Missing CYA?

[TreeFiter]

I've noticed in a few of my pools that after adding CYA, or shocking with trichlor, a week later, the CYA still tests low. One of my coworkers (the only other one paying any attention to CYA) has also noticed the same thing. In one case it was a brand new pool in which we added 12 lb of CYA. I believe the pool was an 18x36 rectangle, so roughly 25,000 gallons. This should have brought the CYA up to about 60 ppm. When tested, it was about 25. It was tested with the tf-100, and about a week had passed since adding it in this case.

I don't know the specifics of some of the other pools, but I know that I have recently added significant amounts of CYA to pools, but when I tested them a week or two later, they were still pretty low. I believe I added 8 lb to one pool, and found it was only at 25 ppm today.

We have been getting a fair amount of rain, but the numbers I'm seeing suggest pretty significant dilution, similar to replacing 25-50% of the pool's water.

Any idea where the CYA is going?

 

[chem geek]

Could be bad CYA. We've seen CYA that was "cut" from some products imported from China. You could test some product in a large bucket of water and verify whether or not its pure CYA or not.

 

[TreeFiter]

How would I tell if it was pure?

The chemicals we use are Nu-Clo by Alden Leeds. In a few of the situations, some of the CYA expected to be in the pools came from trichlor shock, where a 4lb shock would add somewhere around 10 ppm CYA.

 

[chem geek]

1 gram of CYA in 20 liters would be 50 ppm CYA so if you crushed some CYA in a mortar and pestle and weighed it and dissolved it in a tub of water you could verify that it was pure. According to this MSDS the product is supposed to be 90-100% CYA.

How are you adding the CYA to these pools? If you just add it to the skimmer, then it gets caught in the filter and if backwashed too soon it gets discarded before dissolving.

I add CYA in a T-shirt in the skimmer and if I measure it after it is dissolved which is usually the next day it measures nearly the right amount. The long delay only occurs if it gets put into the skimmer directly to be caught in the filter and it dissolves most slowly in oversized cartridge filters since the water velocity through the cartridge material is slowest in such filters.