CyA going away quickly

Chlorinating, maintaining the right chlorine levels,
chlorine problems. Dichlor, trichlor, cal hypo, bleach,
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CyA going away quickly

Postby Eddles » Sun 10 Jan, 2021 07:55

I have an indoor 66,000 litre swimming pool which I sanitise with sodium hypochlorite. I used to have problems with CyA levels being far too high (>300ppm) but got it under control a year ago and it stayed stable at about 40ppm. However now I'm having a different problem, the CyA keep vanishing! Around the start of December, I found no matter how much chlorine I add in the pool, it'd show zero. After a week of this, I decided to check the CyA levels, and it was zero. I added in 1kg stabiliser and after 3 days later, it was 40ppm, ideal. Chlorine started working after this.

But now in mid January, adding chlorine makes no change - FC last night was 0.0, so I put in 2 litres sodium hypochlorite last night and today the FC is 0.0. So, I checked the CyA levels and it's zero! What happened to the CyA?

Note that this pool also has an UV sanitiser - a Blue Lagoon UV-C - would this deplete the CyA?

The pool has no exposure to the sun at all, there's large windows but the blinds are always down.

Would it be better to use stabilised chlorine granules and monitor the CyA and switch back to sodium hypochlorite if CyA gets too high?

Many thanks in advance!

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Re: CyA going away quickly

Postby Denniswiseman » Sun 10 Jan, 2021 10:51

CYA does diminish over time especially with splash out and backwash
However an indoor pool doesn't really need CYA and a level of 20 is quiet sufficient
Having CYA in your water doesn't affect your chlorine sanitation, it's just excessive CYA renders your chlorine ineffective and you have to use more chlorine
The only other thing is Sodium Hypochlorite loses it's strength and you have to use more to get the correct sanitation
For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by at least 7 ppm
To reduce your CYA you have to do a partial drain and refill
Continuous use of Trichlor/Dichlor will raise your CYA which means you have to raise your chlorine level as well

Chlorine / CYA Chart
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