CYA and FC - Just so I have it straight...

[travisr100]

I've read and read and read through pool school and all the various posts. I just want to make sure I've got this straight. My current test results, FC 8, pH 7.7, TA 70, CH 350, CYA 35. I've got a Stenner set up for my bleach. It currently doses 77.2 oz of 10% bleach each day split over 4 dosings at 6 am, 10 am, 2 pm, and 6 pm. Over the course of the last 5 days I get a consistent reading of FC at 8 ppm at around 8:30 in the morning. So it seems things are in balance. I'm dosing enough chlorine to keep the FC pretty much consistent throughout the day. From using pool math it seems I'm burning off about 4.6 ppm of FC over a 24 hour period each day. I realize 8 ppm FC is high based on the CYA/FC chart but I've also read that it won't hurt anything.

Now, I'm about to raise my CYA to 50 and plan to keep my FC at 8 ppm still. If I'm understanding all this correctly the result should be that I need to add less bleach every day once my CYA gets to 50, correct? So if I keep my current dosing schedule but raise CYA to 50 will I see that my FC climbs steadily each day past 8? Maybe from 8 to 9 to 10 etc?

I figure that with a CYA of 35 and the chart recommending a minimum FC of approximately 2.5 (I know it's not linear) it must mean that about 2.5 ppm of FC is bound to the CYA and therefore isn't available to sanitize. So if I raise my CYA to 50 the minimum FC target is 4 which I again assume means 4 ppm is bound to the CYA and therefore isn't available to sanitize. So why by raising my CYA should I expect to see less FC being lost each day?

 

[zea3]

I'll let Chemgeek or Joyfullnoise explain the chemistry and how it works end of things. What I can help you with is that the higher the CYA is the more protection if offer the FC. Kind of like how putting on a thick coat in the winter keeps you warmer than a sweater. You are covering the same surface area, but one protects from the cold much better. However there is also a point when adding another coat on top of the first coat can make you all bound up and miserable, just like too much CYA makes FC "bound up" and unavailable to kill off the algae. When you have the CYA at the correct level for your pool, you will see about a 1.5-2ppm loss per day, maybe a little more with heavy use. Its the combination of sun, shade, climate, and chemistry that determines what the best CYA level is for your pool. For most people it is between 30-50 for a liquid chlorine pool.

 

[duraleigh]

If I'm understanding all this correctly the result should be that I need to add less bleach every day once my CYA gets to 50, correct? So if I keep my current dosing schedule but raise CYA to 50 will I see that my FC climbs steadily each day past 8? Maybe from 8 to 9 to 10 etc?

That is correct. But if you maintain a CYA of 50 and and FC of 8, you will lose about 4 ppm or so daily. If you maintain an FC of 3-5 and the same CYA of 50 ppm, you will lose perhaps as little as 2 ppm certainly not more than three.

So overdosing above the FC/CYA chart somewhat is fine but it is noticeably less economical.....you burn more chlorine for no particular reason.

So why by raising my CYA should I expect to see less FC being lost each day?

I'll let someone else handle that response.

 

[chem geek]

At any given CYA level, the chlorine loss rate will be a percentage of the FC level. So having 8 ppm FC with 35 ppm CYA will lose about 3 times as much as the minimum 2.6 ppm FC for that CYA level. So losing 4.6 ppm FC is not at all unusual for your situation. If you were trying to maintain something more like 3 ppm FC, then you'd probably be using only 2 ppm FC per day under the same conditions (the factor of 3 may be an over-estimate).

If most of your chlorine loss is from sunlight, then having a higher CYA level even with a proportionally higher FC level will lose less absolute FC per day due to non-linear CYA shielding effects. So you could target 3.8 ppm FC with 50 ppm CYA and lose a little less than targeting 2.6 ppm FC with 35 ppm CYA.

