CYA from dichlor not all showing up?

[seilsel]

This is for chem_geek or anyone who wants to chime in.

Coming into an unusually warm Spring after an unusually cold and wet Winter in GA, I've had a lot of SWG customers come in saying, "Yeah, the pool is a swamp."

Well, their salt level is 1600 ppm and their CYA is indistinguishable from 0ppm using the Taylor test... So of course now that the temperatures have skyrocketed out of the blue their pool is green.

So I work out the shock level, dose our sodium dichlor product, give them instructions, and clear up the pool. But when they come back in, I'm not seeing anywhere NEAR a +9 ppm per +10 ppm chlorine increase.

The only reason to recommend the dichlor product, which is ironically more expensive than the cal-hypo product when considering available chlorine and the fact that we pay less for it, is the value inherent in the free CYA. But I've been in dozens of situations (avg of 43 customers per day in the last month) where I should have gotten a CYA increase of 50+ and instead I'm seeing an increase of 10-20 (difficult to tell with the test).

I've called Taylor to ensure that I'm doing the test correctly and I've swapped out reagents to make sure they're fresh. (By the way, you can buy a gallon of Taylor CYA reagent from Leslie's for $24.99).

What am I not accounting for here?

 

[frustratedpoolmom]

I'm not seeing anywhere NEAR a +9 ppm per +10 ppm chlorine increase.

What are you seeing?

 

[seilsel]

But I've been in dozens of situations (avg of 43 customers per day in the last month) where I should have gotten a CYA increase of 50+ and instead I'm seeing an increase of 10-20 (difficult to tell with the test).

Situations just like this. I don't doubt the math. I've spent some time learning and understanding the chemistry and the math. There just has to be something I'm missing.

 

[frustratedpoolmom]

So you aren't seeing the increase in CYA you would expect.
When you mentioned the FC, are they coming in with 0 FC, even after shocking? I'm wondering if the FC isn't killing everything, if the "mysterious CYA eating bacteria" is present and is able to consume/convert the CYA from the dichlor as it's being added....

 

[seilsel]

That's a good theory, FPM.

It's quite common that someone whose last test shows the 60-80ppm CYA level that I recommend comes in the next spring with 40% salt loss (we had a LOT of rain and drain this year) and 100% CYA loss.

The FC levels track reliably but the CYA levels do not. So this is a pretty good idea.

Thanks!

Edit: However, almost no one here covers their pool.

 

[chem geek]

If there is no chlorine in the pool and if the CYA has mysteriously disappeared, then there could indeed have been a bacterial conversion of CYA into ammonia. This will create a huge chlorine demand and if you did add CYA by any means, the bacteria could convert it to ammonia. So in this sort of situation, you want to start off using chlorinating liquid or bleach (or Cal-Hypo if you need the Calcium Hardness raised) to oxidize all of the ammonia and to start measuring a residual Free Chlorine (FC) that will kill off the bacteria and clear the pool of algae. Once that is done, then you can add Dichlor for more chlorine and for CYA.

Note that it is not necessary for the pool to be covered to have this bacterial conversion take place. The bacteria are anaerobic bacteria from the soil, but they don't really care if there is sunlight. If the pool water isn't circulating, the pool water can become poor in oxygen (though usually algae growth would put oxygen back into the pool as they are, after all, a plant).

I know I've added Dichlor in my own pool and achieved the expected CYA increase.

 

[zea3]

The next time one of those samples comes in run a test to see if they have ammonia in the water

 

[bk406]

I opened this year to almost no CYA down from 50-60 when i closed. I had zero CC and litle to no increased FC demand. Go figure :?:

 

[JasonLion]

The current working theory is that bacteria convert CYA into ammonia. If the ammonia is still in the water you will have a significant excess chlorine demand on opening. However, the ammonia can evaporate, given enough time with the pool not covered. So you won't always see the excess chlorine demand when CYA has gone down.

 

[seilsel]

I'll have to make sure that my ammonia test is still good. I buy those out-of-pocket.

Thanks all!