When to shock

Chlorinating, maintaining the right chlorine levels,
chlorine problems. Dichlor, trichlor, cal hypo, bleach,
granules, chlorine pucks and chlorine sticks.
bnairb
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When to shock

Postby bnairb » Fri 15 Jul, 2011 10:32

My pool store says just shock the pool every 7-14 days, but is this really necessary or a waste of chlorine. Also I have to leave the solar blanket off and it takes DAYS for the chlorine level to come back down, therefore I lose a lot of heat.

From what I've read on the reason to shock is to get rid of the combined chlorine. So if my tests show I don't have any, I assume there's no reason to shock it. We don't swim a lot and the auto chlorine feeder seems to keep the total and free chlorine stable indefinately (well i haven't waited more than 2 or 3 weeks to shock so far).

I also I'm guessing that if the above is true about shocking to get rid of the combined chlorine, and I don't have any, that may explain why it takes so long to get the chlorine back down to a safe level.

Thanks.


Lindon
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My Pool: Outside, in ground, vinyl liner with 'hopper'shaped 7' deep end. Solar covered. L=30' x W =15' (approx. 14.000 imp. gallons.) Sand filtered, outside gas boiler heated to 28C in season. FROG mineral cycler. 3/4hp pump. Tap water is TA= 138ppm, PH=7.1 Ideally like to see pool reading CYA = 40ppm, CL = 1.5ppm, TA = 100ppm, PH = 7.7. Test kit is AquaChek digital analyser & 3 way test strips(PH,TA,CL) plus test strips for CYA.
Location: England

When to shock

Postby Lindon » Sat 16 Jul, 2011 15:35

Hi bnairb, as long as all other readings are always consistent i.e. stabiliser/conditioner (cyanuric acid) are in the 30-50 range, TA around 100 & PH near but under 7.8 & CL NEVER allowed to drop below 1ppm, vacuum & backwash regularly & you never have signs of green or cloudy water, with good recycle/flow rates (GPM) & good filter pressure (PSI) then you should very rarely IF EVER need to shockdose or use algaecides. If you 'listen' to your pool (twice a week) & it tells you its fine then give it break, be happy with it... & save yourself a shed load of money!
chem geek
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When to shock

Postby chem geek » Sat 16 Jul, 2011 16:53

Though I agree that if you properly maintain chlorine levels you do not need to shock the pool, I disagree that a minimum level of 1 ppm FC is appropriate for 30-50 ppm CYA. Roughly speaking, for manually dosed pools, the minimum FC should be 7.5% of the CYA level. This will handle pretty much all pools including those with high algae nutrient (phosphate, nitrate) levels. So that's about 2.2 ppm FC for 30 ppm CYA or 3.8 ppm FC for 50 ppm CYA. Saltwater chlorine generator pools can usually operate at a somewhat lower level with an FC that is 5% of the CYA level, so 4 ppm FC with 80 ppm CYA for example.
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When to shock

Postby czechmate » Sat 16 Jul, 2011 18:13

Richard,
is it the salt content, that lowers the FC requirement for correlation with CYA?
I do maintain around 2400ppm of salt in my pool just for the benefit for the skin.
Thanks.
chem geek
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When to shock

Postby chem geek » Sat 16 Jul, 2011 19:28

No, the salt level is not directly related to the lower FC/CYA ratio requirement for saltwater chlorine generator pools. The lower level has to do with 1) having more continuous automatic dosing (i.e. the 7.5% minimum has a little bit of buffer in it just in case it gets a little lower than that and 2) the superchlorination of a portion of the water passing through the cell.

Even when using an automated dosing system such as a peristaltic pump one usually needs to keep the minimum chlorine level closer to the manual-dose minimum rather than the saltwater chlorine generator minimum so there is more than just the continual dosing going on. When hypochlorite sources of chlorine are added to the pool, the chlorine level is locally high so gets some super-chlorination effect, but the pH is also high so the active chlorine level is limited. In a saltwater chlorine generator cell, however, the area near the chlorine generation plate not only gets high FC levels, but the pH is low as well so the active chlorine level is much higher. It's only when such water mixes downstream with the high pH water from the hydrogen gas generation plate that the net effect is similar to that of a hypochlorite source of chlorine.
bnairb
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When to shock

Postby bnairb » Mon 18 Jul, 2011 12:18

Thank you. This is what I hoped to learn. So keep the chlorine level stable and only add shock when combined chloramines are preset. That brings me to two additional questions. 1. I had 0.5 ppm combined chloramines today. From what i've read, i need 10 times that additional chlorine to get rid of it, which would be 5ppm additional chlorine. If my total chlorine is 1.5 ppm now, then that means I only need to shock it to reach 6.5ppm. Is that correct?

