What chlorine level to shock -- now that half my CYA is gone

[JimK]

My appologies if this is the wrong place to post this. I just recently became a subscriber though I've been registered for over a year.

I took the cover off today to start opening the pool. I did a quick check on pH and CYA. pH is 7.2 and CYA is 30, which means I lost about 40ppm CYA over the winter. I know this means I'll probably need more chlorine to get rid of the byproducts from the critters that ate the CYA.

Does this mean I need to shock at a higher level, or should I just shock according to the Best Guess Chart (15ppm CL given 30ppm CYA)?

Thanks.

[PoolDoc]

Exactly the right place to post it!

CYA loss can have 3 end products: nitrogen gas (no problem!), nitrates (not necessarily a problem) or ammonia (the PROBLEM!). The easy way to see what you'll need is simply to shock it, and see what happens. Is your pool really 20K gal? That's small for a 22x40, unless you have no deep end.

If 20K is correct I'd add 5 gallons of 8% bleach tonight, and then test again in the AM. The pH should be raised, too. I'd start with 2 boxes of borax. Low pH makes the byproducts of chlorine oxidation nastier.

If you raise the pH, and add the bleach (about 15 - 20 ppm on a 20K pool) . . . and you have ammonia, you'll have little or no FC in the AM, but may have a high CC. If the pool is slimy but has no ammonia, you'll lose some of the chlorine, but should still have some, and not so much CC. If the pool is just a bit dirty, a lot of the chlorine should last overnight.

[JimK]

Thanks Ben.

The pool installer gave us the 20k gal figure (installed in March 2004). It might be a little bigger, but dosing based on the 20k figure gets me real close. The 22ft width figure is an average of the narrower deep end, 18.5ft, and the shallow end, 24ft I believe. The pool is shaped in what's called a "lazy L" (not a true L, kind of in between a straight rectangular shape an a true L). The shallow end is 3ft (actual water level) and the deep end 5.5ft (they called it a "non-diving pool). I can send you pics if you like.

One winter some years ago I lost all CYA and it took a lot of CL before it would hold. Well, it seemed like a lot but that was when I was operating by pool store rules; I earned here they had me adding too little CL at opening. It was sometime after that when I learned here that CYA could biodegrade and about appropriate CL levels.

7.2 pH is pretty normal for my pool when I first open it. In the past I just let it drift up as the SWCG ran (another lesson I learned several years ago after the first cell died prematurely; only use the SWCG to maintain CL levels, NOT to shock!). I've not noticed any problems in the past doing it this way. Given this is it ok to proceed without the borax (I don't have any borax on hand, just boric acid [I maintain 50-60ppm boric acid in the pool])?

So, I guess the answer to my question above is to shoot for normal shock levels? I'll add the bleach tonight (I bought 10 jugs (121oz) of 8.25% bleach to have on hand for opening) and see how much I lose overnight.

[JimK]

I added five 121 oz jugs of 8.5% bleach around 7:30pm (it's been raining today so no sun) and brushed the entire pool. I just checked CL level (9pm) and it barely registered! This is what it measured:

FC < .5
CC < .5

Strange......... The water is cold (about 60); does this impact the DPD Powder test?

Does this mean I'm fighting ammonia? Should I go ahead and add the other 5 jugs tonight, or wait until morning?

PS -Just found this in a Google search:

"...11. When a DPD chlorine test flashes a little pink then turns crystal clear, your water has almost no chlorine in it.
The opposite is true! More spas are fried and bathing suits bleached because of this error than just about any other. As you know, the universally accepted DPD test for chlorine turns progressively more pink as higher residuals are encountered in the sample… that is until the indicator is itself reverts to a clear resultant. This phenomenon occurs somewhere near 20 ppm chlorine, the actual value varying with different pH or total alkalinity conditions. Many, many untrained operators or “helpers” have dumped excessive amounts of chlorine in their water (most often in hot whirlpools or spas), thinking all along that for some crazy reason they just hadn’t yet put enough in to get a reading! In a small body of water this testing error has resulted in a few hundred parts per million chlorine residual, creating some very unsafe, damaging or at least unpleasant conditions..."

I didn't notice it "flash pink", but I'm wondering if the CL level is over 20 causing a reading error? Out of curiosity, I did another sample and added several scoops of powder (maybe 4 to a 10mil sample?) but that didn't seem to change anything.

Thoughts?

PPS - Found on Taylor site where FAS DPD test can measure up to 20ppm. Hmmmm.... I have a hard time imagining all that chlorine was gobbled up in 1.5 hrs....I'm thinking CL may be higher than 20ppm so I'm not getting a good reading. I'll hold off adding more CL tonight and check it again in the morning.

[PoolDoc]

The 'pink then clear' transition occurs with the DPD tablets; not the with the DPD FAS test.

But . . . while cleaning this up, it might be better to use an OTO drops kit -- OTO is not too accurate, but it's bomb-proof, in the sense of no false-positives OR false-negatives.

OK. Do this:

1. Use borax to push your pH to 7.8 or so.
2. Tomorrow PM late, add (10) gallons of plain bleach.
3. Test with OTO & phenol red in the AM. Add borax as needed to maintain the 7.6+ pH. Remember that phenol red will not return accurate results if the chlorine is very high.
4. Repeat this cycle till you can sustain a strong chlorine reading overnight.
5. At that point, you should resume using the K2006 testing.

