Low CYA

[Missy819]

I read that high amounts of FC will drop CYA. We finally dropped FC to 5 and turned on the SWG. Our CYA is at 40. Pool math is showing I need to add 3.5 lbs CYA to get it to 80. I’m nervous about add that much at once, not wanting to go over. What is the best way to do this?

Add all at once
Half now and test in a couple days
Add sock at a time, retest every cpl days.

I bought a 16oz bottle of CYA testing chemicals so I’m not concerned about testing more often.

 

[duraleigh]

I read that high amounts of FC will drop CYA.

Simply, not at all correct

Rather than go to 80 ppm, I would suggest 60 ppm. Perhaps, if chlorine consumption is too high, you could go to 80 in a few days.

I consider 80 ppm CYA as being in the "safe" zone but at the VERY top end of "safe"

 

[Missy819]

These results show after slamming that it took a while to bring the FC down. What caused the CYA to drop?

 

[pjt]

I read that high amounts of FC will drop CYA

You didn't read that here.

Half now and test in a couple days

I would choose this option. CYA is very easy to add, but draining is the only practical way of lowering it. How much sun exposure does your pool have?

 

[Missy819]

Full sun for about 4 hrs, then pool is slowly shaded over the next 4-6 hrs.

 

[duraleigh]

CYA doesn't drop like that. Testing error is about the only thing that makes sense.

 

[Missy819]

Could it be that it dropped during/after the slam while waiting for the FC to drop?

 

[IceShadow]

CYA has only a few ways to leave pool water.

- if there is a leak and you are refilling
- it does degrade naturally in water but slowly, 5ppm or so in a month
- there is a bacteria that can eat CYA and produce ammonia

One of these is the only explanation, short of testing error. The CYA test is hard to interpret and dependent on lighting, etc.