puzzled about chlorine / CYA ratios, especially for SWG pool

[zeiter]

I have a SWG pool, and have been educating myself about CYA / chlorine ratios. I had not understood the relationship between CYA and chlorine, and have run my pool for the last few years with free chlorine levels of 1-3 ppm, and CYA levels of 20 -50 ppm. The pool water has been fine, with one or two episodes of cloudiness that tested very low chlorine, and responded to winding up the chlorine output for a few days (I have never shocked my pool). I am fascinated & puzzled by the recommendations and posts on this forum for high CYA levels in SWG pools compared to non-SWG pools, as my experience with low CYA levels has been good, with little sign of algal growth.

The CYA / chlorine chart has tables for non-SWG and SWG pools: these are very useful guides, but the obvious question is why is the range of CYA for SWG pools restricted to 60-80 ppm: are these tables saying don't run your SWG pool with a CYA less than 60, or is it OK to use the non-SWG table for lower CYA levels? What extra benefit does high CYA offer (over lower CYA) in a SWG pool? I have seen the posts about less chlorine consumption at high CYA, and appreciate that the free chlorine level is a lower % in the SWG table compared to the non-SWG table.

Apologies for all the questions, but I am perplexed, and trying to figure this all out.

5000gal pool, Zodiac chlorinator, Rolachem acid peristaltic pump, Waterco zeolite filter (in southern part of Australia)
Cool Pooltester

 

[JohnT]

Welcome to TFP.

Most SWCG makers specify a required CYA range for operation. The primary reason is to extend cell life by protecting the chlorine as it is made. There seems to be a shock effect in the cell that allows SWCG pools to operate at lower free chlorine levels relative to the CYA levels than conventionally chlorinate pools.

 

[spishex]

Do you have a lot of shade around your pool? If less light is hitting the surface you'll be able to get away with a lower CYA level easily. The 60-80 recommendation is what seems to work best for most people, but certainly isn't law.

I think there are two things that the recommendation accomplishes. The first is the ability to create and hold a chlorine reserve. This can be difficult to do with a SWG because you're adding such small amounts at a time. If you're manually dosing with liquid bleach and need a 5ppm increase you can do that instantly, but not so with the SWG. In your case it helps to have the chlorine buffer that higher CYA creates in the event that you have a heavy bather load, an especially hot few days, etc. Even if your SWG falls behind in production there's still enough of a residual in the pool to prevent major issues.

The other is pH drift. The more chlorine you're producing through the SWG the more your pH will tend to rise. In a case where a low CYA level has made it hard to maintain a chlorine level, you'll find yourself running the cell longer than usual.

But I didn't make that recommendation and the folks that did may have more/different things to tell you.

Welcome to the forum!

 

[PaulR]

Expanding a bit on what JohnT and spishex have said: In a clean pool the SWG is mainly replacing chlorine lost to sunlight. With a higher CYA, you lose less per day, therefore the cell needs to replace less, therefore the cell doesn't work as hard, therefore the cell lasts longer.
--paulr

 

[dschlic1]

Look at this thread: http://www.troublefreepool.com/effect-of-cya-levels-t17602.html. By increasing your CYA you can reduce the output of the SWCG substantially.

 

[chem geek]

Again, to reiterate, this reduction in SWG on-time with higher CYA levels only occurs if there is a lot of direct sunlight on your pool. If the pool is mostly in shade, then the higher CYA won't have much effect and therefore wouldn't be needed.