Does chlorine really dissipate this fast?

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shellsingleton

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Mar 10, 2008
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Lafayette, LA w/pool in Hot Springs, AR
Pool Size
10000
Surface
Fiberglass
At least I think that's the question I want to ask...
Here's the situation:
Yesterday, zero chlorine in the pool, water slightly greening.
I have been using the 60 algeacide for about a week now, did the original startup dose and followed up with maintainence doses. (Water still foams around the waterfall so I think I have enough residual?)
The filter runs about 12 hours a day, broken up into 4 hour segments.
So, I decide to shock it, poured 3-182 oz bottles of 6% original Clorox into the pool at about 1230 this afternoon.
Around 1830, I broke out my brand new TF-100 test kit and found out that I had a whopping 2.5 ppm FC in the pool.
According to the pool calculator, it should have raised it to 13 ppm. (I know 15 ppm is recommended, but I forgot to include the spa, tanning ledge, steps, etc into my calculations)
So, I retested. Same results.
Can that much chlorine waft away in 6 hours of full sun? :shock:
The water does look a 'lil better, but I'm concerned that I'll need to add another 4 gallons of bleach tomorrow. (Just joking... mostly)

The water's 62*, we aren't using it yet, and won't be until mid May or so probably. What's a decent minimum to shoot for with FC until then?
I read the CYA/Chlorine chart. That's FAR more ppm then I expected to have to maintain. My CYA is at around 90, so that puts my min FC at 6.6 ppm. How much bleach is that per day for a pool such as I have in my sig?
 
Forgot a couple things I wanted to ask.

Thinking I'd get a good sanitizing of the jets' plumbing, I poured my bleach into the spillover spa this afternoon. That was pretty stupid. Even with the jet's aereators closed, it generates a significant amount of super-fine bubbles, and I bet that helps the chlorine evaporate. Right?

Also, half past noon is about the worst time to add chlorine to the pool.

In your opinion, how much effect did these two mis-steps have on my attempt to shock?
 
This sticky from Jason is appropriate to your situation.

With a CYA of 90 ppm, it is most likely that most of the chlorine is being consumed killing algae. That can take a LOT of chlorine and initially it will consume more and then start slowing down as most of the algae gets killed (turning green to gray to cloudy) and then clearing. An extreme sequence upon an opening where the pool was let go over the winter is shown in this thread.

Unfortunately, at 90 ppm, the 6.6 ppm FC minimum you refer to is for preventing algae in the first place, not for fighting algae that has already bloomed. The shock FC level in the table says 35.2 ppm which would be for a relatively fast kill, but that's a heck of a lot of chlorine so in practice one uses less, closer to 20 or 25 ppm and keeps adding chlorine to keep the level high, adding more chlorine as needed multiple times per day if necessary, until the algae clears (you can lower the pH first to around 7.2 before increasing the FC to such high levels after which you won't be able to accurately measure the pH -- high FC interferes with the pH test). It's the high CYA level that makes this process more difficult due to requiring higher levels of chlorine, so another alternative is to do a partial drain/refill to lower the CYA level (while still having some chlorine in the water to keep the algae somewhat at bay). At half the CYA level (i.e. 45 ppm) it takes around half the chlorine level to both fight and keep away algae (about 18 ppm FC for shocking; about 3.2 ppm FC minimum to keep algae from growing).

Though the aeration in the spillover spa will tend to outgas chlorine, this usually isn't significant unless the temperature is much higher. At spa temperatures and aeration, chlorine dissipation from outgassing can be significant, but in pools at the lower temperatures it usually isn't. For pools, the main loss of chlorine, other than fighting an algae bloom, is from degradation from UV rays in sunlight, but that's not your situation since the CYA is protecting the chlorine from that.

The use of PolyQuat 60 algaecide, by the way, is OK for helping to prevent algae from growing, but it is slow-acting so doesn't do much good after growth has taken off and a bloom has started -- it is simply overwhelmed. In the future, it is easiest to make sure the CYA level doesn't climb too high which means not using as much (or any, after a time) stabilized chlorine (Trichlor, Dichlor) and instead using unstabilized chlorine (chlorinating liquid, bleach, or sometimes Cal-Hypo) as a main source of chlorine. Usually a CYA in the range of 30-50 ppm for manually dosed pools is sufficient to protect chlorine from sunlight. Some people keep a higher CYA level for more protection, but if you do then you really need to make extra sure the chlorine doesn't fall below the minimum FC level. If one doesn't mind spending extra money, one can use a weekly maintenance dose of PolyQuat 60 (or a somewhat lower dose each week) as insurance (for prevention) in case the chlorine drops too low for a short time, though this is not necessary if one maintains the FC level at all times.

Richard
 
Thanks for pointing out that sticky. I have read some of them already, but hadn't made it to that one yet. Excellent info there. I feel like my situation is far less extreme than the one outlined by the sticky or documented in the other thread.
Same theory still applies so I will use those steps for fighting the algae.

