Chlorine/CYA Chart

[chem geek]

EDIT: You will find a simplified and standardized version of this chart at Pool School, Chlorine / CYA Chart.

Chlorine/CYA Chart by Chemgeek

FC = Free Chlorine in parts-per-million (ppm)
CYA = Cyanuric Acid in parts-per-million (ppm)

CYA ........... Min FC³ ..... Target FC ...... Yel/Mstrd Min ...... Shock FC ..... Yel/MstrdShock
0 ................. 0.07¹ ........... 0.11¹ ................ 0.15¹ .................. 0.64¹ .............. 1.50¹
10 ............... 0.8¹ ............. 1.2¹ .................. 1.6¹ .................... 4.5 .................. 7.1
20 ............... 1.5¹ ............. 2.4 .................... 3.1 ..................... 8.3 ................ 12.7
30 ............... 2.2 ............... 3.5 .................... 4.6 ................... 12.2 ................ 18.2
40 ............... 2.9 ............... 4.6 .................... 6.1 ................... 16.0 ................ 23.8
50 ............... 3.7 ............... 5.7 .................... 7.5 ................... 19.8 ................ 29.4²
60 ............... 4.4 ............... 6.8 .................... 9.0 ................... 23.7 ................ 34.9²
70 ............... 5.1 ............... 8.0 .................. 10.5 ................... 27.5² ............... 40.5²
80 ............... 5.8 ............... 9.1 .................. 12.0 ................... 31.4² ............... 46.1²
90 ............... 6.6 ............. 10.2 .................. 13.5 ................... 35.2² ............... 51.7²
100 ............. 7.3 ............. 11.4 .................. 14.9 ................... 39.1² ............... 57.3²
120 ............. 8.7 ............. 13.6 .................. 17.9 ................... 46.7² ............... 68.4<sup>2

1</sup>A minimum FC level is needed as a "reserve" for usage so in practice at least 2 ppm FC is required even at low CYA levels. The table above shows the amount needed for disinfecting chlorine for equivalent killing power (rates), but does not take into account the amount needed in reserve to prevent getting used up as this varies by pool.

²The shock levels shown have equivalent disinfecting chlorine amounts (in a column) but at high CYA levels it may be impractical to use such high FC levels. A partial drain/refill to lower the CYA level is usually what is needed or one can shock at a lower level but will take longer to kill the algae.

³Most saltwater chlorine generator (SWG) pools appear to prevent algae at a minimum FC level of 4.5% of the CYA level as compared with the roughly 7.5% of the CYA level shown in the "Min FC" column for manually dosed pools.

NOTE: A reasonable approximation to the above table is the following:
.... "Min FC" is 7.5% of the CYA level
.... "Target FC" is 11.5% of the CYA level
.... "Yel/Mstrd Min" is 15% of the CYA level
.... "Shock FC" is 40% of the CYA level
.... "Yel/MstrdShock" is 60% of the CYA level.
Since chlorine is more effective at lower pH, it is normally recommended to lower the pH before shocking at a high FC level, especially for yellow/mustard algae where lowering the pH to 7.2 before shocking at the above levels is best (and remember that the pH test will not be valid during shocking due to high FC levels).

ACKNOWLEDGMENT: The original chlorine/CYA chart was developed by Ben Powell and is shown here based mostly on experience plus some conversations with chemists. With the help of users at The Pool Forum and this forum, we expanded the chart to include yellow/mustard algae and I made the chart consistent with chemical theory.

 

[gonefishin]

thanks!

dan

 

[stevenbrla]

Thanks !!!!!

 

[AnnaK]

Great! I've printed it out and pasted it in my pool log book.

 

[Navigator]

The chart in the first post looks like it would be very useful but I have not been able to find a post that tells me how to use it. I think I know what the numbers mean, but Min FC and Target FC confuse me. (I have a similar problem with Ben's Best Guess table).

