Picking the most economical CYA level

[glewis29]

So I've been reading a lot of the information on the relationship of CYA to FC and the recommendations that higher CYA levels may be beneficial to pool owners that get a lot of sun and thus experience a lot of FC degradation. I haven't seen anyone do a chlorine consumption comparison of the different levels, so here it is.

Based on the apparent daily solar degradation rates in the following post(http://www.troublefreepool.com/thre...-CYA-FC-levels?p=318157&viewfull=1#post318157), and the FC levels required at varying CYA levels based according to the Chlorine/CYA chart (http://www.troublefreepool.com/threads/2177-Chlorine-CYA-Chart), I put together a spread sheet that shows the amount of FC that needs to be added to the pool daily in order to combat loss due to solar degradation. Note: I added 120 to see where the trend would take me

According to the data provided to me, it looks like running at 50 CYA will use the most chlorine at any given level (min, target, shock, etc.), while running at 20 or 100 (I guesstimated 120 with a value that seems to match the curve) will save you up to 25% of your total chlorine need. Note that percentage of chlorine used daily for a given level are consistent. It scales linearly because the FC/CYA chart scales linearly (within a small margin of error).

Values that require FC higher than 10, and thus are prone to pH testing error are highlighted yellow. My conclusion at this point is that it is better to keep CYA low while correcting pool problems (so that pH can be measured and chlorine levels can fall when desired), but once your pool is well balanced there is very little downside to raising CYA and running higher FC levels (Keep in mind as chem geek noted, that oxidizing power is the same at any given FC/CYA level (min, target, shock)).

Hope this helps some people, I know it helped clarify things in my head. If you have any corrections in my methods, let me know.

Some explanation of the table if it's not clear. CYA is the CYA level and %Daily Loss is the estimated loss in a sunny pool in the middle of summer according to chem geek.

The FC column is the CYA value times the FC percentage at the top (7.3% for Min, 11.4% for Target, etc) which is basically in line with the Chlorine/CYA chart (tweaked to be a little closer to actual values)

The FC Loss column is the amount of FC lost per day. It's the FC value times the %Daily FC Loss value.

% of Max is the percentage of FC Lost versus the worst CYA level (50). 50 is 100% of itself, and they go down from there.

 

[chem geek]

Take a look at the table in this post that's a little better and has some more info. It changes the calculation where 30 ppm CYA isn't actually better than 50 ppm CYA though that's for the case of continual dosing. Besides, you have to factor in the fact that you need to start higher given the loss. As an example, you can't start with the 2.2 ppm FC minimum and lose 1.31 during the day since the FC would get too low. You'd need to start with a higher FC, but that means a higher loss. To maintain the minimum 2.2 ppm FC at 30 ppm CYA, you'd have to start with 2.2/(1-.6) = 5.5, using the 60% daily loss (which is probably more like 67% from my newer data) so you actually lose 0.6*5.5 = 5.5-2.2 = 3.3 so much higher than the 1.31 that you show.

Basically, the main flaw with your calculation is that you don't account for having to have a higher target to make up for the loss. When you do this, then a higher CYA level saves a LOT.

In the newer table in my newer post, I also show what happens with an SWG or automated dosing system that is maintaining the chlorine level. The effect of higher CYA savings is more subtle in that case, but still there. It's when manually dosing where a higher CYA has a huge benefit, mostly because at higher CYA you don't have to start your FC that much higher in percentage terms compared to lower FC/CYA levels.

Also note that these tables are rough estimates. An individual pool's loss will vary a lot based on sunlight exposure, pool covers and other materials to oxidize, bather load, etc.

 

[glewis29]

It's funny, I somehow missed that post you just linked when going over stuff here, but I did find those numbers online quoted in a different forum.

Here's the chart for those numbers, updated using iterative calculations to account for ending at the desired FC level instead of starting there, and you're right, the results are even more dramatic. Those that dose in the evening and want to ensure they stay above the minimum FC levels throughout the day will certainly benefit from higher CYA levels.