Depending on the specific FC/CYA levels, roughly 97-99% of the FC is bound to CYA and not disinfecting nor preventing algae growth, but fortunately it only takes low levels of active chlorine for these functions. The reason that raising the CYA has less loss is due to two factors. First is that the unbound chlorine, hypochlorous acid and hypochlorite ion, is dependent on the FC/CYA ratio so is the same at 3 ppm FC with 30 ppm CYA as it is with 9 ppm FC with 90 ppm CYA. So the chlorine loss from sunlight from the unbound CYA is the same so long as the FC/CYA ratio is constant and that is also true for the loss from chlorine oxidation of anything it finds to oxidize and from any chlorine outgassing (that's usually a small amount we ignore). So that leaves the chlorine bound to CYA that has virtually no disinfection capability and less than 1/150th the oxidation power, but it does break down in sunlight though more slowly than unbound chlorine.

So you might think that the higher amount of bound chlorine would lose more but we find that it doesn't and speculate that this is due to non-linear CYA shielding effects. That is, the higher CYA level (or possibly Cl-CYA bound chlorine levels) shields lower depths from the UV in sunlight so more than makes up for their higher loss near the surface. Mark did experiments (in this post and this post) showing this effect. My best guess as to the net result is shown in the table in this post where you can see that the total chlorine loss is lower at higher FC and CYA levels with constant FC/CYA ratios.

 

[travisr100]

Thanks all for the responses. This is one where I may have to accept a lack of understanding of the chemistry and take it on faith.

 

[BuckeyeChris]

"higher cya ppm means more cya particles in the pool preventing uv rays from penetrating as far into the water, meaning less burn off of chlorine in the deeper parts of the pool".

Because...chemistry.

 

[travisr100]

Buckeye, that seems to make an awful lot of sense if it's correct. Never though of it that way.

 

[bgray9]

So if a higher CYA level saves chlorine, why wouldn't a person want to have a relatively high level of CYA (without getting too high and causing problems)? The general sense I got in reading around here the last couple months is that people generally prefer to keep their CYA level on the lower end of things, like 30 or 40. We recently refilled our pool after a remodel and my CYA is around 30-40, but this has me wondering if I should try to get it up a bit higher?

 

[chem geek]

If one could consistently maintain their FC/CYA level with little risk of letting it drop too low, then one could have a higher CYA level to use less chlorine. This is why the recommended CYA level is near 80 ppm for SWCG pools. The higher CYA level lets one lower the % ontime extending cell life and reducing hydrogen gas bubble generation (aeration), the rate of pH rise, and acid usage.

However, in manually dosed pools it is harder to always stay above the minimum FC/CYA level so at times one needs to recover and elevate the FC level sometimes to SLAM level and doing a SLAM at a higher CYA level requires a lot of chlorine. So while pools in hot sunny climates may push the limits and use a higher CYA level, most non-SWCG pools will target a lower CYA level to reduce risk in the sense of having an easier recovery if anything goes wrong. I'd say that in most moderately sunny climates the CYA is set more at 50 ppm. 30 ppm is pretty low and would only be appropriate for pools with some shade or much further north with less intense sunlight.

 

[gregsfc]

I'm posting to check my understanding of the FC/CYA relationship in a more simplified way. Going back to the original OP scenario taking out the question about changing the CYA level; trying to achieve and maintain FC @ 8 throughout the day with a CYA level @35. Isn't it true, and this was sort of hinted towards in one post, that if one tries to target and/or maintain FC way above what it should be at specific CYA level, want he or she just be using alot of extra product that will just be wasted away by it quickly being broken down by sunlight.

I've been targeting my pool this year at 6; but the chart shows my target should be 5. I've been losing almost 3 ppm per day with no cc's, when the chart shows my loss should be 2. I've been assuming this accelerated loss is due to the fact that by targeting that extra ppm each evening, I'm in effect wasting almost 1 ppm of bleach each day for that extra piece of mind.

In my mind, if on the other hand I was targeting FC @ 8 with the chart indicating my target @ 5, isn't it true that, while I'm not hurting anything, I'd be wasting double or more the amount of chlorine with no benefit other than being super sure I don't ever drop below minimum?

 

[duraleigh]

I'm posting to check my understanding of the FC/CYA relationship in a more simplified way. Going back to the original OP scenario taking out the question about changing the CYA level; trying to achieve and maintain FC @ 8 throughout the day with a CYA level @35. Isn't it true, and this was sort of hinted towards in one post, that if one tries to target and/or maintain FC way above what it should be at specific CYA level, want he or she just be using alot of extra product that will just be wasted away by it quickly being broken down by sunlight.

