CYA Buildup: What Level?

[josephny]

Hi again,

I've been reading and studying and am wondering about CYA buildup from the trichlor tabs I use.

From pool school:

"2. Trichlor - Commonly sold as tablets or pucks that you simply put into an automatic container that passes pool water over them and they slowly dissolve - putting chlorine and CYA into your water and lowers the pH. They are incredibly convenient and incredibly insidious. The CYA that they put into your pool water doesn't get used up, and instead accumulates. Eventually the CYA level will build up to a point that renders your chlorine ineffective. Typically, everything is fine, until one day you start to develop algae and don't understand why."

After 2 seasons of using trichlor, and after a major dichlor shock 4-5 weeks ago, my CYA is 20.

Is there a specific CYA level at which chlorine becomes either ineffective? Or, do we refer to the CYA-FC chart and say that when CYA levels reach a certain point, the amount of chlorine needed to keep the water clean is too high for safe swimming?

At the beginning of each season I raise the water level about 1 foot of my 4.33 foot deep pool. Might this mitigate the CYA levels?

I'm trying to get at how bad my use of trichlor really is.

Thanks,

Joseph

 

[TizMe]

Perhaps a cornerstone element of BBB is understanding the relationship between the chlorine in your pool and the stabilizer you put in to protect the chlorine from the sun. Generally, the more stabilizer, the more chlorine you need to maintain its effectiveness.

Anything less than a CYA level of 20 is considered to be 0. You want your CYA level to be between 30- 50 for vinyl pools. I keep mine on the lower scale of 30.

The pucks you use in your auto chlorination system constantly add CYA to your pool and eventually will get so high the amount of chlorine you use to keep the pool sanitized will be unsafe for swimming.

 

[josephny]

Perhaps a cornerstone element of BBB is understanding the relationship between the chlorine in your pool and the stabilizer you put in to protect the chlorine from the sun. Generally, the more stabilizer, the more chlorine you need to maintain its effectiveness.

Anything less than a CYA level of 20 is considered to be 0. You want your CYA level to be between 30- 50 for vinyl pools. I keep mine on the lower scale of 30.

The pucks you use in your auto chlorination system constantly add CYA to your pool and eventually will get so high the amount of chlorine you use to keep the pool sanitized will be unsafe for swimming.

Any way to know what level (of CYA and Chrloine) that will be?

And, any way to know how soon that will happen? Does putting about 20% fresh water in the pool each year negate (partially or wholly) the buildup of CYA?

Thanks

 

[chem geek]

Any way to know what level (of CYA and Chrloine) that will be?

And, any way to know how soon that will happen? Does putting about 20% fresh water in the pool each year negate (partially or wholly) the buildup of CYA?

Thanks

Joseph,

For every 10 ppm Free Chlorine (FC) added by Trichlor, it will increase the Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it will increase the CYA by 9 ppm.
So if you multiply your daily FC usage by the number of days of such usage, you can figure out the rate of CYA rise. Even at a low 1 ppm FC per day usage (usually it's higher, such as 2 ppm), this is an increase of 18 ppm CYA per month or over 100 ppm CYA after 6 months.

Yes, replacing water with fresh water will dilute the CYA, in your case by 20% when you do that. So if the CYA got to 50 ppm then that would reduce it to 40 ppm. If you backwash your filter, then that would help, but yours is a cartridge filter and I suspect you don't use lose a lot of pool water cleaning it. Rain overflow dilutes the water as well.

How are you getting the 20 ppm CYA number? Is this with your own good test kit such as the TF100 from tftestkits.net? If the number is from a pool store or from test strips, I wouldn't trust it. It's possible you lost a lot of CYA over the winter if you let the pool go by not adding chlorine, but I suspect something else like an inaccurate measurement.

Is there a specific CYA level at which chlorine becomes either ineffective? Or, do we refer to the CYA-FC chart and say that when CYA levels reach a certain point, the amount of chlorine needed to keep the water clean is too high for safe swimming?

