New AGP setup with SWG

[poolsam]

Initial Reading after new water, 3 bags salt, and Intex SWG running about a month - seems clean.

CYA 0
FC 0.5
TC 0.5
Ph 7.0
TA 20
CH 40

After reading pool school, I added 3 cups Cyanuric acid, 3 cups baking soda, and 3 cups bleach - 48 hrs later

CYA 30
FC 1.0
TC 1.0
Ph 7.2
TA 80
CH 40

Didn't add any calcium as vinyl Intex metal frame pool. I am running the Intex SWG 4 hours - manual suggests 2 hours based on volume/temp.

My understanding of cyanuric acid is that it keeps the free chlorine around longer, but reduced the killing effect of free chlorine so you need higher levels of free chlorine. Was my pool dirty before, even though it looked good, and now with increased CYA, do I need higher free chlorine.

 

[svenpup]

You are correct. Use bleach to quickly bring your FC up to 5 and let the SWG take over from there.

Also, 3 cups CYA should bring you up to 40 in 4400 gallons. I would add another 2 cups.

 

[poolsam]

Why is it suggested to have a CYA of 60 or 70 with a SWG, instead of 40. What is the chemistry behind that?

Should I use the pool calc to calculate the amount of 6% bleach to add to bring from 1 to 6, then add all at once?

With a CYA of 60, what FC should I want, or expect, the SWG to maintain with a recommended 2 hr run time?

 

[svenpup]

The appropriate CYA level is always a balance between sun protection and chlorine effectiveness.

With CYA too low, the sun will burn off enough chlorine during the day that FC falls below minimum (or maybe even to zero).

With CYA too high, it becomes difficult to SLAM (if needed) because of the high FC level needed.

The guideline CYA levels here are high enough to protect FC from the sun and ensure residual sanitizer in the water throughout the day, but not so high that shocking is impossible.

In a residential pool, sunlight is the dominant cause of FC loss (as opposed to a commercial pool where bather waste dominates the equation). So by minimizing sunlight losses you keep daily FC loss down.

With an SWG, lower daily FC loss translates to less cell run time, less cell wear and less pH rise.

Running a higher CYA is manageable with an SWG because an SWG provides a very consistent level of FC , and doesn't skip a day or have the daily FC swings typical with manual dosing.

With CYA up at 70-80 your FC should always be kept above 5-6.

Also, your 2 hour run time may or may not be enough. You should adjust SWG percentage setting and/or run time to ensure you maintain your target FC. I run my pump on low for 10 hours with the SWG at about 30%. Your situation is different, but don't be married to the 2 hour run time.


Check out Water Balance for SWGs and Chlorine / CYA Chart.

 

[poolsam]

I've read the two referenced articles.... I'll add more CYA and bleach tonight and retest.....

I am a little gun shy about SWG as I have one in my hot tub that was very powerful when unchecked and corroded two heaters. Both times it was my fault as I left it without checking for 2-3 weeks and FC were very high..... My learning point is to always turn it down several settings if hot tub not being used...

My Intex pool has "appeared" very clean with a FC level of 0.5. With a CYA level of 60-70 and a FC of 5-6, would a swimmer notice any difference? I am confused as to why a free level (meaning unbound Cl which are free to disinfect) needs to be different if CYA present. I would think 6 FC would have much better disinfection than a level of 1 with no CYA?

 

[svenpup]

You are getting into a "Deep End" type discussion, and if you are really curious you can use the search feature and read through a pile of Chem Geek's posts on this topic.

Suffice it to say that FC=5 at CYA=70 is far less harsh than FC=.5 at CYA=0.


The CYA is effectively sequestering the FC so it is not available to disinfect, but also not available to irritate your skin or disintegrate your swim suit.

Though while it is "sequestered", it is also held in reserve to be released as needed.


Check out https://www.troublefreepool.com/~richardfalk/pool/HOCl.htm
At FC=.5 at CYA=0, HoCL is 0.242
At FC=5 at CYA=70, HoCL is 0.029

So with recommended SWG CYA level, the chlorine is 1/10 as "harsh", but there is more available over time, without the sun burning it down to zero during the daylight hours.

If this doesn't scratch your itch, http://www.troublefreepool.com/threads/558-Pool-Water-Chemistry

 

[poolsam]

Readings this am...

FC 1.5
TC 1.5
PH 7.4
TA 90
CYA 60

I added 2 cups bleach for a 4400 gallon SWG pool. I am presuming that the added CYA is still binding the FC, so therefore need to add more until equilibrium and the SWg can keep up with loss?

Any other suggestions... Thank you!

 

[svenpup]

The FC that is "bound" to the CYA is still measured in the test.

2-4 ppm FC loss per day is normal. As I mentioned earlier you need to tune your SWG percentage and run time to make up whatever FC is lost daily. Start high and slowly adjust down.

Another possibility is that algae started to take hold when your FC was low. If that is the case you should SLAM to get rid of the algae before your SWG will be able to take over.

 

[poolsam]

If there was algae, would the TC be high as compared to FC, reflecting combined C. Currently both my FC and TC are 1.5... CYA is 60

 

[svenpup]

CC is an indication that you have something going on, but no CC is not proof that you don't have algae. What does the water look like? Is it sparkly? Water can be clear with algae but may look a little dull.

 

[chem geek]

I am a little gun shy about SWG as I have one in my hot tub that was very powerful when unchecked and corroded two heaters. Both times it was my fault as I left it without checking for 2-3 weeks and FC were very high..... My learning point is to always turn it down several settings if hot tub not being used...

You should not be using chlorine with no CYA in the water and most especially should not be doing that with an SWG (implying elevated salt levels) because the chlorine will be FAR too strong and as you found out that means it will corrode equipment faster. For a spa, I recommend 30-40 ppm CYA and if the spa is covered try and keep around 2 ppm FC though if you want to start a soak at 1 ppm that's OK (some people prefer the minimal amount of chlorine during their soak -- technically a little more risky but this is a residential spa and you'll blast with chlorine right after your soak).

Fortunately, it takes a very low active chlorine level to kill pathogens so you don't need to worry about CYA so long as you don't have it be too high.