Chlorine burn rate at different CYA levels

[Auctor]

Hi all

Something has been puzzling me for years about the relationship between Chlorine usage and CYA. Let’s pretend that we run two identical non-salt pools right next to each other. Pool #1 has a CYA level of 30 and 3ppm Chlorine and Pool #2 has a CYA of 60 And 6ppm Chlorine. its 75 degrees, sunny, pool water temp and other internal chemistry is identical. Do both pools use/burn chlorine at the same rate? or does Pool #2 use/burn more chlorine since it has a higher starting level?

What I’m driving at a few things: 1) I’m not sure how to determine the ideal CYA level; 2) I don’t know if chlorine usage is partly driven by CYA level; 3) why are SWGs able to keep such a low chlorine to CYA ratio?

Thanks in advance for your insight!

 

[Jimrahbe]

A,

1. Use our chart.. See the saltwater section... FC/CYA Levels We recommend 60 to 80 ppm of CYA for saltwater pools.

2. The higher the CYA the less the chlorine will burn off.. But at the same time the higher the FC has to be to keep things sanitized.

3. SWCG pools have a lower target FC because they are generally getting chlorine over a longer period of time. If you manually dose Liquid Chlorine, it has to last for 24 hours until you dose again.. SWCG's generally run 12 hours or longer so you need less reserve to last until the next dose.


I find that my pools work best for me when I adjust my SWCG to maintain my target FC or higher.. I never worry about being a little high after a cloudy day.. I only worry about being low. I view my minimum FC as a cliff that I never what to even get near.. Running hot just has no real downside.. Falling off the cliff, does.

Thanks,

Jim R.

 

[Auctor]

Hey Jim

Thanks for the response. I understand the ratio pretty well. I think I’m asking a question one step deeper, which is whether maintaining a CYA of 20 vs 40 vs 60 impacts chlorine usage? Does the pool use more or less chlorine on a given day because of the CYA level?

I ask because I’d love to minimize chlorine exposure, but I’d also like to minimize chlorine usage. I don’t know if I can achieve both.

 

[Jimrahbe]

A,

Not sure I know the chemistry reasons, but for sure I know that if I let my CYA drop that my pool will start to use more FC each day.. So, my choice is to increase the SWCG's output or to increase my CYA.

Let's see what Matt has to say.. Calling @JoyfulNoise

There is no reason to worry about "chlorine exposure" as my understanding is that TFP chlorine guidelines have a lower active chlorine level than what is allowed in drinking water.

Thanks,

Jim R.

 

[JoyfulNoise]

Everything you need to know is in this thread -

Pool Water Chemistry

This thread presents my findings so far on pool water chemistry including the following: More Accurate Calcite Saturation Index (CSI) to replace Langelier Saturation Index (LSI) Calculation of ppm HOCl (disinfecting chlorine) at various levels of Total Free Chlorine (FC) and Cyanuric Acid (CYA)...

www.troublefreepool.com

4th chart down

 

[Auctor]

Yes! That thread has got a little something for everyone. Thank you. I need to read it a few times to understand, but hopefully this satisfies my curiousity. Anecdotally and on first read, it confirms what I’ve been suspecting, that 20-30ppm of CYA is more than enough, and allows me to keep my chlorine levels between 1.5-4ppm at all times (limiting exposure).

 

[JamesW]

At the same FC/CYA ratio and pH, the burnoff rate in ppm should be roughly equal because the amount of hypochlorite is about the same.

Higher CYA levels seem to have an additional protective effect due to making the water more opaque to UV light.

So, the deeper chlorine is probably more protected from UV with a higher CYA level.

We would need a lab quality test apparatus to be able to know for sure if and how much UV is blocked from going through the water at different concentrations.

If someone has a UV camera, they could try looking through water with various levels of CYA to see if there is a difference.

 

[JeanZ1]

The danger becomes falling below that absolute minimum of 1.5. For example if you start the day with a chlorine of 4.0, the sun (UV and heat) can easily eat up 3-3.5 ppm of chlorine which is below the minimum (call it the sun tax). If you add swimmers to the mix, you will use even more chlorine. You're working on a razor's edge with no room for error. I understand wanting to minimize chlorine but you'll use far more trying to clear algae after the fact. Tread carefully.