I've been targeting my pool this year at 6; but the chart shows my target should be 5. I've been losing almost 3 ppm per day with no cc's, when the chart shows my loss should be 2. I've been assuming this accelerated loss is due to the fact that by targeting that extra ppm each evening, I'm in effect wasting almost 1 ppm of bleach each day for that extra piece of mind.

In my mind, if on the other hand I was targeting FC @ 8 with the chart indicating my target @ 5, isn't it true that, while I'm not hurting anything, I'd be wasting double or more the amount of chlorine with no benefit other than being super sure I don't ever drop below minimum?

Your thinking is correct. One caveat, the FC/CYA chart does not suggest or predict daily FC loss......too many variables. Anecdotally, we find that the average pool seem to lose 2-3 ppm daily when your follow the suggested guidelines we propose.

 

[travisr100]

If one could consistently maintain their FC/CYA level with little risk of letting it drop too low, then one could have a higher CYA level to use less chlorine. This is why the recommended CYA level is near 80 ppm for SWCG pools. The higher CYA level lets one lower the % ontime extending cell life and reducing hydrogen gas bubble generation (aeration), the rate of pH rise, and acid usage.

However, in manually dosed pools it is harder to always stay above the minimum FC/CYA level so at times one needs to recover and elevate the FC level sometimes to SLAM level and doing a SLAM at a higher CYA level requires a lot of chlorine. So while pools in hot sunny climates may push the limits and use a higher CYA level, most non-SWCG pools will target a lower CYA level to reduce risk in the sense of having an easier recovery if anything goes wrong. I'd say that in most moderately sunny climates the CYA is set more at 50 ppm. 30 ppm is pretty low and would only be appropriate for pools with some shade or much further north with less intense sunlight.

Thanks for all the replies. I've truly done a tremendous amount of reading here and this thread really seems to have clarified it for me. So in my situation using a Stenner to dose throughout the day I could lean more toward the SWG CYaa level since I'm not really manually dosing. Finally I get the difference in CYA numbers for SWG pools.

 

[Catanzaro]

Chem Geek:

You write "Depending on the specific FC/CYA levels, roughly 97-99% of the FC is bound to CYA and not disinfecting nor preventing algae growth, but fortunately it only takes low levels of active chlorine for these functions"

This may be over my head. I fully understand that it only takes low levels of active chlorine to disinfect and/or prevent algae, of course a function of chlorine. What happens to the other 97-99% of the FC, that is bound to CYA daily? We dose our pools with enough chlorine to increase the suspected amount of loss daily.

Does the FC (bound chlorine) slowly break away from the CYA for it's intended purpose, and continue the same process throughout the day?

Then by the end of each 24 hour cycle, new FC (with liquid chlorine) is introduced into the pool and I assume gets attached to the CYA that is in the pool?

Funny how pool store employees used to tell me that liquid chlorine is "not stabilized".

 

[pabeader]

I now know the answer to this one. Yes. The bound chlorine slowly releases from the CYA shackles.

The longer story from recent discussion with ChemGeek:
The chlorine bound to CYA is not permanently bound. It's in equilibrium with the unbound chlorine such that roughly speaking the amount unbound is proportional to the FC/CYA ratio while the amount bound is close to the FC level (FC less the small unbound amount). As the FC drops, the bound amount drops but so does the unbound amount -- each dropping proportionally.


Hate to stand up for pool $tore people, but; actually, liquid chlorine, is not stabilized. It gets that way when you put it in your pool along with CYA.

 

[bgray9]

If one could consistently maintain their FC/CYA level with little risk of letting it drop too low, then one could have a higher CYA level to use less chlorine. This is why the recommended CYA level is near 80 ppm for SWCG pools. The higher CYA level lets one lower the % ontime extending cell life and reducing hydrogen gas bubble generation (aeration), the rate of pH rise, and acid usage.

