Conversion of a pool from an ioniser to salt water

[Neil]

I have a 20 year old pebblecrete pool that has always been run on an ioniser with Cu/Ag anodes. While I was away recently a carer got the pH way out of whack (too low/acidic) and after some chlorine was added a black stain appeared on the deepest section of the pool. I would like to convert the pool to salt water but have been advised this will lead to more black staining. Is there a way to treat the water and remove / avoid this reaction and fit a salt water chlorinator. Any advice appreciated.

[Teapot1]

It depends on the concentration of copper and silver. Although you probably test for copper to adjust the ioniser to keep the level around 0.8ppm there is a residual copper level that just builds and builds, I only discovered this when I got my new tester that can test, subject to the reagent for just about anything you can dissolve in water. Over the years of using my ioniser and keeping the level on spec at 0.8ppm the residual copper was more than 4.5ppm. That is a risk factor especially in a plaster type finished pool. So how high is your level?

Whenever the pH reaches 8.1 or above which the bulk water might not but the plaster pebbletec pool wall could easily do the copper/silver drops out of solution and can stain as you have found.

See if you can get the residual inactive copper level tested, I used my Palintest for this. That would determine if you should drain and refill before switching to a chlorinator, chlorinators frequently raise the pH due to gas bubbles produced in the process of making (converting) chlorine. Lowering your TA can help with slowing down the pH rise.

[Neil]

Thanks for your advice. I had to buy another Cu test kit, this was strips and the maximum Cu content on the strip is 2 ppm This was indicated on the test which I repeated a few times so the Cu level is at least 2 and maybe higher. Do you know if there is a procedure, process or guide written down anywhere for conversion or should I engage a professional? Did not really want to spend a lot of money.

[Teapot1]

Ok so if you need to test for possible higher levels, try this. Take a measured sample of pool water and add the same amount of tap water. Mix well and re test, double the answer, so if you test this diluted mix and again get a 2ppm you would actually have 4ppm.
I do not know of any guide but you would need to get the copper level down to 1ppm or below to be safe from issues like staining. Because like me you had a strong copper level staining becomes a very real issue. Once you have lowered the copper level you can go on to install the salt chlorinator.