Chlorine/CYA Chart

[chem geek]

Even under ideal conditions that Jason points out, the reproduction or generation rate (i.e. the time it takes for the population to double in size) for algae is from 3 to 8 hours. This is why the water always turns dull and then cloudy before it turns green (for standard free-floating green algae). However, long before the water even gets dull looking (i.e. not crystal clear) the growth of algae can consume lots of chlorine in the sense that chlorine tries to kill it and there's a lot there even before it starts becoming visible.

Typically, if you completely let a pool go down to zero chlorine then the algae won't start to be visible for a 2-3 days (perhaps a little dull after 1 day) unless the algae was already just below being visible and on the edge of a bloom with the chlorine just barely holding it back.

This is in contrast to bacterial growth where the generation time is around 15 to 60 minutes and is why it is critically important not to let the chlorine drop to zero even for a short time. A pool can look perfectly fine but be teaming with bacteria, some of which may be harmful if swallowed (fecal bacteria, usually).

Richard

 

[DONNIE]

Hey Richard!

I haven't posted in a while but would like to thank you for your help and expertise during my new pool start up back in December 2006. I actually had a "troublefree pool" all summer. Friends and family (and myself) were constantly amazed how clean and clear my pool was. I owe it all to this website. People....pay attention when Richard speaks!

Donnie

 

[frankgh]

Three reasons.

1) Without any CYA then chlorine breaks down very quickly in sunlight -- cut in half every 30 minutes in noontime sun.

2) Without any CYA you are overdosing your pool with chlorine because you can't really control a small 0.1 ppm of Free Chlorine (FC) in your pool.

3) You need a certain minimum FC in the pool to handle local chlorine demand (sweat, leaves, etc.) so around 1-2 ppm FC and since without CYA that would be overdosing chlorine, having some CYA lets one have a higher FC without having the effective chlorine concentration too high.

In short, CYA protects chlorine from breakdown from sunlight and acts as a buffer or reserve for chlorine.

Richard

Thanks Richard! I'm starting to get it now.

We have a new pool and are trying to get it balanced. I got the CYA up to 21 and yes, the cloring is staying in the pool longer, even with the bright sunny days.

One more quick question, I am running ozone too and have been told I can leave the chloring at about 1 ppm. Looking at the chart posted here, I should leave the CYA about where it is at. Does that jive with you? (I love talking to someone who does not have a vested intrest in selling me something!)

 

[chem geek]

Ozone does not provide a residual disinfectant in the bulk pool water so will do nothing to kill pathogens stuck in biofilms to pool surfaces nor will it kill the bacteria going from one person to another until such free-floating bacteria make their way through the ozonator after the many hours it takes to have several turnovers of water. Also, the ozonator can end up having you use more chlorine since the ozone can oxidize the chlorine to chlorates thereby increasing the demand for chlorine. On the other hand, if the ozone is oxidizing organics, then the chlorine demand for oxidation is reduced. Which effect wins out depends on the amount of organics (and ammonia) that needs to get oxidized. All in all, the ozone doesn't help very much for a pool -- especially an outdoor pool that already has the good air circulation, UV rays from sunlight, and regular chlorine maintenance that keeps combined chlorine in check. It might have more benefit for indoor pools or spas, but even that's debatable.

Richard

 

[chem geek]

I received a question (via PM) on the origins of the chlorine/CYA chart and how these values were determined. I responded as follows.

---------------
The values in a column are the same level of hypochlorous acid so in that sense they are calculated from a spreadsheet I made that computes the hypochlorous acid concentration given various water parameters (mostly FC and CYA). However, the targets for each column were determined first by Ben Powell at The Pool Forum where he qualitatively developed his Best Guess CYA chart. Since that time, we determined the higher level of chlorine needed to combat yellow/mustard algae.

So it's really a combination of Ben Powell, me, and others on The Pool Forum and Trouble Free Pool that sorted this out. I will add some info to the chart acknowledging Ben since he certainly deserves credit.
---------------

I'll add a brief acknowledgment and link to Ben's original chart. I can't believe I didn't do that originally and Ben really does deserve a huge amount of credit for figuring this out mostly from his experience plus some communication with chemists. I just put the frosting on the cake by quantitatively working on the underlying chemistry using the equilibrium constants determined in 1973 and published in 1974.

Richard

 

[sks23cu]

Should this chart be modified when using borates?

I believe elsewhere it is said that borates act to retard algae growth, so they reduce the need for FC.

I'm currently trying to lower my TA so I can add more Borax to bring them to the recommended 30-50 ppm.

 

[JasonLion]

Borates normally reduce the chlorine consumption. You still maintain the same FC level, it just takes less chlorine to do that.

 

[mctol]

Hi,

Thank you for your chart - it is appreciated.

