CYA level in pool

Hi all you pool experts
on saturday we had our swimming pool water tested as i suspected we had put the wrong measure of chlore shock in the pool. sure enough the CYA was 65.
The pool people recommended that we waste some water( i felt so bad) and add fresh water. We had our water tested again today and the CYA is 2 points up! This may be due to the temperature because we forgot to read it before going to the shop so guessed.
My question is as 65 is ok as long as it stays there would we be better using eau de javel rather than wasting even more water?
If so where do we buy it and how do we put it in the pool safely?
Any help appreciated. I really dont want to waste thousands of litres of water if i can help it but i feel sure it wont be so simple.
Its not my week for anything water related!

Hi Teresa,
How far south are you, more sun means better to have more CYA up to a point. The CYA binds chemically to the chlorine reducing the free available chlorine to sanitise the water so you’ll need to run with a slightly higher chlorine level to have sufficient free available chlorine to keep the water sanitised. so I would run with a chlorine level between 3.25 and 4.5ppm. Don’t worry as this is a lot less than a swimming baths will run without CYA (indoor pool).

Yes use eau de javel from a Brico shed, around €12 for a 20 litre bidon. you don’t say how large your pool is in m3 so I can’t advise on how much to add. You’ll probably want to buy some hydrochloric acid whilst you are at the brico as well for pH- adjustment. (NEVER LET THE TWO COME INTO CONTACT WITH EACH OTHER) AND PLEASE BEAR IN IMND THESE CHEMICALS CAN BE HAZARDOUS TO HEALTH. skin burns from acid and clothing bleach white from the eau de javel.

Thanks for your reply. We live in Perigord Vert near BrantĂ´me. The pool is approx 10 x 5 metres part of which is called a Roman step ( I think) so very shallow that bit and obviously a graduated depth.
Is the hydrochloric acid instead of the commercial ph -?
Can we not use that?
Also how do I actually dose both these products?
I used to work in a hospital laboratory so am quite used to dealing with hydrochloric acid as we added it to 24 hr urine samples, so yes very good advice. I’ll keep my husband away from it!
Do you recommend a particular testing kit or shall I take the sample to the pool shop.?
Thanks so much.

Also should I ask for a particular strength?
Thanks again

So your pool is roughly 62m3.
Yes hydrochloric instead of the pH- (sodium bisulphate) of course you can use the bisulphate but it adds unwanted sulphates and costs more. You can just measure out in a plastic jug, walk around the perimeter of your pool pouring into the pool as you go, unless you would prefer to use chemical dosing pumps? bisulphate needs dissolving in warm water first.
I always recommend your own test kit be that a titration test or photometer test. I can advise on both as these are scarce in France due to the proliferation of dip strips.

Brico 20ltr bidons are 9.6% active chlorine (almost 10% ) keeping the math easy. 62m3 requires 620ml to raise the chlorine level by 1ppm if it’s fresh chlorine (check dates ) or if it’s a bit weak add a bit more. you’ll soon get a feel for your pool.

Thanks so much.
I looked online at Brico Depot and they have eau de javel but if I’m reading correctly it’s 36%. Am assuming i would divide your calculation by 4 if using that strength. Also they have lots in stock so would have to chk date.
I have a very basic titration tester so if you can recommend either a better one or a photometer I would be very grateful.
Many thanks for your time.

Hi Theresa
The Bricodepot javel is 36° (degrees), not 36%, and so is 9.6% chlore as John has stated, so use his dosage

Aah. Thanks. I only read it online and was looking for percentage and could only see that figure.
Well that’s better can just use dosage as stated.
Off this afternoon to get it.
Thanks again.

If your pool is filtered/cleaned using chlorine, then the stabiliser in the chlorine will gradually build up. You can use non-stabilised chlorine - a bit more expensive to avoid this. Normally you should change 1/3rd of the water per year to avoid this problem…as I found out to my surprise last year !!!

When we bought our house last year it had been empty a while so the pool needed a lot of topping up.
Unfortunately this year my husband didn’t read the blurb properly and rather overdosed. We reduced the pool by about 10 inches and topped up to no avail so eau de javel is our best route for the moment.
I think we have learned by our mistakes rather.
I keep praying for a lot of rain and hopefully backwashing will help too.

There is no need to change 1/3 of the water, when you have sufficient stabiliser just switch to eau de javel from a brico. It is not more expensive.

You do need a decent tester to avoid falling into this trap and do not use dip strips as they cannot accurately measure stabiliser, the most important chemical in your pool.

Theresa if you run on the high side of the chlorine level I gave you, you will overtime oxidise some of the stabiliser out of the pool and over winter sometimes the level drops a bit if there is no chlorine in the pool.

Right my husband went to the local brico with the exact requirements written down by me. He’s managed to come back with a 14% strength. It has a date of 21/07/2018.
It’s Onyx
Hypochlorite de sodium 14%
It says 0.5l for 40m3 for piscine
Is that ok. Thanks.

Hi Teresa, That is fine you’ll probably need a bit less that’s all as it’s allegedly 50% stronger. I can guess it wasn’t €12 though. The one you looked at generally works out less and being slightly weaker doesn’t break down so fast, hence why I said probably.
Approximately 450ml will give you 1ppm

Yes it was double but it was all they had locally. As we had a credit note my husband just got it. Once we have used this I’ll source the 9.6%.
Can I buy a decent pool tester in a brico? The pool shop , strangely, only has strips.
We are off to Riberac in the morning so I’ll look there.
If not I’ll order from Amazon.
Once the chlorine is added to the correct level roughly how often do we add it( I know it will depend on levels to a point)? But is it usually daily, every few days, weekly?
Thank you so much for your time. It’s much appreciated.

No, strangely despite the huge number of pools in france you won’t get a decent tester in a brico or even a pool shop, most pool shops think they are highly advanced if they have an electronic dip strip reader :laughing: it’s a near useless dip strip to start with so adding an electronic reader won’t help.

I can supply a good tester for less than you’ll pay on Amazon but I don’t give Prime delivery option :grin: roughly 7 days. Private message me if you require one.

How long does the chlorine last is a real piece of string question, it depends on how much work the chlorine has to do. Great filtration, low nutrient and it could last for days, I would expect a 50% top up each day, more if it rains.

Sorry. Didn’t mean to repeat first question. I’m on my phone. At the risk of being thick how do I private message you?

Teresa… if you click onto John Withall’s name or face… it will open a box with amongst other things the word Message… click Message and it will open an other box for you to chat and send privately…



Hi Teresa, just read through your questions and answers regarding CYA and pool chemical testing. All the advice given is excellent, I have learnt so much off John Withal over the years, he is the #1 oracle for all things pool wise!
Following John’s advice a couple of years ago I invested in a COLOR Q testing kit which includes a battery operated test unit, together with chemicals, test tubes and clear instructions. They are advertised on Ebay, and usually only available from the USA. About £150 for the kit and P&P but worth every penny, especially in relation to keeping check on CYA.
We’re in the same area as you, near Riberac, so the climate has the same effects on our pool.
Hope that helps, Paul

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Hi Paul, the color Q and all the LaMotte range are available via myself. Ordering via the US means no warranty, whereas we warranty our equipment bought in the EU.
Thanks for the praise though.