To CYA or Not to CYA...

[piku]

Hello all,

I've been doing research on many things but one of the primary things I intend to do is replace my timer with a much less expensive and more convenient computer controlled contactor method as described in this guide: http://www.truetex.com/poolcontrol.htm. I've already successfully built a sodium hypo container using modified instructions from his pages and it's working VERY well.

This article also goes into a rant about cyanuric acid suggesting it's unnecessary if you have a constant source of chlorine addition during daylight hours when UV will deplete (and enough runtime after dark to have a residual build for the overnight hours). It suggests that you will use such little chlorine maintaining the very low needed FC level that it will save you a lot of money in the long run. (no CYA and chlorine is a more powerful oxidizer of algae he says). His setup can be expensive depending on whether or not you have a place to put the chlorine solution ( I don't ) and whether or not you have a cheap ebay peristalic pump or you pay full price. You don't have the temperature reducing benefits of the liquidator with the peristalic pump setup which is a large concern for me.

Is there something to what this guy is saying though? None of his points strike me as wrong or unintelligent and some very intelligent people here recommend absolutely having cyanuric acid. Well perhaps with a liquidator it's unnecessary?

Comments - specifically from waterbear, jasonlion, chemgeek, etc I am interested in

My personal thoughts are simple. If your circulation is good enough that you circulate all parts of the pool within a few hours, you'll be fine. But many pools need a full 8 hours to circulate once. In a pool like this, perhaps the algae might take hold in the uncirculated water since the FC will burn off in just a couple hours.

WHY are SWG pools recommended to have super high CYA?

I'm used to computers and cars - with swimming pools, information out there is SERIOUSLY conflicting. And I'm very obsessive so I want to know and understand everything about swimming pool chemistry now.

PS: My pool *still* won't hold FC, and is *still* perfectly clear at 70F. *grumph*

 

[JasonLion]

His approach can work. You have to add chlorine constantly during the day and that means you need the pump running all day. Bleach will normally mix into the pool within 30 minutes with the pump running, which is fast enough to have some chlorine everywhere before it vanishes. Without any CYA, the amount of chlorine you actually need in the water is quite low, so the whole things works.

However, it is much simpler to keep some CYA in the water and raise the FC level enough to compensate for the reduced effectiveness. The total amount of bleach you need to use will come down significantly, and you no longer need to keep the pump running all day.

His reasoning for not using CYA has several bits of truth in it, but it doesn't accurately sum up the complete picture. CYA does reduce the effective strength of FC, but you can compensate for that by running at higher FC levels. The FAS-DPD chlorine test resolves all of his testing concerns. Using ORP readings for determining effectiveness at killing pathogens is controversial and the study he quotes on this issue is known to be biased. And so on.

Higher CYA levels reduce total chlorine usage. Levels between 60 and 80 are used to maximize the lifetime of the SWG cell.

 

[Guest]

I concur with Jason. Much of this anti CYA rhetoric came from studies done by manufacturers of ORP controllers because CYA lowers ORP (but not the sanitizing ability of chlorine when compensations made for the CYA). ORP control of chlorine does have some problems when CYA is present in the water but the introduction of direct reading chlorine probes has solved a lot of them (with the exception of the extremely high price of these probes at the present time!)

 

[mas985]

One thing I have found with CYA is that I use less chlorine (i.e lower SWG setting) when my CYA was at 80 ppm then when it was at 40 ppm even though I had twice the residual chlorine at 80 ppm. So with higher CYA, the chlorine extinction rate goes down faster than the required increase in chlorine residual.

 

[duraleigh]

One thing I have found with CYA is that I use less chlorine (i.e lower SWG setting) when my CYA was at 80 ppm then when it was at 40 ppm even though I had twice the residual chlorine at 80 ppm. So with higher CYA, the chlorine extinction rate goes down faster than the required increase in chlorine residual.

Mark,

I don't have any data to back me up but I believe I experience the same thing.

However, I approached this subject with either Jason (I think) or Richard (maybe) last year and the take was "No, it doesn't work like that".

Perhaps I did not express my question correctly but I'm pretty sure the answer I got was that the chlorine will reduce at a fixed percentage rate (i.e. 30% of FRC daily) rather than the reduced rate you describe above.

I'm glad you brought this up and I hope we'll all come to a common understanding on this issue.

 

[mas985]

Dave,

I actually have data to back it up although it is a bit of an uncontrolled experiment. I ran my pool at CYA of 40, 60, and 80 ppm last year and targeted 5% of the CYA for residual chlorine. Even with the higher CYA, I had to lower the SWG setting to keep within the 5% target. In fact, the setting went from 60% @ a CYA of 45 ppm down to 30% @ 80 ppm with the same pump run time and a 5% target.

There are a lot of variables which can influence these types of results so this is a bit less than conclusive but I don't need anymore convincing.

 

[Poolidiot]

Now this is one of those "DEEP END" post

 

[JasonLion]

Perhaps I did not express my question correctly but I'm pretty sure the answer I got was that the chlorine will reduce at a fixed percentage rate (i.e. 30% of FRC daily) rather than the reduced rate you describe above.

At any given CYA level and any given sunlight level, you will lose some percentage of your FC each hour to sunlight. More sunlight increases the percentage lost. More CYA reduces the percentage lost.

To my knowledge, no one has every said that chlorine was lost at the same rate regardless of the CYA level. It has been said that the percentage of chlorine lost was lower at high CYA levels but not enough lower to make up for the higher FC level you need to maintain. These days it is believed that the reduction in FC loss at high CYA levels is more than enough to make up for the higher FC levels.

The rate of FC loss at higher CYA levels was controversial as recently as two years ago. There is a many year old, fairly reputable, scientific report, believed by most until about 2 years ago, that shows FC loss at high CYA levels far above what has long been rumored and more recently been measured in pools. A number of us believed that report until some experiments were done showing that it was very wrong in real world pools.

Both that old report and the new understanding agree that the percentage of FC lost goes down as the CYA level goes up. The only debatable point is how much it goes down.

 

[duraleigh]

I remember back probably 5+ years ago on PF, that 20-30ppm CYA was frequently considered close to max and 40 was getting somewhat high. (non-SWG)

It's interesting how the understanding everyone has gained (gaining) has resulted in our recommendations evolving slowly upwards over the years. My own pool was kept at 25 the first year (five years ago) and I'm about to move it to 60 this year as the reduction in chlorine loss has become apparent to me.

 

[Guest]

The difference is that 4-5 years ago everyone was keeing their FC at 1-3 ppm and now we are keeping higher FC levels as our CYA levels increase

 

[chem geek]

My response is here in The Deep End.

Richard