Just a few quick questions...I am currently shocking my pool to get rid of a yellow algae.

When shocking a pool, how long does it take for the chlorine to come down to a useable level?

What is the maximum level that you can still allow people to swim in the pool?

At what reading are you causing problems with bleaching/damaging your vinyl liner?

Do you have a chart that would show how much a chlorine level is raised for each gallon of chlorine is added assuming various percentages of chlorine within the bottle?

I try not to ask a lot of questions that have been covered before, but I fear I am doing that right now...sorry for that.

Thank you for your help! It is greatly appreciated!

 

[CarlD]

On the subject of Polyquat 60% I'm closer to Richard's POV than Jason's. Neither is wrong--just a different style.

If you are really disciplined about your FC levels and never miss a day, or use a Liquidator or something, then Jason's right--PQ may be an unnecessary expense, but it will do no harm.

If you are like me, busy with a young one (3 year old tornado. 40 lbs, same as a wildcat!), a high-pressure job for both me and my spouse, then you may mess up or rush things. Here, PQ is your safety net and when you find your FC at 1 and go "WHOA!" you may dodge the bullet of algae because of the PQ.

It also acts as a flocculant (so that's one other chem you never need to buy).

The ONE thing to watch for with PQ, and it's not documented anywhere I know of (other than when I've posted it), but should be, is that after dosing with it, especially an initial dose or a season-ending dose, your FC will take a MAJOR nose dive within 24-48 hours. Not to panic: Just re-add bleach till it's right again.

My favorite use of PolyQuat 60% is for closing. I run my pool up to shock levels, add a full quart of PQ (OK, that's $20. I'll be a millionaire $20 later), and wait 48 hours. FC plummets and I run it up to shock level again and close. I've done this 5 closings in a row and ALWAYS opened to clear water, never cloudy--with a mesh safety cover, too!

Unfortunately, lately PQ has been rising in price. It may be called lots of things but you can recognize it by the active ingredients: Poly....<something>.....60% and nothing else.

 

[JasonLion]

1) When shocking a pool, how long does it take for the chlorine to come down to a useable level?

2) What is the maximum level that you can still allow people to swim in the pool?

3) At what reading are you causing problems with bleaching/damaging your vinyl liner?

4) Do you have a chart that would show how much a chlorine level is raised for each gallon of chlorine is added assuming various percentages of chlorine within the bottle?

1) That varies with the CYA level. For CYA at 50 or lower, usually a two or three days, sometimes less. For higher CYA levels it takes longer, sometimes much longer.

2) This is really a personal choice. I swim up to normal shock level. But even that is well below the active chlorine level in many public pools.

3) There is a tiny risk to the liner even at normal shock level. I wouldn't go above mustard/high shock level.

4) Try out my Pool Calculator, see the link in my signature. It can figure out how much bleach to use for any given FC change and how a given amount of bleach changes the FC level.

 

[jackbo]

really work for me..thanks

 

[Durk]

I've learned a lot from this forum as I transitioned from 40 years of running a pool we left empty all winter and started with fresh water each spring, then just hit it with Cal-Hypo every evening. Before CYA started to be used, up until 1970 or so, FC always went to zero by end of day yet we had sparkling water and nobody got sick. Calcium was never a problem on a painted steel pool where the water never got older than six months. CYA allowed me to do the same thing, just going to every other day with the cal-hypo. Later we went to pucks, but by the time CYA got too high, the season was over and we dumped the pool.

Now, since 2004, I have a plaster pool that winters full--very light adult only usage. I've been successful with a low (20-30) CYA, low-chlorine (about 0.5 when I add chlorine, every second day, to bring FC up to about 2.5, then let her go 48 hours) plus weekly PQ60. I use a mix of bleach, Cal-hypo and pucks (only when travelling) and juggle Cal and CYA to stay in range. I dump about a third of the pool between closing and start-up to give myself space on CYA and Cl for the new season. Cheap bleach takes expensive gasoline to get to where I am, and I really think this gives me a pleasant, safe and economical pool. I don't think it would work for everyone, but it does work for me. I admit I am maybe one missed dose from a bloom, but I'll take the risk to save some money and the pain of trucking in tons of bleach. If usage climbs, I do bump up the FC just to be safe.

 

[frustratedpoolmom]

I've learned a lot from this forum as I transitioned from 40 years of running a pool we left empty all winter and started with fresh water each spring, then just hit it with Cal-Hypo every evening. Before CYA started to be used, up until 1970 or so, FC always went to zero by end of day yet we had sparkling water and nobody got sick. Calcium was never a problem on a painted steel pool where the water never got older than six months. CYA allowed me to do the same thing, just going to every other day with the cal-hypo. Later we went to pucks, but by the time CYA got too high, the season was over and we dumped the pool.

