Is this possible?

[mccato]

16x32 16,400 gal vinyl pool. CYA has been high because I used dichlor at my pool store's recommendation. On 8/23 their Alex system said CYA was 146; on 8/29 it said 151. When I took a sample yesterday, she told me that Alex can only test to 151, so she added water and tested manually and told me it was 330! I have not put anything except bleach and borax into the pool since 8/23. My chlorinator has always been set to 1;and I turned it off last night. I drained approx. 3 inches of water twice last week and added fresh. Is it possible for CYA to increase that much that fast, with only bleach and borax added to the pool?

I have drained another 2 inches of water today; but if the CYA really is 330, I don't think that's going to help.

How much water can I safely drain from my vinyl pool? It is going to be sunny all weekend and I don't want the liner to shrink.

Thanks!

[larry2338]

Doubtful,
Just looking at your figures, it would make sense to assume that she diluted your water by 50% and then multiplied your CYA result by 100%. If the previous test results were correct??? it would seem more likely that your CYA might be around 165 or half what she told you. If it's possible, I would have another store run the test or have the original store run it again and explain exactly how they are arriving at the 330 number.

[mccato]

Thanks, Larry. She did add water (tap water, not distilled) to the test vial, but she something about multiplying the result by 4. I have ordered a test kit from Ben, but am still waiting for it to arrive. The other pool stores around don't have computerized testing -they usually just test for pH and chlorine. I can't wait to get my kit so I can have accurate results!

[chem geek]

It would take almost 90 pounds of Dichlor to raise your CYA from 0 to 330 (in your 16,400 gallon pool) and this would be adding about 360 ppm FC. Of course, you probably started out with some CYA before using Dichlor, but even so it does seem strange that you got so high. Of course, if you were adding copious amounts of Dichlor regularly, say at a rate to add 2 ppm FC per day, then over 6 months this would be 360 ppm FC cumulatively so one season of dichlor could indeed give you 330 ppm CYA.

Yes, unfortunately you have to drain your pool by much more than a few inches to lower your CYA. There are ways of replacing your pool water safely without lowering your water level. These involve using a large plastic sheet on top of your pool water and then draining from underneath the sheet while adding new water on top of the sheet (letting the sheet get lower in the pool). I can't find the threads that talk about this and related techniques -- perhaps someone else will.

I also agree that it would be good to test the CYA yourself, but even any CYA over 100 is going to require significant drain/refill.

Richard

[mccato]

Thanks, Richard. I used 15 pounds of dichlor when I did use it; so your calculations make me feel better! The idea of using a plastic sheet to add water to the top while draining from underneath sounds good too. I've just not been able to find a sheet of plastic that's big enough!

[chem geek]

Take a look at this thread especially on the third page and later. Though this looks like a scene out of Cocoon, you could use large plastic garbage bags instead of a plastic sheet (or tarp). If you come up with a creative solution and you think it looks funny, be sure to post some pics if you can so we can all get a good laugh. The technique does work, but it can sure look strange!

Richard