My CYA went WAY down after recent SLAM

[borjis]

I do chlorine tests daily sometimes morning then again in the evening.

The other tests I do once per month.

Today for the first time, the CYA is at TFP recommended levels somehow.

Results by date:

FC: 7 7/3
CC: .5 7/3

PH: 7.5 7/3 (my ph is always 7.5 since I moved in the house and balanced it)

TA:
80 5/22
90 7/3

CH:
300 5/2
275 7/3

CYA: (tested back to full sun, with tube in front of me, waist high)
100 5/9 (dilution method)
1 pouch Bio-active cya reducer used: 5/10
80 5/24 (not dilluted)
70 6/13
40 7/3


I did do a successful mustard algae slam a few weeks back.

Other than topping off the pool about every other week to add about 2-3 inches
of water and 3 back wash cycles during the slam, I have had no other water replacement.

I did run out of CYA re-agent and this test today is the first with the new re-agent
so I ordered the 50 ppm CYA calibration re-agent from TFTESTKITS and will test to confirm my results are sound.


This seems a bit odd though. If it is indeed 40 then maybe the Bio-Active did a lot more but
it was a delayed result. With the chlorine up I don't see how....

This would also explain why I'm losing more FC on sunny clear days (pool gets full sun from 11-5)

I've been doing OCLT every day since the slam just to make sure and I'm not losing
any chlorine over night.

 

[chem geek]

While it's possible (though unlikely) that the Bio-Active had a delayed result (the bacteria should have been killed off and enzymes degraded by the SLAM), I think it more likely that your high chlorine levels probably at high pH oxidized the CYA since that is a known mechanism -- it's just too slow to be practical in most cases. See "CYA Degradation by Oxidation from Chlorine" in Degradation of Cyanuric Acid (CYA).

 

[borjis]

Thanks for the reply chem geek, your posts are always interesting reading.

I held mustard shock levels for cya of 80 for several days (open house to sell my home was just a week away and needed tfp looking pool water ha ha)
so if that calibration cya solution arrives and holds true then it must have gotten oxidized. I'll post my results when the re-agent arrives.

yay for my buyer....they'll have a perfectly balanced tfp pool to start september with.

 

[borjis]

I got the CYA 50ppm calibration test re-agent and tested yesterday.

Confirmed it came out as 50!

And good to know my testing method is sound for this difficult test.

So my CYA definitely did go from 70 to 40 after holding mustard shock levels several days in a row.

The new buyer of my house will inherit a "perfectly balanced in every way" pool.

A far cry from the condition it was in when I took possession.