Now, since 2004, I have a plaster pool that winters full--very light adult only usage. I've been successful with a low (20-30) CYA, low-chlorine (about 0.5 when I add chlorine, every second day, to bring FC up to about 2.5, then let her go 48 hours) plus weekly PQ60. I use a mix of bleach, Cal-hypo and pucks (only when travelling) and juggle Cal and CYA to stay in range. I dump about a third of the pool between closing and start-up to give myself space on CYA and Cl for the new season. Cheap bleach takes expensive gasoline to get to where I am, and I really think this gives me a pleasant, safe and economical pool. I don't think it would work for everyone, but it does work for me. I admit I am maybe one missed dose from a bloom, but I'll take the risk to save some money and the pain of trucking in tons of bleach. If usage climbs, I do bump up the FC just to be safe.

Welcome to the forum

I have to say though, that while I understand your position on driving to buy bleach and that your routine works for you, I don't necessarily agree that your routine is "economical". PQ60 is expensive, and really unneccessary in a properly balanced pool. But if you'd rather continue with your routine that's your decision of course. Just wondering though, is your water free? Where/how to you obtain the other pool chems? Where do you buy groceries, can't you just get the bleach on the weekly grocery excursion?

While my pool is is only about 13,000 gallons or so, last year I spent about $60 all season, having only to put in bleach, nothing else was required. Shocked once, when going on vacation, and again when I winterized. I don't think I could get any more economical than that....

Anyway, to each pool his own, I say! Welcome to TFP! :-D

 

[chem geek]

There were two problems in the years before CYA when you used Cal-Hypo in the evening. First, is that the chlorine level in the morning was too high. It would oxidize swimsuits, skin and hair and also potentially create more irritating disinfection by-products (nitrogen trichloride). Then later on in the afternoon when the chlorine was very low or even zero (consumed from bather waste, not only from sunlight), the risk of person-to-person transmission of disease was higher. You prevented runaway bacterial growth by effectively shocking the pool every night with the Cal-Hypo and having some chlorine in the pool most of the time except possibly a few hours. It's not a terrible situation, but you were lucky that no sick person spread a disease in the afternoon -- maybe some actually did, but unless it was a full outbreak people may not have noticed. At least when you started using CYA, the chlorine level would be more consistent though you lost some of the "shock" effect since the active chlorine level would be lower all the time so would be riskier getting towards zero especially for algae growth.

As for your current situation, using PolyQuat 60 weekly or using a phosphate remover or even copper ions (ignoring staining issues) are all ways to control algae that let you have a lower FC/CYA ratio. What you save on chlorine costs you lose in algaecide costs. PolyQuat 60 weekly for a typical pool is around $2-3 so around $8-12 per month compared to incremental chlorine costs in the $10-15 per month range (for an extra 1 ppm FC per day usage). In your case, the costs are probably quite close. There's no one "right" way to do this -- there are options and it's really a personal choice as to the tradeoffs.

Richard

 

[frustratedpoolmom]

What you save on chlorine costs you lose in algaecide costs. PolyQuat 60 weekly for a typical pool is around $2-3 so around $8-12 per month compared to incremental chlorine costs in the $10-15 per month range (for an extra 1 ppm FC per day usage). In your case, the costs are probably quite close. There's no one "right" way to do this -- there are options and it's really a personal choice as to the tradeoffs.

Richard

I agree, there are several options, but I still feel BBB is the most economical.

In addition to the Polyquat 60, Durk is using cal-hypo, pucks and dumping water mid-season. I guess I don't see how that is saving him money? Especially since bleach is available at the grocery store?

I bought a bottle of Polyquat 60 from Leslie's...it was almost $24 for 1qt. It says for weekly maintenance, add 2-4 oz per 10,000 gallons. So assuming Durk put in 12 oz a week, he could go through a bottle of Polquat in a month, which would be almost $24, for the polyquat only. I think its always a possibility people think "more is better" and do the higher dose of the preventative. So I think people may even put in more than they need to, spending more money than necessary...

Then factor in the other products Durk is using, and possible additional costs as a result of shocking if an algae bloom occurred.

Anyway, I get this question alot, what is the cheapest, most convenient method....Richard, I recall you did a cost analysis before...it was liquid chlorine bleach, right?

For me my personal experience had been a no brainer compared to what my former routine was, the amount of money I spent was insane.

 

[JasonLion]

what is the cheapest, most convenient method

There are two issues inherent in this question. First, "most convenient" is subjective. For example, some people think that replacing all of their water once a year is convenient, some don't. Likewise, some people find spending a couple of minutes each day to add chlorine convenient, and other don't. But, even if we ignore that aspect, the cheapest method is not what most people think of as the most convenient.

The most convenient method is to hire someone to take care of the pool. Then all you have to do is write them a check once a month. That is convenient. The cheapest method is going to involve spending at least a little time working on the pool every day. Compared to hiring someone that is not convenient.