Would it mean that if I bring the FC level up to the Target value at dusk, that I shouldn't let it drop below the Min value throughout the next day? (This would either mean picking a higher Target value appropriate to my pool, or continually monitoring it and adding chlorine if necessary to keep it above the Min value).

This is a wonderful forum -- and I was actually able to register. Thanks for any help.

 

[chem geek]

You are correct that I took the style of the chart from what Ben had in his Best Guess Chart so inherited the concept of "Min" though I did not use "Max" since it really isn't applicable (the actual maximum where pool's are not safe to swim in is far higher as most indoor pools exceed even shock levels in terms of disinfecting chlorine concentration).

The "Min" is easy to understand as it truly is a minimum FC level that a pool should never fall below. The chart was done for manually dosed pools so this "Min" column is perhaps a little conservative, but not by that much as I can attest from my own pool. The "Target" column is roughly the midway point between Min and Max of Ben's chart so is closer to what most people would try and shoot for, but as you point out this depends on how much chlorine gets used up each day and how quickly you add chlorine. The "Target" is also a safer number to use as it provides some leeway for error, both in measurement and in not being diligent or having some unexpected extra bather load or strong sun.

So perhaps we should just get rid of the Target and refer to the Min as a "hard" minimum so each pool user needs to add more chlorine the night before to ensure they never drop below that minimum.

To complicate things even further, I'm getting more info on how phosphate levels and CYA-like organics can mess things up in a small number of pools. There is a limit to algae growth rates even with all the phosphates and nitrates (i.e. algae food) that algae can handle, but it's not clear whether Ben's table (which is what I based mine on) is for the worst-case fastest-growth algae conditions. I suspect it is not, but handles the vast majority of pools, but I really don't know for certain. The CYA-like organics is a stickier issue since there are no current tests for those, as far as I can tell, and the closest proxy is with ORP or HOCl sensors.

So the basic rule is 1) make sure you have an FC level appropriate for CYA level according to the chart; 2) if the rare cases where you consistently get algae anyway, try higher FC levels (such as for mustard/yellow algae); 3) if even that does not work, check for phosphates and if high, use a phosphate remover or a weekly PolyQuat 60 algaecide (and if the phosphates are already low, then use the PolyQuat 60 algaecide).

Some pool users may not want to nor be able to maintain consistent chlorine levels above the minimum. In such cases, they should use a weekly PolyQuat 60 algaecide or a phosphate remover, but the latter is a far more expensive one-time dose with ongoing unknown maintenance. The weekly 4 fluid ounces of PolyQuat 60 algaecide is on the order of around $2 per 10,000 gallons with an initial dose about double that (typically, 9 fluid ounces). Though the recommended weekly phosphate remover dosage (which removes an additional 125 ppb) is also around that price, the initial dosage is about $15 (for Sea-Klear) to remove 1000 ppb in 10,000 gallons so could be more or less depending on phosphate level. Just keep in mind that phosphate removers are the latest way for pool stores to make money and are usually not needed.

Richard

 

[Navigator]

chem geek,

Thank you for your very informative answer. It answered all my questions, and I think it will be useful to a lot of other newbies.

You also touched on another subject I've been wondering about: the uses of PolyQuat60. It seems to be a common opinion on this board that PolyQuat is effective and doesn't hurt anything, but doesn't do anything that adequate chlorine doesn't do. So why incur the additional expense? Nevertheless, it seems like good insurance (how much do you dislike cleaning up an algae bloom)? And it's convenient -- I'd be more comfortable asking a neighbor to put a couple of ounces of PolyQuat in my pool once a week while I'm on vacation than a couple of pints a day of chlorine.

And, as you calculated, it isn't that expensive. I just bought a quart of it for $18.95 on sale at Leslie's and the bottle recommends a maintenance dose of "2 to 4 ounces every 5 to 7 days". At the minimum dose, which I suspect is adequate, that's only $1.19 per week for a 10,000 gallon pool.

Potentially, it could save money if it could be substituted for chlorine's algae-prevention function and allow the chlorine levels to be reduced to just a sanitizing function (perhaps especially so at high CYA levels).

