Borates - Refreshing Qty Question

[linen]

I just read this whole thread and had a question. Doesn't CYA float near the top of the water and that is why it blocks the UV from zapping the free Cl? If that were the case when you have to drain the pool to reduce CYA isn't it mostly at the top and therefore you don't have to reduce much volume to get a large decrease in ppm?

Urban legend. See: http://www.troublefreepool.com/high-cya-t11183.html#p89862

 

[chem geek]

No, CYA doesn't float at the top of the water. It is a chemical that dissolves in the water so is thoroughly mixed with it. The CYA protects chlorine through a combination of binding chemically to most of it and it also may have some shielding effect but that effect can still happen with it mixed in the water. Think of how the visibility through cloudy water gets less and less if the water gets deeper, even though the cloudiness may be dispersed throughout the water.