FC degradation rate

[dombey]

Hi guys,

I have been patiently adding CYA to my pool to bring it up after a nearly complete drain & refill. After adding 12lbs of CYA I'm expecting a reading around 45-50ppm once this last dose has fully dissolved and circulated.

Last test I ran showed CYA at somewhere between 25 and 30ppm and I'm still having to add chlorine on a daily basis. I've been slowly ramping up the FC level to correspond to the CYA in the pool - once I hit 50ppm of CYA I'm going to be shooting for 6ppm of CYA and letting it drop to 4ppm before adding chlorine.

Based on experience over the last few weeks though, I'm seeing degradation of about 2ppm/day. It seems pretty consistent regardless of how much CYA is in the pool - so basically I'm still gonna need to add chlorine every day. Is that normal? I was hoping to at least get myself to an every-other day routine. My dog swims literally every day so I'm hesitant to load it up on Monday and let it degrade throughout the week...

Is there any rule of thumb for degradation rates and CYA/FC levels...?

 

[bk406]

Yes, chlorine is a consumable and even with CYA in the water, chlorine is still consumed by sunlight. With CYA in the water, it's just not as fast. 2 ppm loss is about average and what has been reported by many folks on TFP. Of course, higher swimmer loads and/or more sun, YMMV.

 

[waste]

Just to, hopefully, help clarify the above post.

Anything that is or ever was alive will cause chlorine consumption, no matter how large or small.

UV rays will consume chlorine.

Chlorine will be used up every day, thereby necessitating adding more, fresh chlorine.

 

[chem geek]

The actual daily chlorine usage from sunlight isn't a consistent fixed amount for every pool. First, you should eliminate the possibility of any non-sunlight causes for the degradation by doing an overnight test to see the FC drop with no sunlight. Then you'll know the sunlight effect on its own.

2 ppm FC per day is not unusual though some with higher CYA levels (even with proportionately higher FC levels) can see a lower 1-1/2 ppm FC per day. In experiments made by Mark, the drop in chlorine usage was when going from 40 to 70 ppm CYA. Since you are in Arizona, you might need 70-80 ppm CYA. You could try experimenting in a large bucket or basin at that level since it's hard to lower the CYA if you decide you don't want it that high.

Note also that the loss depends on temperature as well, though that's more for the loss that isn't from sunlight.

Unless you have a pool cover (and in Arizona you'd want that to be a white or reflective cover since you don't want the water that much hotter), you will likely need to add chlorine every day or two. With a cover, possibly twice a week (I have a cover and add chlorine twice a week and use around 1 ppm FC per day).

 

[dombey]

The actual daily chlorine usage from sunlight isn't a consistent fixed amount for every pool. First, you should eliminate the possibility of any non-sunlight causes for the degradation by doing an overnight test to see the FC drop with no sunlight. Then you'll know the sunlight effect on its own.

2 ppm FC per day is not unusual though some with higher CYA levels (even with proportionately higher FC levels) can see a lower 1-1/2 ppm FC per day. In experiments made by Mark, the drop in chlorine usage was when going from 40 to 70 ppm CYA. Since you are in Arizona, you might need 70-80 ppm CYA. You could try experimenting in a large bucket or basin at that level since it's hard to lower the CYA if you decide you don't want it that high.

Note also that the loss depends on temperature as well, though that's more for the loss that isn't from sunlight.

Unless you have a pool cover (and in Arizona you'd want that to be a white or reflective cover since you don't want the water that much hotter), you will likely need to add chlorine every day or two. With a cover, possibly twice a week (I have a cover and add chlorine twice a week and use around 1 ppm FC per day).

Thanks. To clarify, Mark went from 40-70ppm of CYA and the FC usage dropped from 2 to 0.5-1.0?
Sounds like I need a more granular & larger scale chlorine test... I just have the Taylor DPD kit from Leslie's...only goes to 5 ppm on the chlorine and it's a guess when you're between colors...

 

[chem geek]

Actually, I was off a bit. As described in this post, Mark went from 45 ppm CYA to 80 ppm CYA and the FC drop went from 50% (5 to 2.5 ppm) per day to about 15% per day. However, he didn't scale up the FC level as one would normally do for the higher CYA level. If one does this, then it's more like comparing 50% per day with 30% per day loss, so still a significant improvement. Remember that one can't really compare absolute numbers since every pool is different, but the basic principle of higher CYA lowering FC loss even at proper FC/CYA ratios is sound.

Since the non-linear effect may be from direct CYA shielding, then deeper pools may show more of a benefit. We first noticed this effect when a few people reported losing less chlorine at higher CYA levels, even keeping FC up, and these pools were deeper (8 foot at deep end). Mark's experiments were in an attached spa and in buckets so the effect still shows up even without great depths.

 

[dombey]

If one does this, then it's more like comparing 50% per day with 30% per day loss, so still a significant improvement. Remember that one can't really compare absolute numbers since every pool is different, but the basic principle of higher CYA lowering FC loss even at proper FC/CYA ratios is sound.

So, what you are saying is that when you increase CYA and FC to maintain your ratios, your FC degradation in ppm will slow?
The last thing I want to do is add a bunch more CYA and FC, then still lose 2ppm/day of FC and have way more chemicals in my pool.
In other words:

Current Scenario:
50ppm CYA
5ppm FC
40% FC loss/day
2ppm loss per day

New Scenario:
80ppm CYA
8ppm FC
25% loss per day (improvement)
2ppm loss per day (no improvement in actual terms)

 

[chem geek]

Well, your example isn't like Mark's data. Mark's data is more like the following:

5 ppm FC
45 ppm CYA
50% loss per day
2.5 ppm FC absolute loss

5 ppm FC
80 ppm CYA
15% loss per day
0.75 ppm FC absolute loss

so convert to scaled up FC amount to keep the FC/CYA constant.

8 ppm FC
80 ppm CYA
15% loss per day
1.2 ppm FC absolute loss

I shouldn't have written 50% vs. 30% since I wasn't really comparing percentages anymore after I did the scaling. That was just confusing. From the above, you can see that the absolute loss drops in spite of needing a higher FC at the higher CYA level. This is because the % rate of loss dropped faster than the proportion that the CYA was raised.

The problem is that the above effect isn't precise nor the same in every pool. Also, the drop is only about the loss from sunlight. If you've got other losses in the pool, those probably won't change. That's why getting a baseline overnight FC loss number is helpful since that number isn't going to change (and you can scale up that number to get a 24-hour result). Mark's experiment didn't factor out the non-sunlight losses and those vary a lot from pool to pool anyway. I do know from my own pool that such non-sunlight losses seem to be very temperature dependent and are quite a bit higher at higher water temps. I even notice a difference in overnight chlorine loss between 88ºF vs. 82ºF.

Hopefully some other people with pools in your area or similar hot sunny desert climate area will chime in with what they do.