adjust cya before or after shocking

[Kateyru]

CYA = 40 this morning so according to "Best Guess" I need to shock at FC = 15 and stay there or above.
Dave, algae is mostly on the bottom and lower sides of pool. Thanks for recapping the steps.
I'll edit signature.
Thanks!

[BigDave]

Do you still have the dichlor? You can use that to move toward your shock level and your CYA goal but remember as CYA goes up so does your FC goal for shock and normal operation.
How much dichlor do you have on hand?
Is there still CYA in the stocking?
Do you bleach / Liquid chlorine on hand? How much?

[Kateyru]

No more dichlor on hand. There is still CYA in stocking but I have removed it from the pool. I don't have any bleach on hand so was heading to the store. Should I use dichlor or bleach? I haven't done a cost analysis. If I use dichlor, do I need to raise pH to 7.6 before I start?

[BigDave]

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

[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.

[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!