Why need CYA

[pgjohnson]

Hi

I understand what CYA does but not sure why its required and do not understand fully why I can't maintain pool without it. I have a fairly new pebblecrete (plaster) pool about a year old with a SWG. The reason I am asking is I have a basic drop test pool kit that can only test FC up to 5. Now if I set the recommended levels for CYA to +- 70 then I would need the FC to be higher than 5 and would therefore not be able to test myself and rely on pool shops unless I purchase a better test kit (limited in Australia).

So if I run the SWG for 8 hours during the day (sunlight) and maintain FC at a level lower than 5 but have a lower CYA (0 or even like 30 or so) then can't see how the FC would lower during night (no sunlight). Keeping this routine should be able to maintain the FC as a reasonable level?

Any help showing me what I am missing here would be great.

PH 7.8
FC 2
TA 70
CYA - not sure maybe around 20/30
SALT - not sure around 3000
CH - not sure think around 220

Thanks

 

[dfwnoob]

I'll let others who are a lot smarter me explain the chemistry side of things, but good test kits are readily available in Kangarooland via Clear Choice Labs, there is no need to rely on pool stores.

 

[ajw22]

@mgtfp will provide a good explanation to a fellow Aussie.

 

[Jimrahbe]

pg,

I can tell when my CYA gets low as I have to crank up the output on my salt cell to keep the FC up.

You can run at 20 or 30 if you want, but you will have to run your cell at a much higher output to maintain the FC you want.

And if you are not testing all the time, the FC can easily drop to zero and then you will get a visit from Mr. Algae...

Thanks,

Jim R.

 

[pgjohnson]

Understand that it may drop to 0 which can cause an algae problem.

But if you have say CYA at 70 and then need FC to be around 8, then would the same not happen if not testing all the time if FC say dropped to like 4?
Not really understanding why the need to monitor would change but do get the need to run SWG at higher output although seem to be fine running at level 6 out of 8 so not maximum anyway?

Thanks

 

[pgjohnson]

Actually this link Why such high CYA recommendations for saltwater pools? helps a bit to understand more.

 

[JoyfulNoise]

Try splashing some pool water with zero CYA and an FC of 4ppm into your eyes … you’ll understand really quickly why CYA is important in pools

Actually don’t …

CYA not only protects chlorine from UV loss but it also acts as a chlorine buffer. By holding the chlorine atom in reserve by reversibly bonding with it, the active chlorine species (hypochlorous acid and hypochlorite anion) are kept at much lower concentrations. Even though this buffering chemistry slows down oxidation and disinfection, it greatly helps keep chlorine levels manageable and less harsh to skin, eyes, hair, bathing suits, etc. It’s a benefit that is necessary for comfortable swimming and it’s the main reason why so many people hate public swimming pools. In many countries, public pools aren’t allowed to use CYA and so the water is very harsh and forms very irritating combine chlorine compounds which is what most people would attribute to that “gross chlorine pool smell”.

CYA, when used properly and with the right ratio to free chlorine, is a friend …

 

[pgjohnson]

Awesome, that helps with the harshness point.

 

[mgtfp]

Sorry, came late to the party, had a busy day.

Matt already touched on a crucial point, that CYA acts as a chlorine buffer.

When you look into Chem Geek's original FC/CYA chart (hold the phone landscape):

Chlorine/CYA Chart

EDIT: You will find a simplified and standardized version of this chart at Pool School, Chlorine / CYA Chart. Chlorine/CYA Chart by Chemgeek FC = Free Chlorine in parts-per-million (ppm) CYA = Cyanuric Acid in parts-per-million (ppm) CYA ........... Min FC3 ..... Target FC ...... Yel/Mstrd...

www.troublefreepool.com

... then you'll see that there is a row for CYA=0. That will tell you that without CYA, target FC is 0.11ppm and SLAM-FC is 0.64ppm. That's the appropriate range to maintain a pool without CYA. They do that for example in public pools in Germany, works nicely when you have the professional equipment for automatic testing and dosing.

If you go above this range, the water starts feeling very unpleasant. FC=3 without CYA is actually pretty tough stuff, like 5 times SLAM. They do that for example here in Oz or in the US in indoor pools, not so great.

How do you maintain such a small FC range in a private setting? Producing enough with the SWG in full sun and not too much on a cloudy day. Just one kid peeing in the pool will probably annihilate FC down to zero.

Without CYA, you also can't ignore the pH influence on the HOCl concentration. From pH 7.5 to 8.0, the HOCl concentration drops by 50%.

That's where CYA comes in. The vast majority of the FC will be bound to CYA, where it's protected from UV, but useless as a sanitizer. The uselessness you compensate by maintaining the required FC/CYA ratio.

Benefit one is the UV protection.

Benefit two is the flattened pH dependency. Already with CYA 30, the HOCl concentration drops by just 15% between pH 7.5 and 8.0.

Benefit three is the chlorine reservoir. All that "useless" chlorine that's bound to CYA stays in reserve. As soon as some HOCl gets used up, the equilibriums reshuffle (pretty much instantly), effectively releasing fresh HOCl.

Let's say you maintain FC 0.3 without CYA (that's like half way up to SLAM). A kid pees into the water and FC goes down to zero - a loss of 100%.

If you maintained instead half way up to SLAM with CYA 50ppm, you'd be at FC 10ppm - same thing in terms of readily available HOCl. You lose 0.3ppm due to a pee incident and you are still at FC 9.7ppm. A loss of just 3% and you are basically still half way up to SLAM - the party can keep going without the water turning cloudy or even green.

CYA gives you a much bigger FC working range without the water feeling too harsh.

The question now is, how much CYA is the optimum. The TFP recommendations are a good baseline, essentially based on experience.

Someone with a SWG in a desert climate will benefit from a higher CYA level around 80ppm. Not just in terms of UV protection, but also in terms of CYA working range. In these climates you have account for up to 10ppm CYA degradation per month. Starting higher gives you more buffer until the pool starts behaving oddly.

In a less sunny climate you probably get away with less CYA. Just try out what works best for you.

Being able to maintain FC a bit higher with the higher CYA levels makes pool maintenance so much easier, once you get to know your pool a bit. FC and pH testing maybe once or twice a week, just to see that everything is still fine. Lots of wiggle room. Occasional SWG adjustments with the seasons. Sometimes a bit of acid.