However, in manually dosed pools it is harder to always stay above the minimum FC/CYA level so at times one needs to recover and elevate the FC level sometimes to SLAM level and doing a SLAM at a higher CYA level requires a lot of chlorine. So while pools in hot sunny climates may push the limits and use a higher CYA level, most non-SWCG pools will target a lower CYA level to reduce risk in the sense of having an easier recovery if anything goes wrong. I'd say that in most moderately sunny climates the CYA is set more at 50 ppm. 30 ppm is pretty low and would only be appropriate for pools with some shade or much further north with less intense sunlight.

So for someone like myself in a hot sunny southern climate (Arizona), what would you generally say is the best CYA level to target? Thanks for the discussion.

 

[Catanzaro]

Pabeader:

Sorry, what I meant is that the "pool store" claims that liquid chlorine, because it is not stabilized will just burn off and disappear in the water, not necessarily that it is like a form of stabilized 3" pucks, and that because it does not container stabilizer, it is not bound. I explained to them, that liquid chlorine attaches to the CYA in the pool, but they did not want to hear it.

They basically were trying to have me shy away from liquid, and stay with the FROG. The pool stores charge $6 a gallon for 12.5%, now I pay roughly $3 for the same liquid chlorine at Ocean Odd Lot in Shrewsbury NJ.

 

[travisr100]

Not to get OT, but I pay $1.68/gal of 10%.

Again, best thread I've seen on this subject so far without a highly technical discussion that's above my head. Particularly the comment from pabeader. Now it's all becoming more clear.

 

[chem geek]

So for someone like myself in a hot sunny southern climate (Arizona), what would you generally say is the best CYA level to target? Thanks for the discussion.

You have to decide that for yourself with some experimentation. Most likely you need at least 50 ppm CYA. Whether you go higher is up to you. While you would use less chlorine at higher CYA levels even with proportionately higher FC target levels, you increase your risk if anything does go wrong -- by "risk" I mean you need to use a lot more chlorine if you need to do a SLAM. Again, that's up to you.

The bound chlorine slowly releases from the CYA shackles.

As for chlorine release from CYA, I wouldn't call it slow. If needed because the unbound chlorine were instantly consumed, half of the FC bound to CYA is released every 0.25 seconds. I'd call that pretty fast. That's why the chlorine test measures the chlorine bound to CYA as well as that unbound. It is only the unbound chlorine that reacts with the DPD dye in the chlorine test (and reacts with the FAS titrant drops to reduce it to chloride salt), but when it does that it is no longer free so more gets released from CYA quickly to replenish the unbound chlorine.

The CYA doesn't "know" to release anything. What is really happening is that chlorine is going back and forth between being bound and unbound very quickly where the rate of its release is many times slower than the rate of attachment (how much slower depends on the CYA amount). This is why so much more chlorine is bound to CYA than is unbound because it releases more slowly than it attaches so at equilibrium there must be a lot more attached than unbound. If you then consume the unbound chlorine, the chlorine still gets released but there is no unbound chlorine to get reattached so it appears like the CYA is releasing chlorine when the reality is that it was ALWAYS releasing chlorine -- the change is really that the unbound chlorine was removed/consumed so that there was no unbound chlorine reattaching to CYA.

At equilibrium,

"Chlorine bound to CYA" <<<-------> "Chlorine unbound to CYA" + CYA

where it is going back and forth from left-to-right more slowly than from right-to-left so more ends up being on the left (i.e. chlorine bound to CYA) than on the right (i.e. chlorine unbound to CYA). When you rapidly use up unbound chlorine you are left with:

"Chlorine bound to CYA" ----------> 0 + CYA

so one can only go from left to right (there is nothing for CYA to combine with to go the other way) so unbound chlorine gets replenished and one ends up back close to where one started because there is much more chlorine bound to CYA than unbound to CYA. If one continues to use up unbound chlorine, then you'll notice the FC dropping since the drop in the amount of chlorine bound to CYA will become significant enough to be measurable. The amount of unbound chlorine is fairly constant and proportional to the FC/CYA ratio.

 

[BuckeyeChris]

rate of its release is 30-100 times slower than the rate of attachment

like a lot of my relationships!