It may take a LOT of chlorine to recover. Unfortunately, with a vinyl pool, draining and refilling is not really an option. If you'd like, you can probably drain to 1 foot above the highest horizontal surface, and then refill. (Ie. you can't drain a pool with vinyl covered steps!).

However those are your options, so you just have to suck it up, vow to NOT let 'slime' (CYA-eating bacterial and algal biofilms) grow next winter, and dose with chlorine till you're done, no matter how much it takes.

It's possible to figure out approximately how much total chlorine you'll need. But other then scratching a mental itch, that information has no value: you do NOT want to do an all-at-once dose; there's too much chance you'd bleach your liner.

So, dose, check, adjust pH, repeat . . . till it's all done.

Here's hoping it won't take much.

[BigDave]

I hate to sound like I'm contradicting PoolDoc, but, I do see a value (beyond my itchy brain-scalp-very itchy) to figuring how much chlorine will be needed and that's how much to buy. The logistics of buying, moving, and storing enough chlorine for an algae / ammonia clan-up may be well worth considering; especially for us weekend warriors who can only collect the materials on Saturdays.
My two cents

[JimK]

Thanks Ben.

Looks like I've got a few more items to pick up.

The info I found on the Taylor site referred to the FAS-DPD test using the powder and mention a max test limit of 20ppm:

https://www.taylortechnologies.com/C...P?ContentID=11

.The latest trend among service professionals and a significant number of health officials is testing chlorine with FAS-DPD. This variation of the traditional DPD method allows users to measure both free and combined chlorine levels as low as 0.2 ppm—the maximum allowable level for combined chlorine according to most health authorities as well as the Association of Pool & Spa Professionals (ANSI/APSP)—and as high as 20 ppm. By contrast, the color comparators used with the standard DPD test generally allow readings at the low end of 0.5 and 1 ppm and at the high end of 5 or maybe 10 ppm.

(Bold added)

I'll test again tomorrow morning and see what I get.

Btw, I understand high CL will give false pH readings, so how do I accurately measure pH while shocking? Also you said, vow not to let CYA eating slime grow next winter.... How do I prevent it? Seems like whether or not I lose CYA over the winter has been a crapshoot in the past.....most years no/little loss, a couple times big loss. I don't know what I may or may not be doing that causes this.

Btw, I took a look at what I did at opening last year (I keep records of my pool maintenance) and I added the same amount of bleach, but I didn't test CL until the next day (I got a reading of 16.5). I'm hoping the issue this time is I'm not getting a good reading because CL is over the FAS-DPD test limit, and not that there is so much ammonia that I'm just pouring bleach into a black hole! I should have an answer for this tomorrow.

Oh....forgot to ask.....is it ok to leave CYA at 30 until CL holds? Is this high enough to prevent sunshine from completely depleting CYA?

[JimK]

I hate to sound like I'm contradicting PoolDoc, but, I do see a value (beyond my itchy brain-scalp-very itchy) to figuring how much chlorine will be needed and that's how much to buy. The logistics of buying, moving, and storing enough chlorine for an algae / ammonia clan-up may be well worth considering; especially for us weekend warriors who can only collect the materials on Saturdays.
My two cents

Thanks Dave.

I'm also limited to when I can buy supplies and try to have everything on hand in advance (I figure what I need based in last seasons).

[PoolDoc]

Losing CYA is a bio-degrative process: no "bio"; no CYA loss.

Covering a pool stops solar loss of chlorine, and it mostly stops algae, but it does not stop bacteria, especially once the chlorine is gone. Maintaining minimal chlorine levels under the cover, will almost certainly stop the process. BUT, one class of the operative organisms is Pseudomonas genus, a group notorious for forming sanitizer resistant biofilms.

Following this process of winterization should do the trick:

1. Stop using the pool at least 1 week before closing (to allow all human added organics, etc to be oxidized and/or filtered).
2. Physically clean the pool at this time, removing all leaves and debris, and brushing thoroughly to break up any forming biofilms.
3. Run the filter 24/7 during the final week. (Same reason). Clean the filter 1/2 of the way through the week, to remove all goop from the filter.
4. Raise the pool to maximum chlorine levels at the beginning of this period. Allow it to drop to high-normal levels by week's end.

5a. Winterize the pool OR
5b. Cover the pool, but do not winterize that that time.

6a. Check weekly while water temps are > 60 degrees to maintain chlorine levels OR
6b. As above, but circulate the water for several hours (or for 1 hour per day every day)

7a. --
7b. Winterize the system, once freezing temps are imminent, or water temp is below 50.

8. Continue to maintain chlorine levels all winter, with monthly checking.

9a. Once water temps rise above 60 degrees, check chlorine weekly.
9b. As above, but also open the circulation system, while leaving the pool covered.

10. Open the pool.

[PoolDoc]

Dave has a point about accumulating chlorine.

If it's not convenient to purchase bleach every other day or so, it may be worthwhile to do a chlorine demand test: http://pool9.net/bucket-demand