Shock process and funny numbers

[doncaruana]

Had some unusually high chlorine demand (for no apparent reason, but that's another issue) so started down the shock path. Mind you, through this entire process, CC has never been over 0.5.

I lowered the pH to 7.2 or 7.1. First question - that's a good level to start with, right? Is it okay if I slightly overshot the pH lowering?

Now, the numbers...When I do the math myself, 121oz jug of 8.25% bleach in my ~15,500 pool (mind you I'm 100% sure on the pool volume), I get that it should raise the FC by 5.03ppm. But the pool calculator says it should be 5.2. Does anything else affect the FC numbers?

So, about that bleach...I measured the pool at a little over 6FC and, with my CYA at 50, the pool calculator says shock level would be 16. So I figure 2 bottles of the 8.25 to start and go from there. But after the 2nd bottle and letting it circulate (poured it slowly in front of jet, let pump run at full speed) for 30-45 minutes, the level I got was 21! I took my sample from the opposite end of the pool, but did I let it circulate long enough? And by overshooting so much, is that an issue?

Regardless, we're off now, so I let it go over the day yesterday and check the level at night. It's all the way down at 14. So, using the pool calculator, 72 ounces of 8.25% bleach should raise me by 3.1, which will get me to 17, which should be good. But, after the same process (slowly pour in front of jet, let circulate for 30-45 minutes), my FC is all the way up to 21! Instead of raising it 3, it raised it 5! Huh?

Okay, well, either way, it's at shock level, let it roll. I test it this morning and it's at 18.5. CC was 0.5 or 0 (honestly don't remember right this second), but a 2.5 drop in FC. I'm still over shock level, so I'm not adding any more bleach. But can anyone make sense of all this for me? I'm extremely confused about the above...

 

[Bama Rambler]

A lot of builders, pool stores, owners overestimate the volume of their pools a lot. That sounds like what's happened with you. Keep track of what you add and how much it affects the pool and over time you can adjust the volume to match the changes.

 

[Bluechip]

There is no need to be that precise. 5.03 vs 5.2? Close enough in my book. How are you 100% sure of pool volume? It could be a few hundred gallons on either side of 15,500. Lot's of variables in a pool dig. 8.25% bleach could be a little stronger than that if it's fresh or a little weaker if it's been sitting around for a while. After you have been doing this for a while you will know that one jug raises the FC by x amount. Don't worry about the exact numbers. You sound like my son who has to have everything work out to a precise answer. He's studying to be an engineer. There are times when preciseness is needed but pool chemistry is not one of them.

Was the 2.5 ppm drop in FC overnight with no sun on the pool? If so then you have something consuming the FC and need to continue with the shock process. What part of the country are you located. This heat we are experiencing can cause a larger FC loss than is normal. It sounds like you have a good grasp of what you need to do. Just trust your tests and do what the results tell you to do. Good luck.

 

[doncaruana]

I'm 100% sure of the volume because I know every dimension in the pool (I had it built) and I've calculated it out and modeled it multiple times. No one told me the gallons, I calculated them and the math otherwise has consistently reinforced the numbers. Yes, it could be little bit here or there (every inch the water is higher or lower is 319 gallons), but that's a 2% difference and in the grand scheme is close enough. The point is, there's no way I'm more than a few hundred gallons off.

What I'm trying to do is understand why the numbers are so different. If I strictly had a 32x16 flat pool with no deep part (the shallow is 42 inches), I'd have over 13,000 gallons. These numbers are acting like the pool is only 10,000 gallons or like it's 12.5% bleach. Can there be that much fluctuation in bleach? It would degrade, not get stronger if it sat around.

As far as the drop, no sunlight. And in southeastern Michigan it hasn't been that hot the last couple of days anyway.

Yes, I'm continuing the shock process, but one thing I don't understand is why no CC if it's using up that much FC? Shouldn't I still be getting some CC showing up?

 

[harleysilo]

Do you have a drawing/diagram you've made that we could use to double check you pool capacity calculations?

 

[Richard320]

Sometimes the bleach is made overstrength so that as it degrades sitting in warehouses and on shelves, it still meets the stated strength. I've had batches where it was WOW that FC is high!

 

[doncaruana]

Sometimes the bleach is made overstrength so that as it degrades sitting in warehouses and on shelves, it still meets the stated strength. I've had batches where it was WOW that FC is high!

Thank you! That makes sense! Am I letting it circulate long enough before testing though? For liquids, is 30 minutes with pump running and decent rule of thumb?

 

[Richard320]

Sometimes the bleach is made overstrength so that as it degrades sitting in warehouses and on shelves, it still meets the stated strength. I've had batches where it was WOW that FC is high!

Thank you! That makes sense! Am I letting it circulate long enough before testing though? For liquids, is 30 minutes with pump running and decent rule of thumb?

Should be. If there's any doubts, get in there with the brush and work your way around the walls. That will set up enough cross-currents to really get it dispersed. Especially if you use a wall whale. If I go down one side of my pool, a few seconds later I see an eruption of water on the opposite side.

 

[doncaruana]

If there's any doubts, get in there with the brush and work your way around the walls. That will set up enough cross-currents to really get it dispersed. Especially if you use a wall whale. If I go down one side of my pool, a few seconds later I see an eruption of water on the opposite side.

Yeah...I don't actually have a brush of any type, much less one that cool...Maybe that's one of my issues...

Will that one work with my vinyl liner?

 

[Richard320]

If there's any doubts, get in there with the brush and work your way around the walls. That will set up enough cross-currents to really get it dispersed. Especially if you use a wall whale. If I go down one side of my pool, a few seconds later I see an eruption of water on the opposite side.

Yeah...I don't actually have a brush of any type, much less one that cool...Maybe that's one of my issues...

Will that one work with my vinyl liner?

Yep. It's got nylon bristles.

 

[doncaruana]

Do you have a drawing/diagram you've made that we could use to double check you pool capacity calculations?

Here's the final specs. I've tape-measured enough it to call it good enough at least for pool calculations. I could go into all the gory detail on the math, too, if anyone likes. Keep in mind, the water level is typically 6 inches from the top of the pool, so the water depth in the middle (deepest part) is ~5'6". And the corners are 2' radius rounded (negligible difference) and I have steps that are outside the outline, which would give it a few extra gallons, but not many, but at least offset the rounding on the corners. Going by all that, I get 15,439.79 gallons...give or take . So I just roll with 15,500 and often round to 16K in conversation.