adjust cya before or after shocking

[BigDave]

Use bleach, it'll be eaiser, unless toting it around is an issue.

[PoolDoc]

CYA = 40 this morning so according to "Best Guess" I need to shock at FC = 15 and stay there or above.

Yes! And, brush all the algae spots AFTER adding a chlorine dose.

(Thanks, Dave!)

[Kateyru]

no problem toting bleach...easier than toting 40 lbs. bags of salt. yes, brush, brush, brush! My new mantra: Yes I can...defeat algae!!!
Thanks!

[aylad]

Your pH is fine where it is. Don't bother with adjusting it until it needs to be. If you get down to 7.0, then just adjust it up with Borax.

In a 22K gallon pool, 5.5 gallons of 6% bleach will get you up to 15 ppm. After that, each 1.5 quarts will raise your FC by 1 ppm, so you can use that as a guide. With your SWCG off, raise your chlorine to 15 ppm, (I would do this in the evening), then test in the morning and sometime during the day, adding whatever amount of bleach necessary to get back over the 15 ppm mark. Brush the pool daily, ( I would do this in conjuction with your bleach additions) so that you're knocking all the algae loose, getting better exposure to the chlorine. Hold that 15 ppm until you're not losing any chlorine from night time til in the morning, and then you can let it drift back down. Keep the pump running (but not the SWCG) and keep an eye on your filter pressure, so you can backwash it as needed (when the pressure rises 8-10 psi over your clean pressure).


This procedure is not difficult; but it does take persistence on your part to get it cleared up. If you don't fully get rid of the algae before you let the Cl levels come back down, or if you aren't consistent about maintaining your Cl levels, then you're going to be fighting algae all summer.

[Kateyru]

Okay, I think I've got it. Added 5.5 gallons of 6% bleach this evening then brushed all the sides and as much of the bottom as I could reach. SWG is off and pump set to run. I'm going to look back over the posts from my last attempt to shock and try to figure out where I went wrong. Maybe I didn't go long enough with FC at 15 and not losing any chlorine from pm till am.
Thanks.

[Kateyru]

It's clear to me now that I didn't keep FC at or above 15 (shock level) long enough even though I got to the point where I didn't lose more than 1 ppm and CC was < .5 from evening till morning. The FC was just too low to kill all algae. Once FC got down to 6, the algae was obvious. Thanks, Aylad.
Question for Aylad--why are the chlorine amounts you said are needed to shock different than what I calculated with the Pool Calculator?
Thanks.

[PoolDoc]

Question for Aylad--why are the chlorine amounts you said are needed to shock different than what I calculated with the Pool Calculator?

I'll answer that:

(1) The Poolcalculator is NOT ours, but it is based on Chem_Geek's chlorine spreadsheet, which laid out the analytical basis for my Best Guess chart, which was partly analytical, but partly based on my field experience.
(2) Both the calculator, and Chem_Geek's spreadsheet attempt to be EXACT. We don't, and not by accident. Pool volume calculations, test kit errors, and dosing errors all come together to make the sort of exactness suggested by those tools, unreachable in practice.
(3) So . . . we try to give estimated doses, that are calibrated to avoid problems. In practice, this means we estimate CHLORINE doses HIGH, and everything else LOW.

What Janet did was give you a dose for 15 ppm MORE chlorine -- in practice, that's the right thing to do, since she didn't know how much chlorine you'd have in your pool, when you added your dose. But, when you used the pool calculator, you probably took the chlorine level from your LAST test -- even though that value was unlikely to be correct by the time you added more chlorine -- and entered that into the calculator. So, the calculator told you how much MORE chlorine you should add to get there.

Mathematically, that's correct. But, practically that's wrong.

Richard (Chem_Geek) doesn't agree with me on this point. But, I think that's partly personal error. Richard is EXTREMELY careful and meticulous in his personal habits, and tends to assume others will be, too. He recently told me he'd NEVER had even a drip when using muriatic acid, as we discussed how we should tell people to add acid to pools. If anyone else had told me that, I'd assume they were exaggerating or outright lying. But, I'm pretty confident that he may be the only pool person who is that precise.

So, the pool calculator and his spread sheet work out for HIM, as exactly as they imply. Of course, if there's another pool owner in the USA as exacting and precise as he is, I'd be surprised.

My view is that the pool calculator is more exact in theory (apart from a few errors it has embedded), our approach is more correct in actual practice.

[Kateyru]

Thank you so much for taking the time to explain that. Makes perfect sense!

I kinda screwed up as I ran out of titrating reagent this morning BEFORE I could test. Should have been paying attention to that... I ran to the store and got distilled water to test that way. My best guesstimate using distilled water testing is that FC is at least 15. I actually did test FC before I added bleach last night so I could be fairly accurate with Pool Calculator calculations but then ended up adding a total of 5.5 gallons of 6% because that's what Janet said to do (and now I understand why). So, last night, FC should have been at about 21.5. All that to say that I think FC is at least 15 now. So, should I add more bleach now for insurance? If so, how much? Should I be testing pH when I test FC?
Thanks!!!

[aylad]

Leave the pH alone and don't worry about it. With the chlorine level that high, the pH result won't be accurate anyway, but if you're just using bleach, the pH is not going to change enough to be an issue. Focus on your chlorine and forget about the rest for awhile.

If you "add more bleach for insurance", then you may just be wasting bleach, so I wouldn't do that. Test your water again, estimate the level, then add bleach if you're under the 15 ppm mark. (And if you did get to 21 ppm last night, it won't hurt the pool--it would just be more available chlorine to work on the algae).

[Kateyru]

Ignore previous post as I think I am muddling through. Here's another related question. I reviewed the thread about my previous efforts to get rid of algae. Quick recap: in process of shocking, and after turning off SWG, FC test results were:

4/11/12, 5PM, FC =15.5
4/12/12, 6AM, FC =14
4/13/12, 7AM, FC =10
4/13/12, PM, FC = 8
4/14/12, AM, FC = 7
4/14/12, PM, FC = 7
4/15/12, AM, FC = 7
4/15/12, PM, FC = 4.5
4/16/12, PM, FC = 7.5
4/17/12, PM, FC = 9.5
4/18/12, PM, FC = 8
4/19/12, FC = 10.5
4/20/12, PM, FC = 8.5
4/21/12 PM, FC = 7
4/22/12, PM, FC = 4.5 (I turned SWG back on at this point)
4/23/12, PM, FC = 5 (I added 48 oz. of CYA, don't know if I added bleach)
4/25/12, PM, FC = 11.5 LEFT TOWN
5/4/12, PM, FC =10
5/5/12, PM, FC =9 (added 48 oz. of CYA)
5/6/12, Am, FC = 8
5/7/12, AM, FC = 6.5
5/7/12, PM, FC = 6
5/8/12 I"VE GOT ALGAE FOR SURE!
5/8/12, PM, FC = 6.5 and shocking starts

Also, re: test results, just fyi, when tested during shock period, CC was always .05, < .05, or 0.

As I said in previous post, it is clear to me now that I didn't keep FC > 15 long enough.
So, here's my thought/question:

**** Can algae, over time, raise its tolerance to FC? That is to say, if you don't get all the algae when you shock (due to human error or whatever), at some point in time after repeated unsuccessful attempts to shock, will the FC need to be maintained > 3 - 5 ppm in the future to keep algae at bay? I am not a chemist (obviously) and have never had much interest in chemistry but am somewhat fascinated by all of this.
An aside, it seems like there could be some really interesting science projects out of pool water study. I'll have to remember that when my grands are a little older