Thanks for the feedback, hope this helps.

 

[chem geek]

Yes, that now looks more like what we see since generally speaking for manually dosed pools that try to stay above a certain minimum level, the higher the CYA the less total chlorine that they use. The only reason we limit the CYA to 80 ppm is due to the risk of having to deal with shocking at that level if anything goes wrong. If one never had a problem needing a SLAM, then one could have even higher CYA levels, but we just think the risks are too great. Things happen -- pumps fail, people forget to dose, something gets dumped into the pool consuming chlorine, etc.

 

[glewis29]

I created another version for continuous feed systems like SWCG, Liquidator, & Stenner pumps based on the hourly rate in your table. I assume this hourly rate only looks at peak loss, but I would think the relative hourly loss between different CYA levels should be the same for other time periods and thus work for establishing relative chlorine consumption rates for various CYA levels.

While the data doesn't lean as hard towards elevated CYA levels, it certainly still supports it. In light of this, I wonder why one would not want to run higher CYA levels as a general rule (which goes against the TFP standard recommendation of 30-50 ppm)

Thanks.

 

[RobbieH]

We do. We recommend 70-80 for SWGs.

 

[glewis29]

Robbie, the data shows that all pools benefit from CYA levels greater than 50, not just SWG. In fact, pools with daily dosing and thus greater swings in FC benefit even more with higher levels of CYA.

 

[svenpup]

What your data doesn't show is what happens when FC accidentally drops below minimum and algae takes hold. What is your shock level when your CYA is 100? 150? 200? If there was no possibility that something unexpected could to cause the need to SLAM we would all run CYA at 200+.

Go back and reread Richard's post:

...The only reason we limit the CYA to 80 ppm is due to the risk of having to deal with shocking at that level if anything goes wrong. If one never had a problem needing a SLAM, then one could have even higher CYA levels, but we just think the risks are too great. Things happen -- pumps fail, people forget to dose, something gets dumped into the pool consuming chlorine, etc.

 

[duraleigh]

In light of this, I wonder why one would not want to run higher CYA levels as a general rule (which goes against the TFP standard recommendation of 30-50 ppm)

1. How much higher? 60-80? 80-100? 100-120? You apparently have no upper limit of benefit...........it just doesn't work like that.

2. By far, the single biggest issue on this forum year after year is having too much CYA in your pool water.

If you manually chlorinate your pool you should follow the guidelines established by thousands of anecdotal reports and years of experience along with some rock solid science..

If you chlorinate your pool with an SWG, please do the same thing...........stay within the suggested guidelines published here.......they work.

 

[chem geek]

Well, the benefit of less absolute loss does increase at higher and higher CYA levels which is how some pool services get away with using 100 or even up to 200 ppm CYA and dose once a week without as much chlorine loss, even with proportionally high FC levels. The problem is that this is very risky because IF something happens and you need to shock the pool then this is a very large amount of chlorine to get the FC up and keep it there during a SLAM.

All the problems with people reporting pools with high CYA levels are ones where they did not proportionately keep their FC up high -- new people who used Trichlor or Dichlor regularly and got their CYA high but still kept FC at 1-3 ppm (or certainly less than 7.5% of the CYA level) as recommended by "the industry" or "pool store".

 

[glewis29]

Thanks for the feedback all, I was one of those people who just dumped in pucks, had ridiculously high CYA, and wondered why 3ppm wasn't cutting it. At least until I found TFP.

I would probably not go too much higher than 80-90 due to pH measurement issues and the inability of shock levels to fall in a timely manner.

One last thought is that I believe the risk of the FC dropping below desired levels would actually be lower at higher CYA levels due to the buffering action of CYA. As available chlorine is consumed by oxidation of organics, etc. the hit will then release more active chlorine, but the drop in FC will be a lower percent of FC than it would be at lower levels. It seems to me that once you've got your pool dialed in, higher CYA, within limits, would actually make things more error proof.

