chlorine vanishes

first off i use calcium hypochlorite shock and trichlor tabs and have an 18,000 gal pool. my pool has been beautiful with crystal clear water. tuesday early morning hours we had aprox 6 inches of rain overnight and into the daytime. tuesday morning before work i noticed a residue that appeared to be the beginning of yellow algae(which i had in the spring and subdued with the bromine yellow algae treatment and all has been good for three months) in a spot on the pool bottom. we let a total of six inches of water out to accomodate the rainfall. tuesday night i shocked the pool with 24 oz of shock and all was well. on wednesday afternoon the pool was a deep blue green and you could barely see the drains. we used two packs of the yellow algae treatment and the pool looks great. here is the problem. I came in friday evening and the chlorine was zero. shocked the pool again. saturday morning the pool is clear and the chlorine is about 10ppm. checked it saturday night and it is zero again. I put 3 tabs in the chlorinator and turned it to full tonight and shocked it again. the water is clear. what is causing the chlorine to dissipate so quick?
thanks

The chlorine is disappearing because you have something growing in the water that it is fighting. The sodium bromide is just a band aid, not a cure. I suspect that your CYA levels are way too high for your current FC level. If you could post a full set of test results it would help us determine just what is going on.

"Boiler plate" content that is directly relevant to your questions - Ben

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+ It's much easier to answer your questions, when we have the details about your pool in one place. We often 'waste' the first few posts back and forth collecting information. So, please complete our new Pool Chart form -- it takes about 30 seconds, but will save much more than that.

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+ Get a cheap OTO (yellow drops) / phenol test kit, or if available at YOUR Walmart (check availability), get the HTH 6-way DROPS test kit, which is compatible with the Taylor K2006. Test the pool as soon and you can, and post the results. If you get the 6-way kit, ALSO test the water you FILL the pool with, especially if it's a well, and post THOSE results as well. (The HTH is the best available kit you're likely to find locally, but it's not the K-2006. It can only provide rough measurements chlorine levels above 5 ppm, and it measures "TOTAL" hardness, rather than "CALCIUM" hardness, which is not ideal.)

+ Having a good test kit makes pool care easier for EVERYONE. A good test kit means a kit that can test chlorine from 0 - 25 ppm, pH, alkalinity, calcium hardness, and stabilizer with reasonable accuracy. Test strips (AKA 'guess-strips' ) do NOT meet this standard. Some pool store testing is accurate; most is not. The ONLY way you'll know whether your pool store is accurate or bogus, is by testing accurately your own self. On the other hand, pool store 'computer' dosing recommendations are NEVER trustworthy -- ignore them. They are designed to sell more chemicals than you need, and WILL cause many pool problems.

+ We recommend the Taylor K-2006 test kit, which meets the requirements above, for many reasons. The HTH 6-way drops kit is a great starter kit, and is compatible with the K2006 (it's made by Taylor). There are a few alternatives; for example Lamotte makes an FAS-DPD kit that's OK -- but it costs 3x as much. But, we're not aware of any test that is better, and since we are all familiar with the K-2006 (and can help you with it) we recommend it exclusively ( Test kit info page )

One caution for the 2012 season: Amazon does not stock the kits directly. So when buying at Amazon, Amato is our current preferred seller. However, they often don't list enough stock to last the whole day, so try order mid-morning. You should expect a delivered cost under $60 for the K2006A and under $95 for the K2006C. If you can't find that, wait a day.

+ Here are links to the kits we recommend (you can check local availability on the HTH kit, using the Walmart link):

HTH 6-Way Test Kit @ Walmart
Taylor K2006A (3/4 oz bottles) @ Amazon
Taylor K2006C (2 oz bottles) @ Amazon

I had my water tested to back up my tests this morning. confirmed FC 3.9%, ph 7.0, TA 34, cya 100, total hardness 133. chlorine is now holding steady. bought 25 lbs sodium bicarbonate to raise the TA. water is now crystal clear. have not useen any more signs of the yellow dust on the steps for several days.

i guess it was the excessive rain.....

Quote:

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Originally Posted by CharlieB

i guess it was the excessive rain.....

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The only reason rain causes algae is that it causes pool owners to ignore their pools. ;) Chlorine is just as effective at destroying algae spores when it's raining, as it is when the sun is shining!

i'm through....

Quote:

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Originally Posted by CharlieB

i'm through....

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????

Quote:

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Originally Posted by CharlieB

I had my water tested to back up my tests this morning. confirmed FC 3.9%, ph 7.0, TA 34, cya 100, total hardness 133. chlorine is now holding steady. bought 25 lbs sodium bicarbonate to raise the TA. water is now crystal clear. have not useen any more signs of the yellow dust on the steps for several days.

i guess it was the excessive rain.....

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It's worth mentioning that with a CYA of 100 you need your chlorine to be between 8 and 15 to be effective. Also, most (all?) tests do not register above 100 for CYA so your cya level could be significantly higher. If you just drained off 6" and replaced it with rain, and you're still registering 100, it's a safe bet it's well above. I would stop using the trichlor pucks (these add CYA to the water) and try to figure out what your real cya level is. AFAIK there is only one way to actually reduce CYA levels and unfortunately that is do a partial drain and refill with fresh. Alternatively you can try to compensate for the high cya by maintaining correspondingly high chlorine levels.

To test for CYA > 100 with the Taylor kit you should be able to mix your pool water sample 50/50 with fresh distilled water, then run the CYA test and multiply the result by two; assuming it's not over 200 that should get you a reading you can work with.