Roughly speaking, the same ratio of FC/CYA has the same chlorine effectiveness. So if you raise the FC as the CYA goes up, then the chlorine is able to kill pathogens and prevent algae growth at the same rate as before. It's not that a higher FC is unsafe for swimming (unless you were to drink LOTS and LOTS of water), but rather that it becomes impractical to maintain a very high FC level. More importantly, if the FC level did drop, then the pool becomes very difficult to treat if algae does start to grow since the required FC level for shocking is extremely high.

One can certainly use Trichlor pucks as their source of chlorine if one understands the extra costs associated with pH Up products that are needed, the acidity of the pucks that can damage what is nearby if in a floating feeder, and the costs associated with an algaecide or phosphate remover needed to supplement the pucks when the CYA gets high (without having a higher FC). If one is able to dilute the water or otherwise keep the CYA lower, then that avoids the high CYA issues.

Richard

 

[josephny]

Richard,

Thank you so much for the detailed explanation. While I enjoy and aspire to learning this stuff, I question whether I'll remember and be able to implement all of it.

I had read about the .9ppm and .6ppm CYA increase for each 1ppm of FC increase for dichlor and trichlor, respectively, but I didn't (and probably still don't) know how to calculate my daily usage/consumption of FC.

For example, last summer I used 2 floaties simultaneously each with 3 tabs in them. I replaced the tabs when they were gone or almost gone, which was somewhere in the 7-14 day range (I took no notice or notes of what I was doing, something I hope to change this season).

A random google search finds a random (other) forum post that says these tabs each introduce a total of 4.6ppm FC into 45,000 liters of water (11,900 gallons). Doing the math: 11,900 divided by 14,700 gallons in my pool results in .81, or 3.726 ppm for my pool.

Assuming 6 new tabs (22.36 FC ppm) every 10 days, I should be raising CYA by 13.4 every 10 days.

I doubt that's the case with my current reading of 20 -- maybe the pool store fancy tester if off?

My other confusion is how I would lose CYA over the winter? I thought the only way to reduce CYA is to replace the water? Clearly some water is being replaced over the winter, but I wonder how much and to what effect.

Thanks again,

joseph

 

[josephny]

Correction/clarification:

The figures were for an 8 oz 3" trichlor.

From: http://www.opacific.com/chlorine.html I see that each 1.5 oz of trichlor will raise FC 1ppm in 10,000 gallons of water, resulting in 4.67ppm for a single 7 oz trichlor tab.

6 of these every 10 days should introduce 28ppm of FC and 16.8ppm of CYA.

I'll be using my new TF100 on on Friday and hopefully all will be well.

 

[TizMe]

There have been several posting about people opening up pools this summer with no CYA readings in the water.
This is one of the postings here about it.

 

[josephny]

Wow, that's some good chemistry in the link and the link within the link.

Too late to test for ammonia for me, but hopefully I'll remember next spring.

In the meantime, I look forward to keeping my eye on the water chemistry.

Thanks!

 

[chem geek]

josephny,

You can use The Pool Calculator to calculate the effects of adding chemicals. For your 11,900 gallon pool, 8 ounce of Trichlor will raise the FC by 4.6 ppm and the CYA by 3 ppm. However, 8 ounces is a large 3" tab/puck. Such tabs/pucks come in different sizes and weights. The larger 3" ones are 6, 7 or 8 ounces weight. There are also 1" ones are 3 ounces and the very small 1/2" ones are 1 ounce. With 6 tablets every 10 days, I'll bet those tablets were smaller, probably 3 ounces 1" or maybe 6 ounces 3" -- do you know their size?

It is quite possible that the "pool store fancy tester" is off. The CYA test is often done improperly by pool stores. This is why we recommend getting your own good test kit such as the Taylor K-2006 you can get online here or the TF100 you can get from tftestkits.net here. A comparison of these test kits is in the Pool School here.

It is possible to lose CYA over the winter, or at any other time, if you let the chlorine level get to zero. Then, bacteria can grow and convert the CYA into ammonia. This usually doesn't happen so you can't rely on it and the ammonia takes a LOT of chlorine to get rid of.

Richard