Chlorine/CYA Chart Questions

[steveg_nh]

Split from HERE. Please start a new thread to ask questions and not revive a 4 year old thread. Thanks, jblizzle

After reading this whole thread (which was incredibly interesting), I still don't understand a few things...

So CYA slows the loss of chlorine. I get that. But you need MORE chlorine as your CYA levels rise. Wouldn't it be the inverse? If CYA reduces chlorine loss, you would need less. That's what my gut would think.

I read that some of the FC binds to the CYA, so is it unavailable to sanitize. Right? Is that why you need more as your CYA rises? You really aren't "raising" the level of chlorine available to sanitize, but offsetting the amount "wasted" to being bound to the CYA? Basically your FC = Chlorine to sanitize + chlorine bound to CYA?? Something like that?

If so, why would you not want to keep your CYA levels low then - just enough to have the benefits of stabilization, without requiring excessive bleach dosing or SWG usage. I think it was mentioned that that is a goal, so why then, with a SWG vinyl pool, are recommended levels 60-80 for CYA, but about half of that for non SWG vinyl pools? If SWG pools seem to stay clear of algae at lower FC levels, wouldn't it make sense that you could have less CYA?

I'm a bot confused. Sorry if my logic as to why I'm confused isn't clear.

 

[JasonLion]

You need higher FC levels, but use less total chlorine to maintain those levels.

Using low CYA levels means more chlorine lost to sunlight. Using higher CYA levels means more trouble SLAMing the pool, should you need to do so. The levels we recommend are designed to balance out those conflicting factors.

 

[AimeeH]

Ok...really waiting on chem geek here, but my simple answer is:

CYA helps stabilize the chlorine so that you lose less to sun. It does increase the amount of FC you need to kill off bio hazards but basically allows you to keep it at a more stable level.

In other words ....with no CYA and full sun....you are burning off all of your chlorine.

 

[chem geek]

You generally want to have a consistent level of disinfection and oxidation from chlorine so that means you want a constant level of active chlorine not bound to CYA. That amount is roughly proportional to the FC/CYA ratio. So to keep the active chlorine level constant as the CYA rises, you need to proportionately raise the CYA level.

The FC/CYA ratio we chose is that which prevents green and black algae growth since bacteria are generally far easier to kill so it's the killing of algae that determines that minimum active chlorine level. So now the question becomes what FC or equivalently what CYA level is desirable. CYA protects chlorine from breakdown from the UV in sunlight and it turns out that even with the same active chlorine level a higher CYA level slows down this rate of chlorine breakdown even with the higher FC. This is where there is the trade-off that Jason wrote about -- too low a CYA and the chlorine loss rate from sunlight is too high, but too high a CYA level and the risk for having a harder time SLAMing a pool if you need to for whatever reason becomes greater.

So indoor pools can use a lower CYA level, but should still use some because the active chlorine level would be too high with no CYA since one cannot easily maintain 0.1 or 0.2 ppm FC in a pool. Outdoors, how much CYA is needed depends on how much sunlight. A pool with a pool cover that keeps out a lot of light could have 30-40 ppm CYA while a pool in moderate sunlight might have 50 ppm while a pool in full intense sun may need 60-80 ppm. A saltwater chlorine generator (SWCG) pool needs closer to 80 ppm to minimize the chlorine loss as that allows for lower output to have the generator last longer. If you use a large generator, you could have a lower CYA (and FC) level, but it's just wasting chlorine generation with chlorine breakdown in sunlight. Also, the higher chlorine output from an SWCG when the CYA is lower has consequences including faster rising of pH and therefore more acid that needs to be added to counter this rise.

 

[steveg_nh]

Split from HERE. Please start a new thread to ask questions and not revive a 4 year old thread. Thanks, jblizzle

I thought about that, but since it was a stickied thread, I figured it was ok.

So thanks for the explanation. So the higher levels is because of bound chlorine + active chlorine. That raises the number you need. I get that.

This still seems interesting though - if CYA is low, you burn chlorine off easier. If you raise CYA, you need more chlorine...it's not a wash though I assume?? The ratios and numbers (if I broken them down) must be saying that my statement is true, but you use less extra chlorine at higher CYAs than you would adding more chlorine to keep up with loss to sunlight at lower CYAs...

I'm hovering around 60 in my pool, and was determining if I do want to go higher. My pool gets A LOT of sun each day.

 

[JasonLion]

If you are using less than 2 ppm of chlorine each day, and don't have a SWG, there is no real value to raising the CYA level above 60. If you are losing more than 2 ppm of chlorine each day to sunlight, then you might want to bump CYA up a little.

 

[chem geek]

This still seems interesting though - if CYA is low, you burn chlorine off easier. If you raise CYA, you need more chlorine...it's not a wash though I assume?? The ratios and numbers (if I broken them down) must be saying that my statement is true, but you use less extra chlorine at higher CYAs than you would adding more chlorine to keep up with loss to sunlight at lower CYAs...

In theory, if only the unbound chlorine were broken down by sunlight and the bound chlorine was not affected by sunlight, then the rate of chlorine loss would be the same for the same FC/CYA ratio. So 3 ppm FC at 30 ppm CYA would be the same as 6 ppm FC at 60 ppm CYA would be the same as 9 ppm FC at 90 ppm CYA because these all have the same unbound chlorine since the FC/CYA ratio is the same in these three examples. However, for reasons we do not fully understand but have speculated from some experiments that were made, there may be an additional CYA shielding effect that protects lower depths from the UV of sunlight. So a higher CYA level has a lower chlorine loss even with the same amount of unbound chlorine (i.e. at the same FC/CYA ratio).

If the FC were around 10% of the CYA level and at a pool water temperature of 85ºF and a pH of 7.5, then the unbound chlorine level is roughly 0.16 ppm. The loss rate in a pool of average 4.5 foot depth would be around 1.2 ppm over a day assuming 8 equivalent hours of noontime sun (the half-life of a 50/50 mix of hypochlorous acid and hypochlorite ion in 4.5 deep water is 1 hour; at the surface it's 35 minutes; so I used 45 minutes as an average). There is also additional loss from chlorine oxidation of CYA itself, but that should be around 0.2 ppm per day. Since we see higher losses in practice, it is possible that the UV in sunlight also breaks down the chlorine bound to CYA, just not as readily as the unbound chlorine. However, if that were the case, then the CYA shielding effect must be fairly strong so that a higher CYA shields faster than the higher FC produces a greater loss.

As you can see, this is a complicated subject that is not fully understood. It's easiest to just use the rule of thumb that Jason gave. If you are losing around 2 ppm FC per day, you're in pretty good shape with your CYA level. It's your pool and you can always experiment with different levels, but 2 ppm FC per day is pretty typical and good -- some pools are higher in loss rate even with somewhat high CYA levels.

 

[steveg_nh]

While maybe not well understood, certainly interesting, and it does help me understand the ratios a bit more now. Thanks!