When to shock

[bnairb]

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]

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]

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.

[czechmate]

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]

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]

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]

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]

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?

[czechmate]

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!

[chem geek]

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).

[chem geek]

Your demand on FC will drop to half of todays (4.5 instead 9 ppm) and save you money daily.

That isn't true if one maintains an FC that is proportional to the CYA level. The FC drop with an FC of 6.8 ppm with 90 ppm CYA is lower than with an FC of 3.4 ppm with 45 ppm CYA (see this post for an experiment that shows this). Though at a given CYA level the amount of chlorine lost from sunlight is proportional to the FC level, higher CYA levels protect chlorine from sunlight more through additional mechanisms such as shielding lower depths from UV (i.e. not just from binding to chlorine itself). So having a higher CYA level often results in somewhat lower chlorine usage in spite of the higher FC level needed to prevent algae growth. This is the main reason why 80 ppm CYA is the recommended level for SWCG pools even though the 4 ppm FC is higher than 2 ppm FC with 40 ppm CYA. It is to lower the absolute chlorine loss (FC drop) to reduce SWCG on-time which reduces hydrogen gas bubble generation and associated carbon dioxide outgassing as well as any undissolved chlorine gas outgassing. This reduces the rate of pH rise in SWCG pools.

[bnairb]

So I understand I need a lot more chlorine at 90ppm cya. So today I decided to do a partial drain/fill to get the level down to reasonable. According to what you said and the calculations I've done I need to drain about 3' which should be roughly half or 10000-14000 gallons. In working on this, I have discovered a pink slime inside the skimmer that I've never seen before. I attached a picture. In briefly looking on the net, I've read that red/pink is a bacteria and not a true algae. I've never had this before, but I've never had this cya prob before so I'm only guessing that, as you've explained, my chlorine level is not high enough in relation to the high cya this year to combat the bacteria. I have not had an algae problem, but I attribute that to using Pool Perfect+Phos free to keep the phosphates out of the pool, so to me it makes sense a bacteria would grow but not an algae. I hate to start the red/pink algae? in another post as i'm sure it's related to this chlorine/cya issue.

So, what is it?
Once I get the pool refilled, chemicals balanced, do I need to do something special for this red stuff?

Also, until I can afford a SWG(maybe next year), how much water per week do you think i need to backwash/refill to keep the cya in the tablets from building back up this season?

[chem geek]

It's hard to tell if that's pink algae. Is it slimy? If so, then it's likely to be pink algae or some other bacterial biofilm. You can add bleach directly to the skimmer during some chlorination and can wipe it with a scrub sponge. Once you start maintaining normal chlorine levels, it should get killed off though if biofilms are present elsewhere (such as in pipes), then you may need to get to shock levels for a while or use a biofilm-breakup product. Unfortunately, there's not enough info to be definitive about the best product to use nor if any of them absolutely work reliably, but as described in this post the Aquafinesse pool tablets did seem to help in this one situation. That's not enough to make a recommendation (we usually wait until we see consistent results from many users), but there's not much else to go on. Again, usually shock levels of chlorine work for pink slime and regular maintenance levels with an appropriate FC relative to CYA keep it away.

[bnairb]

New problem, see viewtopic.php?f=21&t=10471. Pool water yellow after refill and shock.

[Lindon]

Fi-Chlor are marketing tubs of slow dissolving CYA free 'Supercapsuls' each with its own special wrapper which is not removed & which they say should be placed in the skimmer basket. The wrapper is supposed to ensure an equal amount of water flows over the capsules surface area & dissolve at equal rates They claim they can be used in previously stabilised water & also in conjunction with stabilised tablets if necessary providing the pool is not dosed from the same place. I've not used them yet but hoping this may be the answer.