Sorry to ramble on, but your answer was thought-provoking. Good thing I have no idea what 'CYA-like organics' are.

 

[chem geek]

You are absolutely correct about the full maintenance dosage of PolyQuat algaecide letting one use lower chlorine levels (relative to the chart) and typically that means higher CYA levels -- so more like the 3 ppm FC and 60-80 ppm CYA one finds with SWG pools. You are also correct that really PolyQuat 60 is insurance -- not needed, but there in case the chlorine gets too low for whatever reason.

I disagree with using the low-end of the recommended dosages for PolyQuat IF you are also not going to go with the chlorine chart most of the time. That is, if you are going with lower FC/CYA ratios than in the chart, then I would stick with the high end of the PolyQuat recommended range, especially if you have dilution as with a sand filter that is regularly backwashed and the pool water volume is smaller (< 10,000 gallons). Chlorine breaks down PolyQuat over time which is part of the reason for the maintenance dose (the other is dilution and the last is that it is a clarifier so will consolidate algae and other particles and get caught in the filter). I've seen some reports where PolyQuat was used but algae formed anyway, but the usage was spotty. So if you are going to use it in place of chlorine on vacation, then I'd hit an extra heavy dose of PolyQuat -- and really it's better if someone could add chlorine anyway, but if you MUST go without chlorine, then I'd go heavy on the PolyQuat. And of course when you get back, you have to add chlorine again and wait a bit (an hour should be fine) before the water is sanitized with respect to bacteria (PolyQuat also kills bacteria, but slowly, so may not prevent growth by itself -- only chlorine will do that).

You may have noticed that most pool maintenance "programs" that use Trichlor pucks/tabs also use an algaecide, usually PolyQuat 60 (or sometimes an algaecide that has that and an additional different algaecide in it). That's how they can get away with guaranteeing that you won't get algae, even though your CYA level gets very high (over 100 ppm). So this is certainly an option for people, but it's usually more expensive and has you add more chemicals (i.e. algaecide) than you really need to if you maintain sufficient chlorine levels. I know that there are those that feel very strongly one way or the other (i.e. chlorine alone vs. maintenance programs), but I prefer to just lay out the pros/cons and let people decide for themselves. The worst part about having the CYA get high, in my opinion, is that IF you get an algae bloom for whatever reason, then it's really hard to get rid of with chlorine alone since it takes so, so much. So then you're left with drain/refill or using an expensive method such as a phosphate remover (with chlorine as well, since you still need to oxidize the existing algae somewhat) or an algaecide with a "green pool or hair" side effect such as copper, etc. And if the CYA gets really, really high, above 150 ppm, then degradation of plaster pools becomes an issue. [EDIT] This last point about high CYA causing plaster degradation may not be true as the study that showed this was not reproducible. [END-EDIT]

If PolyQuat is just used as insurance in a pool that normally follows the chart, then a small amount even a little below the recommended range is probably adequate. That's what Carl does for example.

Richard

 

[JasonLion]

I don't see the point in bothering with PolyQuat except for special situations like winterizing or the ascorbic acid treatment. I don't get algae without it except over the winter, and then I usually get algae even with it.

If I did mess up the chlorine level I would rather get algae as a reminder instead of risking worse things growing in the pool and no obvious indicator of that. I would probably test the water and notice but if I'm away the day the SWG breaks the family is going to hop right in unless things look obviously wrong. So I like risking algae in order to have a low chlorine warning system that anyone will notice.

 

[chem geek]

I don't use any algaecide (other than chlorine, of course) in my own pool. I see what you mean about the algae being an indicator for low or no chlorine, but it's clearly not as safe an indicator as testing as a pool could have no chlorine with lots of bacteria and not show algae, but perhaps it's better than nothing. I think that if people can be responsible and maintain chlorine levels, then an algaecide isn't needed, but adding chlorine every day (or every other day) does seem to be too much for some people. Of course, they could get an SWG, but now we know they could also get The Liquidator as being talked about in some threads. Or they could use an opaque pool cover like I have (so I add chlorine twice a week).

