my test kit. This value is amazing to me because with all the wind and rain we've had lately I'm not only getting rain dilution but lots of leaves to clean out. I would have thought this would have put me in a chlorine demand but the pool is holding its own. Just a thought to share with you all since things are so quiet around here.

I am new to pool ownership too and have an SWG. It stopped producing chlorine about 10 days ago due to the cold weather. The water then tested at 7 ppm of FC, Since then we had lots of rain and I tested it yesterday and it still reads about 5ppm. I also had the swg at its lowest setting for the past 4 weeks. I would say, next to pool noodles :D :D :D an SWG is the next best investment,

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Originally Posted by chemistrydropout

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.

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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

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!!!!

You'll find that as your water temp lowers, so will your chlorine usage...just don't make the mistake of getting complacent about testing and chlorinating your water--algae blooms CAN still happen!

Janet

I haven't added bleach since the first week in september, since then it's been sitting there saparkilg clean, bored, no one swimming in it, I'm surprised it hasn't gone and left us for the little attention we pay to it now.
I guess I should get around to closing it sometime soon......before it freezes up....

We're down to about 1.5-2 cups a day from a high of 6 cups during the summer. Muriatic acid use is also down to 1.5 cups a week from 3 cups during the summer.