The 2nd question is what should my "constant" ppm of chlorine be. I thought is was 1-3, but the posts suggest it is also based on how much cya is there. I use 3" trichlor in an auto feeder. My cyanuric acid is somewhere around 40-80.
chem geek
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When to shock

Postby chem geek » Mon 18 Jul, 2011 20:38

If you are using a FAS-DPD chlorine test using a 10 ml sample size, then 0.5 ppm CC is OK since it's really <= 0.5 ppm. Also, the 10x rule for shocking is wrong -- that only applies to the amount of chlorine in ppm Cl2 units needed to oxidize ammonia in ppm N units. CC is in ppm Cl2 units, just like FC, and is not in ammonia ppm N units -- there is a factor of 5 difference in those units. Also, CC already has one of the 1.5 chlorine it takes to oxidize it.

If you do get higher CC, then a reasonable shock level is that used to get rid of algae which is an FC that is 40% of the CYA level.

As for your FC level, it should be 7.5% of the CYA level since you don't have a saltwater chlorine generator. It sounds like you aren't using a proper test kit (perhaps you are using test strips?) so the first thing you should do is get either the Taylor K-2006 or the TFTestkits TF-100 . Then read the Pool School to learn more about how to maintain your pool. Note that for every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm. With a 2 ppm FC per day chlorine usage, this is over 35 ppm PER MONTH increase in CYA if you don't have any water dilution.
bnairb
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When to shock

Postby bnairb » Thu 21 Jul, 2011 10:16

I have a taylor kit i use every week or two, in between i use strips just to see if anything is way off. I had not dne the cya test in the kit so i did last night. 90ppm. I backwashed for a while and refilling and will test tonight. Which brings up the next question. Obviously as you stated, the stabilized trichlor tabs are putting in more stabilizer than i am backwashing out on a regular basis. Since I don't need to backwash that often, is there another form of 3" tabs I should use that won't keep increasing the stabilizer? ( my pool store is a little rinky dink, which is why I'm getting better answers on this forum) Also, I backwashed a good 3 or 4 inches out and added fresh water, but is there a more calculated way to know how much water needs taken out/added back to get the cya down. pool is 18X36, deep end 8-9', shallow about 3". I think it's in the range of 22,000-24,000 gallons, or is what I'm doing (drain, refill, retest) what I should be doing?
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When to shock

Postby czechmate » Thu 21 Jul, 2011 11:24

To drop CYA to comfortable level of 40-45 would require to drain 50% of your water.
Now in hot summer is best time to do it.

Here is why:
Water table below your pool is very week an low so you can drain 70% without much worry about hydrostatic pressure.
Your water will get a fresh cool supply of water and it will be more pleasant to swim in.
Your demand on FC will drop to half of todays (4.5 instead 9 ppm) and save you money daily.
Your cooler water will melt chlorine puck a bit slower.
Also pool tile area water will be a bit cooler .
That is the first place algae starts, due to very little chlorine and warmer water.
It will cost you about 14000 gal of water and of course salt and borates if you have it in now.
Do not add conditioner, you will have plenty in there to start with allready!
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When to shock

Postby chem geek » Thu 21 Jul, 2011 12:02

bnairb wrote:Since I don't need to backwash that often, is there another form of 3" tabs I should use that won't keep increasing the stabilizer?

Unfortunately, there are not slow-dissolving tabs of chlorine that do not add something extra in the water. All Trichlor tabs add CYA since it's part of the chemical itself -- CYA isn't "added" to the tabs. For every 10 ppm Free Chlorine (FC) added by Trichlor, it will increase Cyanuric Acid (CYA) by 6 ppm. There are Cal-Hypo tabs that can only be used in special chlorinators or feeders designed for them (since they dissolve at a different rate -- also one NEVER puts Cal-Hypo in a Trichlor feeder or vice-versa or explosion can result). However, Cal-Hypo tabs tend to fall apart as they get smaller and they leave more gummy residue (binders). For every 10 ppm FC added by Cal-Hypo, it also increases CH by at least 7 ppm.

There are ways of automating the addition of chlorinating liquid or bleach by using The Liquidator or a peristaltic pump. One can also get a saltwater chlorine generator (SWCG).

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