The same point was made by chem geek about CYA in indoor pools. It's not valuable for solar degradation, but it does give a buffer and reserve of FC to pull from.

<Edit> So because I can't turn my brain off, here's a simple example. Let's say that you're running at Target levels because you want to have a bit of reserve in case there's an event that eats up FC. Let's say you have an even that consumes 3 FC. At 30 CYA, you'd be running at 3.4 FC, and you would drop to .4 FC, which is definitely below the low. If you're at 80 CYA however, you'd be running at 9.1 FC, so you'd only drop down to 6.1 FC, which is still above the 5.8 FC Min level. That sounds like a better situation for when something unexpected occurs.

Additionally, when considering SLAMing, if you look at the chart for daily dosing above in post 3, in order to ensure you maintain an appropriate FC level, for 30 CYA you have to add 25 additional FC to ensure you have enough FC at the end of the day. At 80 CYA, you add 30 additional FC. But in the 30 CYA case, you lose 23.7 FC to solar degradation, while at 80 CYA, you only lose 8 FC. If the SLAM goes more than one day you're going to put in much more chlorine at 30 CYA than at 80 CYA to ensure you maintain an appropriate FC level. In this sense, even SLAMing at higher CYA seems to be easier and more economic because the FC you put in the pool that you want to kill organics is buffered and protected form the sun and released when needed instead of floating in the pool exposed, getting consumed by solar degradation. </Edit>

 

[engrav]

interesting discussion, thank you
may or may not change my CYA level but the discussion is helping me to understand how CYA works

one question please
what does Yel/Mstr refer to?

 

[glewis29]

Yellow/mustard algae. Apparently it's more resistant to chlorine.

 

[chem geek]

Yellow/mustard algae needs roughly twice the active chlorine level (proportional to the FC/CYA ratio) as green algae. Black algae in theory is in between, but seems to be kept at bay with the same chlorine levels used to prevent green algae.

 

[chem geek]

Additionally, when considering SLAMing, if you look at the chart for daily dosing above in post 3, in order to ensure you maintain an appropriate FC level, for 30 CYA you have to add 25 additional FC to ensure you have enough FC at the end of the day. At 80 CYA, you add 30 additional FC. But in the 30 CYA case, you lose 23.7 FC to solar degradation, while at 80 CYA, you only lose 8 FC. If the SLAM goes more than one day you're going to put in much more chlorine at 30 CYA than at 80 CYA to ensure you maintain an appropriate FC level. In this sense, even SLAMing at higher CYA seems to be easier and more economic because the FC you put in the pool that you want to kill organics is buffered and protected form the sun and released when needed instead of floating in the pool exposed, getting consumed by solar degradation.

Usually if you need to SLAM you lose a lot of chlorine killing algae, but your points are only valid if you were to add chlorine just once during the day. If you add chlorine regularly to replenish that lost from sunlight, it's not as extreme since in the 30 ppm CYA case the regular shock level is only 12 ppm FC (yellow/mustard 18 ppm FC) whereas for 80 ppm CYA it is 32 ppm FC (yellow/mustard 48 ppm FC). If you are adding chlorine every hour, then you still use more chlorine in the 30 ppm CYA case but not as bad as your example.

Anyway, it is true that something like 30 ppm CYA can be difficult unless you have a mostly or partially opaque solar cover (to UV). We usually recommend something closer to 50 ppm as a sweet spot, but yes one can run with 80 ppm CYA if they understand the risks you've pointed out. There are many pools operating at 80 ppm with saltwater chlorine generators, but those are automated systems so are much more likely to be maintained with chlorine more consistently. If someone can ensure regular daily dosing, then they can manage an 80 ppm CYA pool reasonably well. I think our recommendation just comes down to the reality that people make mistakes more often than automated machines fail. Certainly if someone wants to minimize their chlorine usage IF most loss is from sunlight in their situation, then a higher CYA level (with proportionally higher FC level so same FC/CYA ratio) will help them do that.

 

[glewis29]

Thanks for the feedback.