CYA Behavior

[hAltonJones]

I'm a newbie to pool maintenance, but I'm a chemical engineer. With that said, more out curiosity than anything else, I'd like to understand the behavior of CYA. I keep stumbling across posts that say if the CYA level gets too high you can't maintain adequate levels of free chlorine. However, if CYA only serves to chemically "tie-up" the chlorine and act sort of like an electrical capacitor by permitting slower release of the chlorine, then it seems logical to me you should be able to attain proper FC levels regardless of the CYA content. It's not like the CYA eats the chlorine and turns it into raw asparagus, it just holds it for slower release.

Why can't you say double the CYA level, have it "hold" double the chlorine and then establish a correct FC level? It seems it would merely provide a greater buffering capacity for chlorine and what's wrong with that?

 

[Ryno98]

Your thought process is not wrong. It's just that your required chlorine levels for higher CYA get to be cost prohibitive and very difficult (e.g. an entire truck bed full of bleach), especially when you consider shock levels. The legwork has already been done for you on this forum. Check out the FC/CYA chart if you haven't already. The other thing to consider is where your optimum point is with regard to chlorine expense. Too low CYA - all your chlorine gets burned up and you spend a bunch. Too high CYA - you spend too much to shock and maintain. You want the sweet spot in the middle which can be a bit different in different environments/parts of the country. Usually CYA 40-50 is good for most people in sunny locations.

 

[kcindc]

I'm sure chem geek or other more knowledge folks will be around soon to give you specifics. Here are a few things for you to consider:

There is a break point where too much CYA will not allow the chlorine to be released fast enough to keep the water sanitary.

Also, very high levels of chlorine cause a person to use more testing agents and at some point, FC can't be tested reliably. Also, high levels of FC can impact the ability to reliably test for PH. When FC is above 10, the PH test can falsely read high.

Chlorine/CYA Chart:
http://www.troublefreepool.com/content/128-chlorine-cya-chart-slam-shock

Recommended levels of FC/CYA:
http://www.troublefreepool.com/content/134-recommended-levels

Try searching for "chem geek"'s posts on CYA... I'm sure there are a few of his posts with more detailed info.

 

[svenpup]

There is a break point where too much CYA will not allow the chlorine to be released fast enough to keep the water sanitary.

There is no breakpoint. If you maintain FC in proper ratio to CYA you still get disinfection. Your other comments are correct though. It does become more difficult to maintain such high CYA, pH measurement is affected, and you go through tons of reagent. High CYA really becomes a problem if you let it go and have to SLAM the amount of bleach to get to shock level are staggering.

 

[chem geek]

(I wrote a post, but svenpup really summed it up best so no need to repeat)

 

[prs]

I understand that daily FC loss decreases as CYA increases. Limited experience with my SWG indicates that daily FC loss is reduced by as much as 50% if CYA is increased from 40 to 80 ppm. I don't see why this wouldn't apply to a non-SWG pool, so there is the potential for non-SWG pool owners to cut their chlorination costs by half if the TFP recommended CYA level was increased.

chem geek explained in another post that TFP keeps the non-SWG recommended CYA at 30-50 because manual dosing of a pool is more problematic than an automatic SWG, and if the pool owner isn't able to keep up with manual bleach additions then an algae bloom is more likely. So in order to SLAM the pool only has to be raised to 16 FC at 40 CYA but at 80 CYA it has to be raised to 31 FC.

I guess the lesson for SWG owners is that if your cell ever fails go get some bleach ASAP as you don't ever want to deal with an algae bloom!

 

[SunnyOptimism]

I guess the lesson for SWG owners is that if your cell ever fails go get some bleach ASAP as you don't ever want to deal with an algae bloom!

+1

As an SWG operator I now always have 2-4 gallons of 10-12.5% liquid chlorine on hand. SWGs are great at maintaining a constant background level of FC but they are totally unable to make rapid or large increases to FC levels despite what the sales literature says about "super-chlorination" or "Boost Mode". Anyone operating an SWG pool would be wise to keep a small cache of bleach on-hand for a FC bump when needed.

 

[chem geek]

There are some in very hot sunny areas that operate above 50 ppm, some as high as 80 ppm, and they just need to be more diligent. Some pool services have the CYA at 100 ppm or more and use higher FC levels of around 14 ppm which then drops to around 4 ppm by the end of the week. The blast up to 14 ppm kills off anything that might have started growing in the couple of days at the end of the week.