Dilution & Replacement: CYA vs Sodium

[emike93309]

Forgive me if this is addressed elsewhere, my searches regarding sodium all came up with posts about SWG pools. This is a two-part question:

1. I'm going to be out of town for two weeks, and I'm going to have to revert to trichlor tabs for the sake of the person who will be taking care of the house while I'm gone. I've been running numbers on Poolmath, and i'd love it if someone could double check my math: it seems I should expect the CYA in my 22k gallon pool to rise 1.5 ppm for every 8 oz trichlor tablet they add. That means that for every tablet that's added to my pool, I'll need to replace 440 gallons of water in my pool when I get back if I want to stay around my current CYA level. Does that sound correct?

2. That got me thinking about all the bleach I dump in my pool. Wouldn't 8.25% sodium hypochlorite add salt to my pool every time I use it? How do I measure the salinity of my pool, and at what levels should I be worried about replacing water?

 

[chem geek]

Your numbers are correct if you currently have 75 ppm CYA and want to maintain it. That's from 1.5/(440/22000) = 75.

Sodium hypochlorite does add salt to the pool as does any source of chlorine (Trichlor does as well) because when chlorine is used/consumed it becomes chloride salt. For every 10 ppm FC added by chlorinating liquid or bleach, it also increases salt (after chlorine consumption) by 17 ppm. However, salt is rather innocuous and it takes very high levels before it would become any sort of issue.

So normally you would use winter rain overflow to dilute your pool water annually and that should be enough to keep the salt level in check. It might get up to 1500 ppm or so though it depends on the amount of rain or other dilution you get. Looks like Bakersfield only gets 6.45" per year so in a pool with average 4.5' depth that would only be a 12% dilution. You have a sand filter so backwashing that regularly dilutes the water somewhat as well. If you had 2 ppm FC per day, then over 6 months that would be around 600 ppm salt so a 12% dilution would not be enough as the long-term steady-state would be around 5000 ppm salt. With 100 gallons of backwashing per week, then by itself that would be a long-term steady-state of again over 5000 ppm salt so it would take both combined to get down to 2500 ppm long-term.

You can see how even though the rate of CYA buildup is less than that of salt, a CYA of 100 or more is a problem whereas with salt you don't even start to think about it until it's at least 2000 or more and even then 3000 is the usual level of saltwater chlorine generator pools in the U.S. CH buildup from Cal-Hypo is in between because normal CH levels are in the 300-400 ppm range. So use of chlorinating liquid or bleach requires the least amount of water dilution, but it still requires some.