 

[Navigator]

Thanks for all the information.

I'll probably just follow chemgeek's chart pretty conservatively. I only bought the Polyquat because it was on sale and I understand it keeps well. Maybe when I go on vacation next year it'll come in handy.

I'm the owner of a brand new pool and a brand new Taylor K-2006 test kit (running low on FAS titrant). I test my pool water three times a day: in the morning to see what the chlorine did overnight, at dusk to see what I have to add, and an hour later to verify that the levels came up to what I expected. So I feel pretty safe even without insurance.

I do not have the situation JasonLion has. My family (kids and grandkids) don't come visit when I'm not home, and my wife won't get in the pool by herself. And, unfortunately, I don't have an SWG to break down. If I get one, I think I'd look around for a kinder gentler litmus test than an algae bloom.

 

[matj6876]

Folks

I'm a little confused and wonder if you could clarify.

Pentair say to run the intelliflow SWCG with 2ppm FC and CYA at 70ppm

Using the table above and referring to note 3 I should keep the FC at a minimum level of 3.15 with CYA of 70ppm and I would guess target it somewhere around 5pmm?

Who's right?

Thanks in advance

 

[JasonLion]

With a SWG and CYA of 70 I would target a FC of 3 in the early evening. Since the SWG produces chlorine most of the time that means your peak FC won't usually need to be much above 3 (unless you only run your pump at night). Keep in mind that there is no one true answer. Different pools are different and often you can be fairly far out of the ideal range and still not have a problem for some time.

I think Pentair is being a bit optimistic. AutoPilot is similar; they suggest 1 to 3, which at least allows that it might need to be higher. I talked to an engineer at AutoPilot once when I was having a problem and he was shocked that I was aiming as high as 3, said I should aim for 1 (my CYA was 50 at that time). When I let FC go below 3 nothing goes dramatically wrong but after a few days the water has a dull look to it. If you are feeling adventurous give 2 a try and tell us what happens. In the mean time I am going to stick with recommending 3.

 

[chem geek]

There have been a few pool users who reported seeing algae when their SWG pools were at 2 ppm FC so I would say that 3.2 ppm FC is closer to the correct number in the 70 ppm CYA range (though doing things like regular brushing might help one run at a lower FC level since the main problem is for algae stuck to surfaces that don't pass through the SWG cell to get superchlorinated and killed). There are even some SWG users where even 3 ppm was not enough, but that was for mustard/yellow algae (if I recall correctly). Because of the variety of factors that affect algae growth, including temperature and nutrient (phosphate, nitrate) levels, any recommendation is going to be something that works in most, but not all, situations. Ben's original chart was his "best guess" based on experience and knowledge and works for almost every pool for green algae prevention (mustard/yellow algae prevention needs closer to his Max column to prevent). I just translated such experience into consistent chemistry based on what is known about the disinfecting chlorine level at various FC and CYA levels.

The Pentair recommendation probably comes from the pool industry "standard" recommendation of 1-3 ppm FC that more recently has been changed to 2-3 ppm FC (I think -- could be 2-5) as the recommended range, but that standard says nothing about chlorine as a function of CYA. A new standard is apparently being developed (see this link), but I doubt that it will be any better regarding any discussion of Free Chlorine relative to CYA as the industry continues to point to the Pinellas study (that I discuss in this thread) as to why CYA doesn't matter.

You are welcome to try 2 ppm FC and let us know if your pool remains crystal clear, but with algae the recommendation has to be based on the chlorine level relative to CYA that has virtually all pools be free of algae. If some pools can operate with less chlorine, that's nice, but not useful as a general guideline.

Richard

 

[Titanium]

I see that Ben's best guess chart showed a shock level of 25 PPM FC for CYA levels of 100-200, whereas ChemGeek's chart shows a shock level of around 47 ppm FC for a CYA level of 120.

