CYA measurement units

I’ve just purchased a Scuba II photometer, and was alarmed to find it says the CYA level in my pool is 130mg/l. To check the meter I carefully made up a test solution of 80mg dry CYA/ l. This measures (consistently) as 150 mg/l on the scuba II! Is the meter bad or are CYA levels conventionally defined in some other units (eg chlorine is as gas, alkalinity is as CaCO3 units etc, none of which you’d know from the documentation of the test kits).

Hello Glenn and welcome to the forum. I’m sure someone will click-in with some info soon.


Apologies, I should have introduced myself. We are in the Vendee and have a Christal double bubble pool, put in 15 years ago. Still pleased with it, although we’re thinking of replacing the original filtration system with a conventional post pump filter.


Sounds like the meter is bad.
PPM is close to mg/l which is how we normally express it. Chlorine is really being measured as hypochlorite/hypochlorous acid. Alkalinity is all forms but predominantly bicarbonate.
CaCO3 is the measure of hardness although some magnesium could also be present.
I stopped being a fan of the Scuba range mainly because of internal O ring leaks, I prefer testers you don’t dunk.

Thanks, I certainly hope it’s the meter. With the water restrictions in place there’s not a lot I can do about it now though either way.
I didn’t like the idea dunking the Scuba either, so I bring a jug of water in from the pool to test in the kitchen, out of the sun. It seems to produce sensible numbers for chlorine and pH anyway. Can you recommend something better for CYA?

As you have the scuba and it’s good for somethings then a “vanishing dot” CYA tester would do. I doubt you’ll find one in France though. I can help with that if you are stuck.
There is an enzyme treatment that can have some success for lowering CYA but you’ll need zero chlorine for it to work or a dechlorinator like sodium thiosulphate. There are mixed results but people may not have removed the chlorine before trying.