Here's my plan if you care to comment:
Backflush my sand filter today and clean out any leaves.
Use some Muriatic acid to drop the pH to 7.2 overnight tonight.
Tomorrow morning, because my CH is only at 170 ppm, I could use some pre dissolved Cal-hypo to intially shock it, killing 2 birds with one stone. Is a 100 ppm TA high enough to cause the Cal-Hypo to cloud the water?
I plan to maintain the FC levels all day with my Clorox.
I will be brushing the pool once the initial shock is underway.
Maintain as needed until the FC levels stabilize as described in the sticky.
 
Calcium clouding from Cal-hypo depends on the calcium saturation index (CSI), which is calculated from all of the numbers from a full set of water tests. If the index is positive there is some chance of clouding but the amount of clouding is not usually significant until the index is above 0.5 or 0.6. I don't see enough information in your post to calculate your CSI. You can use my Pool Calculator to calculate it, see the link in my signature.
 
I have some confusion concerning the pool calc.

It mentions Cal-Hypo in the FC section, and it mentions calcium chloride and calcium chloride dihydrate in the CH section. Are those chemicals both considered Cal Hypo?
What are the diferences in their effects?

Here are my numbers and my current CSI as of last night.
FC 2.5
CC .5
pH 7.6
T/A 100
CH 170
CYA 90
Temp is 62*.
CSI is currently at -0.26

If I change my pH to 7.2, raise the FC to 30 ppm, and get my CH to 300, my CSI would be -0.5 during the shock and -0.14 after I put my pH back to 7.6 afterwards.

So, the other question is how do I determine how much the CH will be raised by the amout of Cal-hypo I plan to add? Will it end up anywhere near my 300 target range?
How can I get my csi positive, like a plaster pool needs?
 
Cal-hypo is calcium plus disinfecting chlorine. Calcium Chloride is calcium plus salt (not really, there isn't any sodium, but close enough). Calcium Chloride Dihydrate is calcium chloride plus water (strange as that sounds).

For every 1 ppm of chlorine you add with cal-hypo you also add about 0.7 ppm of calcium. You need to use cal-hypo over a period of time to have a significant effect on calcium levels. You can use my Pool Calculator to do the math for you. Look at the bottom for the "Effects of adding chemicals" section, enter the amount of cal-hypo you intend to add, and it will tell you how much chlorine and how much calcium that is equivalent to.

You don't need to get the CSI positive. Anything between -0.6 and +0.6 is fine. You usually need to go much further than that to have significant effects. It is good to have the CSI more or less near zero when PH is 7.5 so that any PH swings will still leave CSI within reasonable bounds. That is why Pool Calculator cautions you when CSI is outside -0.3 to +0.3. But when using CSI to figure out what happens when you raise or lower the PH intentionally for short periods of time you can allow the CSI to range much further.
 
I hit it with 120 oz of calhypo last night, which should have brought it up to about 30 ppm FC in my 14,100 Gal pool. This morning, the FC is at 13.5, and I have no CC. The water's clear, should I shock it back to 30 FC one more time since it didn't quite retain 1/2 the FC overnight or can I just maintain the FC at the minimum suggested by CG's CYA/FC chart?
 

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This evening you should test the FC and bring it back up to shock level. Then, about an hour after you add the chlorine, measure the FC level again and then yet again in the morning.

There are two possibilities. Either the Cal-hypo is not having the effect you intend or you have something consuming chlorine. The steps above will tell you which one is happening. If the FC test tonight reads low than the cal-hypo is either spoiled or a lower percentage than you thing or the pool is larger than you think. If the FC test tonight shows FC where you expect it to be then there is something consuming chlorine and you need to keep the pool at shock level till whatever it is goes away. It is rare, but quite possible, for something to be consuming chlorine even when the water is completely clear.
 
Not trying be dense or argumentative here, just trying to follow your train of thought and learn something.

I thought that no CC indicated no substantial activity regarding the FC?
I used 48% CalHypo that I purchased yesterday for this treatment. Also took detailed depth and spa measurements which led me to adjust my volume to 14100 from the estimated 21000 earlier.

Are you saying that, with a CYA of 90 ppm, it's not normal for 30 ppm FC to drop to roughly half that level overnight? Even if the filter is left on overnight?(The spillover spa keeps the water moving pretty well...)
 
The lack of CC doesn't really tell you anything. Depending on the chlorine level and the amount of sunlight and what particular thing the chlorine is breaking down there may or may not be any CC even when there is something the chlorine is fighting. Algae isn't the only candidate, particularly if you just opened the pool for the spring. If there is CC then that tells you something, but the lack of CC isn't really an indicator.

Cal-hypo can break down if it is stored incorrectly. This could have happened at the pool store or between the manufacturer and the pool store. It is also possible that you got some from a batch that was mixed incorrectly or labeled incorrectly and so contains less than the usual 48% of cal-hypo. Both of these things are fairly unlikely, but they do happen.

Pool volumes are always estimates. Even if you had an exact number from the builder, the pool isn't always filled to the same level so the actual amount of water varies. I wouldn't expect an error in the pool volume nearly as large as it would take to account for your measurements, but it might be contributing.

