Trying to make sure I understand FC/CYA relationship

[chem geek]

You are thinking the same thing I was at one point. What I had to realize is this; Higher CYA does not mean higher FC. It does mean higher base FC but NOT higher FC usage. Daily usage can be similar regardless of CYA level. (too a certain point, and oversimplified like crazy)

The main way to look at it is the following. There is chlorine unbound to CYA and it is lost from sunlight fairly quickly, losing around half of its amount every hour in direct noontime sun. Then there is chlorine bound to CYA and its loss in sunlight is much slower typically losing less than half over an entire day. So at the same FC/CYA ratio which determines the unbound chlorine level its loss will be the same, but the amount of chlorine bound to CYA lost will be higher at the higher FC level. However...

If the above were all that were going on then one would conclude that a lower FC and CYA level would be better due to less chlorine bound to CYA, but that would be wrong because there is another factor that dominates. Higher CYA levels shield lower depths from the UV in sunlight and this is a rather strong and non-linear effect such that higher FC and CYA levels even with the same proportionate FC/CYA ratio actually lose less absolute FC. Mark did experiments in this post and this post that showed this effect that we had seen in pools but did not understand. We have yet to find any scientific paper confirming this since unfortunately there are none that show the UV spectrum of CYA or chlorine bound to CYA in the relevant 300-370 nm range. I do have detailed UV molar absorption data for chlorine unbound to CYA.

 

[TexKSAH]

Thanks Chem Geek. It seemed to me the information was hinting that lower CYA/FC was better (less chlorine loss ratio) but the recommendations were pointing to higher CYA/FC. That was the part that was nagging me. Glad you cleared that up.


So, if I'm going to use a stenner pump and automate gradual chlorine delivery throughout the day do you think a CYA in the range of SWG is ok?

Ordered my test kit yesterday. It will be 3-4 weeks til my pool is ready, but I can practice on the neighbors pools

 

[TexKSAH]

I went with the bigger pump so it can deliver it's chlorine in the least amount of operating time (I have mine come on 3 times a day for about 15-20 min per cycle). This should prolong the life of the pump and the tubing.

I was thinking the same thing, especially since they both cost the same. But I was leaning towards trickling it in anytime the filter pump is running which I think I will have mine running on the lowest setting 16-18 hrs per day (mainly for the waterfall effect from the spa overflow, not cause I think I will need to clean that much). It probably would prolong the pump if I did 3-4 injections vs 16 hours of trickling.

 

[chem geek]

So, if I'm going to use a stenner pump and automate gradual chlorine delivery throughout the day do you think a CYA in the range of SWG is ok?

Why don't you start at 50 ppm CYA and see what sort of chlorine usage you get with the automated dosing maintaining a chlorine level. You can then increase your CYA level and see if that makes a difference and that will also give you an idea of the amount of savings vs. CYA level so that you can judge better for your pool whether it is worth it. The only risk for the 80 ppm CYA level is if you run into a problem and need to SLAM the pool, but that risk is presumably lower since you will have the automatic dosing so be less likely to "miss" a dose and have the FC get too low.