 

[chem geek]

I bought a bottle of Polyquat 60 from Leslie's...it was almost $24 for 1qt. It says for weekly maintenance, add 2-4 oz per 10,000 gallons. So assuming Durk put in 12 oz a week, he could go through a bottle of Polquat in a month, which would be almost $24, for the polyquat only. I think its always a possibility people think "more is better" and do the higher dose of the preventative. So I think people may even put in more than they need to, spending more money than necessary...

Then factor in the other products Durk is using, and possible additional costs as a result of shocking if an algae bloom occurred.

Anyway, I get this question alot, what is the cheapest, most convenient method....Richard, I recall you did a cost analysis before...it was liquid chlorine bleach, right?

My $2-3 estimate was from info I describe in this post where the price of the PolyQuat 60 was $17-20 per quart with a recommended dose of 3 ounces in 10,000 gallons. You are right that I was figuring on a 10-15,000 gallon pool. I now see that Durk's pool is larger at 28,000 gallons, but that also increases the cost of chlorine (i.e. my $10-15 per month for an extra 1 ppm FC would really be almost double that amount).

The cost comparisons I did were mostly for chlorine as shown here, but that just shows that Trichlor isn't as inexpensive as it first seems. Chlorinating liquid and especially discounted bleach are the least expensive though Cal-Hypo is also close (sometimes less expensive, but increases CH over time). I did not do a full analysis of "low chlorine" with supplemental algaecide. It's roughly order-of-magnitude similar, but will depend a lot on the specific situation. When sunlight is not as much of an issue, as with many covered pools, then the non-algaecide chlorine-only route becomes much less expensive. When there is a lot of direct sunlight, the methods are closer in cost, but it can be tricky to properly maintain the lower chlorine level without automation.

Richard

 

[Durk]

Thanks for the polite response to my 'heretical' post. I have a great well drilled in 1937, luckily zero mineral hardness up here in the Watchung hills proved by $500 worth of pro testing, so cost of H20 is just pump watt-hours plus some Costco baking soda at $5/12lb. bag (have you seen that??--Great deal!) to adjust TA. We could do the old 40000G pool in about 18 hours.My insolation is intense, east-west pool on a Southern slope, so 7AM-7PM in June.

I use 6 oz per month of PQ60 plus 24 .oz initial dose in my 28000G IG pool and it costs about $22 per quart. Total for May 1-Sept 15 season is 4 quarts or $88. That would buy an extra $4 worth of bleach per week, not counting gas and hassle because I have to go 18 miles to Wal-mart for a decent price on bleach and nearest carboy supply is in the same area. That $4 is probably less than what it would take for me to get to chart target at any CYA. I use the other products in place of spending $ on CaCl (cal-hypo) or CYA(pucks) and in that way they are cheaper than bleach but ONLY when used that way up to my need for Ca or CYA (ignoring use of pucks when I travel, which is a lot cheaper than paying someone to chlorinate.)

I heartily endorse the BBB approach--and I consider myself a believer--I just try and operate it toward the low-CYA low-Cl end of things. IMHO, based on one man and one pool, higher CYA levels (40+) and the Cl needed to go with it, leads to a huge bleach demand and unnecessary excess costs and hassle even if it eliminates any need for PQ60. Again, thanks for your polite responses. I just wanted to provoke some thoughtful discussion and I got that.

I agree with Richard that it is 'tricky' to maintain a low Cl; my K-2006 gets a workout. Thank goodness for FAS-DPD; otherwise my approach would not be possible.

Finally, I like dumping H2O mid-season, 55oF well water allows me to maintain the 74-78 oF level I consider ideal, to the disgust of some of my female patrons. :-D

 

[frustratedpoolmom]

Thanks Durk - I really do appreciate you sharing your pool experience with us to show me and others that there is more than one way to do things as for 74-78 degrees, you won't see me in your pool! LOL my toes don't go in unless it's 85 degrees!

I'll second your "Thank Goodness" for the FAS-DPD! I'd be totally lost without it because I can't differentiate between 2 and 5 on the comparator tube for the OTO.

 

[Durk]

Thanks Durk - I really do appreciate you sharing your pool experience with us to show me and others that there is more than one way to do things as for 74-78 degrees, you won't see me in your pool! LOL my toes don't go in unless it's 85 degrees!

I'll second your "Thank Goodness" for the FAS-DPD! I'd be totally lost without it because I can't differentiate between 2 and 5 on the comparator tube for the OTO.

Hehe--I figured from your user name you would be an 80+ type. When it's hot out, I want to COOL off when I jump in the pool and 80+ just doesn't do that. I deliberately do NOT have a heater, because the girls would try and make me use it :lol: !