CyA going away quickly

Chlorinating, maintaining the right chlorine levels,
chlorine problems. Dichlor, trichlor, cal hypo, bleach,
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Eddles
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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!


Denniswiseman
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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
Let's have your numbers
FC:
TC:
pH:
TA:
CH:
CYA:
Eddles
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My Pool: 66000 litre indoor swimming pool with UV and sand filtration, and 1200 litre hot tub.

Re: CyA going away quickly

Postby Eddles » Sun 07 Feb, 2021 10:33

Hello thanks for your reply! Sorry for delay in my reply. I've restored the CyA levels and it now seems stable, but chlorine seems to be vanishing into a black hole. I'm using brand new sodium hypochlorite as sanitiser. Latest readings are:

FC: 0.0
TC: 0.0
pH: 7.3
TA: 130
CH: 280
CYA: 40

I'm adding in 2.5 litres sodium hypochlorite now (15:30 my time), and will retest tonight.

Your advice would be greatly appreciated!
Eddles
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Posts: 38
Joined: Mon 10 Jun, 2019 16:09
My Pool: 66000 litre indoor swimming pool with UV and sand filtration, and 1200 litre hot tub.

Re: CyA going away quickly

Postby Eddles » Sun 07 Feb, 2021 10:41

By the way, to make doubly sure I didn't mess up anywhere, I am using the same sodium hypochlorite from the same container to sanitise my hot tub filters in a bucket. So I took my test sample from this bucket and it instantly turned red. Therefore, chlorine is present in the sodium hypochlorite I'm using and the test process should be working properly.
Eddles
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Posts: 38
Joined: Mon 10 Jun, 2019 16:09
My Pool: 66000 litre indoor swimming pool with UV and sand filtration, and 1200 litre hot tub.

Re: CyA going away quickly

Postby Eddles » Sun 07 Feb, 2021 17:26

Tested just now - 22:30 my time, FC is 1.5, TC is 0.0.
Eddles
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Re: CyA going away quickly

Postby Eddles » Mon 08 Feb, 2021 05:01

Forgot to mention that I did add in 2.5 litres bleach last night. 10:00 now, figures are now FC 2.5, TC <0.5. Better, but that took 5 litres of bleach in less than 12 hours to achieve. Have put in a further 2.5 litres.
Denniswiseman
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Posts: 1890
Joined: Tue 06 Sep, 2011 05:48
My Pool: 10k inground fibreglass, Telescopic Cover, Hayward Powerline pump, Quality filter with glass media, 27kw output heat pump, K-2006C test kit
Location: United Kingdom

Re: CyA going away quickly

Postby Denniswiseman » Mon 08 Feb, 2021 06:12

The only thing that springs to mind is that the sodium hypochlorite strength has diminished
What initial strength was the sodium hypochlorite
Try using Pool Maths to calculate what dose is required according to the strength of the sodium hypochlorite /plain bleach
Get some fresh supplies just to check
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Re: CyA going away quickly

Postby Teapot » Mon 08 Feb, 2021 15:17

Sodium hypocrite liquid doesnt usually contain CYA so gradually it will oxidise any CYA out of the pool. If you do not have CYA in your indoor pool and why would you unless its in the sun? You could comfortably run the pool on 0.2- 0.3ppm of free chlorine and still be on the high end of chlorinated water.

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