What prompted the large increase in recommended shock levels at high CYA levels? 47 ppm FC is a big difference from 25 ppm FC. Especially since I have a CYA level of around 160, and I assume that ChemGeek's chart, if it was extended into this regime, would have even higher shocking levels than 47 ppm.

Thanks for any help.

Titanium

 

[chem geek]

My chart is based on the chemistry of chlorine and CYA (described technically in this thread) so the shock column shows equal amounts of disinfecting chlorine (hypochlorous acid). Ben's chart was (mostly) from experience which worked well for the minimum chlorine levels to prevent algae and tracks my chart reasonably well for that, but for shocking there isn't a single number that works. If you use more chlorine, you will kill algae faster. So in theory, my chart gives a somewhat consistent kill rate.

Of course, there are assumptions I am making such as the chlorine attached to CYA (known as chlorinated cyanurates) won't kill the algae even at high concentration. So that's why I put the ² superscript for any FC shock level above 25 ppm. Unfortunately, Ben is out of communication so we can't ask him if there is some other reason why his shock numbers are lower or if they should be lower for some reason. I know that there is this visceral feeling of not wanting to put in 47 ppm FC chlorine even when the CYA is at 120 ppm, especially in a vinyl pool, and unfortunately I cannot "prove" that this is as safe as 12 ppm FC at 30 ppm CYA. The chemistry says that the disinfecting chlorine (hypochlorous acid) levels are the same in these two situations and there are studies that show that it is the disinfecting chlorine that matters for disinfection and oxidation while experience shows it is what prevents algae, but there aren't good studies on that (there is one bad study that says CYA doesn't matter for algae prevention, but its methodology was flawed -- and there's the Pinellas County commercial pools study the industry uses to claim that CYA doesn't matter here.

In practice, when you've got a very high CYA, it is usually much better to do a partial drain/refill, even with algae present, as it is far easier to kill the algae at lower CYA levels since much less chlorine is needed. You can certainly try using an FC level of 25 ppm and see what happens. It should still kill the algae -- just more slowly -- and to be most effective you should circulate as much as possible including brushing pool surfaces. The key is to expose all the algae to the high levels of chlorine, and this becomes more important when that overall disinfecting chlorine level is lower. In an algae bloom, there is so much algae that local regions can get to low chlorine levels and then the algae doesn't get killed off. In fact, some algae (typically black algae) creates slime layers that essentially "sacrifice" themselves as chlorine attacks them while protecting the algae underneath. A similar principle holds with free-floating green algae in "clumps" or filament bundles. This is why it is so much easier to prevent algae when it's a relatively small number of cells at modest chlorine levels than it is to kill a bloom which requires higher chlorine levels.

Richard

 

[frankgh]

If lower CYA levels are preferred because less chlorine is needed, why would you want any CYA in your pools at all?

Yes...noob here...

 

[chem geek]

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

 

[JCJR]

How long (in hours) does it take for algae to grow if your CL levels stay below the minimum? Example, a pool with 40 cya and maintaining a 2 ppm and dipping into the 1ppm.

Also, does water movement play a part in the development of algae? In other words, longer pump times keeps water moving. Will that help algae not to grow?

Reason for both questions is due to my experiences. That's how I ran my pool for 7 months. I have never had an algae bloom and my CL levels were always low due to my inexperience and I guess exposure to a lot of sun. I run my pump over 10 hrs daily and I did add CL daily to get 3ppm in a 40 cya pool. Now equipped with a Liquidator it is easier for me to maintain a 4ppm pool.

 

[JasonLion]

There are several factors that enter into the rate at which algae grows. Algae requires sunlight, food (mostly phosphates but also some other things), low chlorine levels, etc. Any one of those factors can constrain the rate of growth. Poor water circulation enters in because algae can sometimes consume all of the chlorine in one local area allowing the algae enough time to form a biofilm in that area and thus resist higher chlorine levels in the future. Good circulation keeps the water mixed up enough to help prevent the biofilm from forming. Under ideal growth conditions algae can grow very quickly, turning a pool green in two or three days. But conditions are rarely ideal.