Regardless of the CYA level there shouldn't be any significant drop in FC overnight. If you had a 600 GPM frothing waterfall there would be some drop, but that doesn't sound like what you describe having. With a CYA level of 90 I would expect you to lose perhaps 20% of your FC over a full 24 hours on a sunny day. The CYA measurement might be wrong, but if no sunlight was involved that doesn't make any difference.

Now if algae, or something else that is consuming chlorine, is involved, then the CYA level make a large difference. High CYA levels will dramatically slow down the rate at which the chlorine fights algae/etc. With zero CYA and a massive algae bloom the chlorine would all be gone in under an hour. But with CYA around 90 the chlorine will work more slowly and 50% could still be left over after 12 hours even when there is plenty more algae/whatever to be fought. With clear water it presumably isn't algae, but there are other things, several of which you can't see, that can consume chlorine in a similar way and which you want to get rid of before you swim (urea, ammonia, etc).
 

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JasonLion said:
This evening you should test the FC and bring it back up to shock level. Then, about an hour after you add the chlorine, measure the FC level again and then yet again in the morning.
Click to expand...
shellsingleton,

Please follow the advice I quoted from Jason above. This will really tell us a lot about what is going on. If the Cal-Hypo is weak, then there will be an almost immediately measurable lower FC than what is expected. If the one-hour measurement is OK (not a large drop from what is expected) and there is a drop in FC overnight, then the chlorine is still getting consumed -- my guess would be algae even though it's no longer visible. If the rate of drop overnight declines over time (i.e. the next overnight drops less), then this means chlorine is oxidizing whatever is consuming it and will eventually be OK. It may just be that the cooler water of 62F is making this process of getting rid of all the algae take longer.

Your plan of cleaning (backwashing) the filter is also good since lots of organics in the filter will consume chlorine for a time until they break down (especially "small" high surface area organics such as pollen).

If, after doing the above at shock levels, you go back (in the future, not now) to normal FC levels and find the FC consumption to be higher than expected, especially overnight, then the last thing to check would be the phosphate level. If it's extraordinarily high (>> 3000 ppb), then that might explain the excessive chlorine demand as algae growth is more rapid at such high nutrient levels.

Richard
 
Lots of great info to chew on.

I'll definitely check the water this evening, then shock, then recheck after an hour.
I'll double check the CYA also to make sure it's accurate as well.
There has been a ton of pollen released in the last 2 weeks around here. I didn't think about how the organic matter in the sand would still consume FC, so I'm glad you mentioned that. I thought I was fine with just the backflush before the shock cycle. I'll backflush it tonight before I start the process.
BTW- my sandfilter gauge read 34 psi before I backflushed 2 days ago, and read 22 psi afterwards. Still reads about 22-23 psi. Is that kinda high for backpressure?
 
22 psi after backwashing is not a problem in and of it's self. The question is what the psi reading was when the system was new and the filter was clean. If it was 22 psi back then then everything is fine. What pressure you have with a clean filter depends on the size of the pump, size of the pipe, how the plumbing runs, solar, in floor cleaners, etc. The "normal" range is anything from 2 psi to 40 psi depending on those things.
 
Guess I'll never know about the original backpressure, seems to be fine though. Consistently drops to the same psi after a backflush, and it's well below the 50 psi rating of the sand filter.
I had 10 ppm FC this evening before I backflushed and re-shocked. I added 94 oz of Calhypo in an effort to bring it back up to 30 FC. An hour later, I had 24.5 FC. CalHypo was pre-dissolved (mostly) in 5 gal buckets, 4 doses, each poured partially thru the skimmers and then the remainder washed out and thrown into different areas of the pool, including the spa. I'm pretty sure it was well dispersed.
Thing is, as I drove home, I figured out that I had made another error in calculating my pool's volume. It's actually larger than 14,100 gallons. So, the apparent lack of FC is most likely due to this error.
I will remeasure in the morning. I have a slope on both sides of my pool, and then it levels off for a stretch in the middle. So, I can't just take the deepest and shallowest and divide by 2. I need to calculate the volume of the flat section in the middle of the pool seperately from the ends.
I still have all the details of the amounts I've added so I should be able to refigure everything, taking the new volume ito account. Sorry for all the wasted effort so far, but if it makes you feel better, I'm definitely learning alot along the way. :cheers:
I checked the CYA outside in shaded sun this evening and got 75, checked it again in the kitchen like last night and got 85-90. Test is VERY subjective. I think I may just take a sample to a pool store and have them run it through their machine.
Although... the first time I did that, they said I had 40 for CYA, I went back and threw 4 - 3" discs of Di-Chlor in the skimmer. I had them recheck it in 3 days and te CYA was up to 100. The discs were only half dissolved too. I took them out immediately, but is that even possible? Could those 4 half dissolved discs have raised the CYA 60 points in 3 days? Or is there machine no better at reading the CYA than I am?
 
Shell, that CYA test can be more difficult for some of us to read. Richard (Chem Geek) actually posted a link with photos of what the dot is supposed to look like! It helped me lots. Maybe he'll see this and post it up again.

Also, doing our tests in a consistant manner is important. For many of us, it is the kitchen table with the same overhead lighting and similar amount of sunlight from windows.

As Jason said, trust your numbers over the pool store. You're doing a great job! :)
 

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