My goal is to have a CYA level of 20ppm or less

[axum]

I have a question regarding the CYA/FC levels in a salt water pool.

I would like to keep the CYA levels lower than what is recommended by the manufacturer. My goal is to have a CYA level of 20ppm or less. Personally I don't see a problem with that but I'm curious why the recommended levels are a lot higher than that. I am using a Hayward Sense and Dispense system to maintain 650mV ORP and the pump is running 24/7 (variable speed pump).

 

[chem geek]

Re:

Welcome to TFP!

The reason for the higher recommendation for the CYA level of SWG pools is to reduce the loss of chlorine to sunlight. Is the pool you are referring to indoors or otherwise not exposed to sunlight? If it's exposed to sunlight, then your chlorine usage will be high due to the UV in sunlight breaking down chlorine. That will require you to turn up the on-time on your SWG and it may not even be able to keep up. Also, if you do have your pump running 24/7 to try and output enough chlorine, that's very wasteful for electricity since you do not need to have that many turnovers of water for your pool.

If you follow our recommendation for the FC/CYA level, then you will use less chlorine, have a lower SWG on-time and a lower pump run time, a lower rate of pH rise (since running the SWG tends to make the pH rise) so will use less acid, and all of this will be accomplished with the same active chlorine level. The ORP for that active chlorine level varies by sensor -- Chemtrol might report 640 mV while Oakton might report 600 mV but the chlorine level will be enough to prevent algae growth and kill pathogens quickly, especially for a residential pool where the risk for transmission of disease is very low.

 

[axum]

I am aware of the role of cyanuric acid, but I want to keep the amount to a minimum because it has a huge impact on the sanitizing power of chlorine. My previous pool that I had for many years didn't have any CYA at all and used a liquid chlorine pump with an ORP sensing system. It worked great, the chlorine levels could be kept very low (<1ppm) and the water was great.

Now that we have moved to a "sunnier" part of the world, I accept the fact that we won't be able to get away with having no CYA in the water, but 80ppm is ridiculous IMHO. There's a famous paper on cyanuric acid "Cyanurics, benefactor or bomb?" that contains a lot of useful scientific information on stabilized pool water. It's impossible to argue with the findings of that paper.

About the ORP sensors: I calibrated the sensor with a ORP buffer solution so that's not an issue.

My goal is to have pool water that is balanced and safe. The balanced part is easy. I keep it safe by maintaining at least 650mV ORP (calibrated). The only variable is the CYA levels. The more CYA, the more FC is needed to keep the water safe. I don't like the idea of having 5ppm+ of chlorine in the water. After all, chlorine is toxic (that's why it kills algae, bacteria and other pathogens).

Given the fact that I'm fine with spending a little more money on electricity and muriatic acid, is there any reason why keeping 20ppm or less CYA is a bad thing in a SWG pool?

Specifics regarding my installation:

- Chlorine generator is oversized for the pool (generator is rated for a 40,000 gallon pool but our pool is only 13,000 gallons).
- pH is controlled by an automatic acid feeder and muriatic acid is cheap.
- We have a variable speed pump that is very efficient (using less than 200W of power to maintain a flow of more than 1,000 gal/h).

 

[JasonLion]

While the article you link to has some very good information in it, also contains several mistakes and misleading statements. We have commented on that article several times here. If you do a search I am sure you can turn up more specific information about what they got wrong.

While CYA does reduce the effectiveness of chlorine, this can easily be compensated for by raising the FC level, resulting in the exact same sanitizing activity level.

Very low CYA levels do present a problem in some situations. When a couple of hot sweaty people get into the pool the chlorine can be completely consumed. Using higher CYA levels along with proportionally higher FC levels you get the exact same sanitizing power, but keeps extra chlorine held in reserve by the CYA which will then be available when the active chlorine is used up. This results in better sanitation, and a more stable overall situation (small mistakes are much less likely to lead to problems).

Higher CYA levels can also dramatically reduce total chlorine usage in outdoor pools because CYA protects chlorine from sunlight so effectively. This increases the lifetime of your SWG significantly.

 

[axum]

I hear what you're trying to say. The article tries to tell me to lower CYA levels to single digits but that's not really what I'm planning to do. With all the information I have available at the moment, 20ppm doesn't seem to be such a bad number. Since I'm using the Hayward Sense&Dispense system, lower CYA levels will make the ORP probe more reliable because of the effect of hydrogen gas on the ORP readings. With lower CYA levels, the positive effect on ORP of raising FC is stronger than the negative effect of increased hydrogen gas.

I'm a big fan of automated chlorination using ORP and I think it would work well for us because we never have high bather load in our pool (and if it would happen in the future, I would take the necessary precautions by manually raising chlorine levels in advance). IMHO, lowering CYA levels is the only way to make the ORP sensing system work. I accept the fact that it will cost more because of the increased need for chlorine and acid.

Which brings me to another (related) topic:

What I don't understand about Hayward is that they are marketing a system that simply can't work because of the effect hydrogen gas has on the ORP sensor. With the Hayward recommended CYA levels, once you start generating chorine, the ORP starts to drop and you enter a vicious cycle. The chlorinator simply won't stop running because the ORP will never go up enough to shut it off.

 

[bridgman]

The more CYA, the more FC is needed to keep the water safe. I don't like the idea of having 5ppm+ of chlorine in the water. After all, chlorine is toxic (that's why it kills algae, bacteria and other pathogens).

I can never find it when I need it, but someone recently posted a table that linked CYA, FC and "active chlorine" levels... where "active chlorine" is what does the sanitizing. The idea of higher FC levels as CYA goes up was to maintain a consistent active chlorine level. Reading between the lines, I *think* your concerns are related to having a high level of *active* chlorine, and I don't think that is the case when CYA levels are higher.

Does anyone remember where that table is hiding ?

 

[chem geek]

The active chlorine (hypochlorous acid) level is proportional to the FC/CYA ratio. So our recommendation for SWG pools is an FC that is roughly 5% of the CYA level so 4 ppm FC with 80 ppm CYA which has the identical active chlorine level as 1 ppm FC with 20 ppm CYA. However, with the 4 ppm FC with 80 ppm CYA you will lose less absolute FC per day to sunlight. You'll be able to run your pump a lot less and the SWG will last longer as well.

As for the "famous paper" you refer to, that is NOT a peer-reviewed scientific paper in a respected journal. Nevertheless, Kent Williams of the PPOA gets most things right, but not everything. He came from an ORP manufacturers background and is biased in that way and ignores the consequences of higher active chlorine levels with respect to the creation of more disinfection by-products. See the thread Find a way to collaborate with the PPOA? for more info.

For the truth on the relationship of FC, CYA and the active chlorine level, the definitive paper is the one in this link. You can also read the "Chlorine/CYA Relationship" section in the first post of the thread Certified Pool Operator (CPO) training -- What is not taught.

As for your ORP readings, they will be largely affected by the active chlorine level so should be the same at 4 ppm FC with 80 ppm CYA as they would be with 1 ppm FC and 20 ppm CYA.

What you are attributing to higher CYA levels is completely made up for by having a proportionately higher FC level. Remember that the PPOA is dealing with commercial/public pools, not residential pools. You don't need the active chlorine level to be higher than that which prevents algae growth. It will already kill pathogens quickly, but you won't have the large amounts of bather waste that would require higher oxidation rates though in practice one should use supplemental oxidation when bather loads are high in order to minimize disinfection by-products. You can look at kill times for chlorine where the FC is roughly 10% of the CYA level (about double the level we're talking about for an SWG) in this post.