Vanishing FC

[EndlessScott]

Hi all!
Just starting up my (wife's) new 3800gal vinyl liner Endless pool.
Initial chemistry seemed pretty easy following the advice here: pH started 7.8 down to 7.5 with a few oz of dry acid that came with the pool. TA is sitting at 150 nicely, CH is 150, and water is clear and feels good.

I can't seem to keep FC in the pool, though. I add 16oz of 5.25% Clorox, which should give me 2ppm, and a few hours later test none there!

I checked for total Chlorine, and still none. No chlorine odor either, not likely CC/organics using it up.
I though maybe my reagents were old in my test kit, but I can get measurements with a dilution of the bleach solution, so I don't think so. It's only a Taylor DPD kit, I have the FAS-DPD kit ordered, but we are stuck until it gets here.

Then I realized I was likely burning off the FC in the UV sanitizer. Is this a reasonable guess?

I probably need 20-30ppm of CYA if she wants to continue to use the UV, right?

Any experienced suggestions?

Thanks,
Scott

 

[JVTrain]

Without CYA, FC will rapidly be broken down by sunlight. The UV lamp is also contributing to this as well. I'd recommend getting that CYA up first. You could do this in several ways. Purchase pure CYA, use Pool Math to determine how much and get it dissolving in a sock placed in front of your return or skimmer. 16 ounces looks like it would get you to 32 ppm CYA. Then maintain 2-4 ppm FC during the process of dissolving the CYA.

You could also use dichlor to bring the FC and CYA up at the same time. In your pool, each 4 ounces of dichlor would bring your FC up by 4.4 ppm and your CYA up 4 ppm. If you did this process 5 times, not all at once because that would raise your FC too high, then you'd have 20 ppm CYA while maintaining chlorination in the process. If you did it 7-8 times, your CYA would be at 30 ppm. This would be your "dichlor start up" method. This is typically done with spas and small pools.

 

[EndlessScott]

That's what I was thinking.
They did send me a 1lb bag of "multi-clear shock" dichloro-s-triazinetrione with no instruction about when to use it.
The only instruction is to add a half cup of bleach each day after swimming. Not real helpful.

Is the dichloro that I have the same dichlor that you discuss that will produce CYA? No wonder people get confused about what to use for chlorination if they don't mention CYA anywhere on the package! Add the general misunderstanding people seem to have about pool chemistry and things really get messed up!

Scott

 

[zea3]

Anything labeled dichlor or trichlor contains CYA.

 

[chem geek]

For every 10 ppm Free Chlorine (FC) added by Dichlor, it also increases Cyanuric Acid (CYA) by 9 ppm.

 

[JVTrain]

That's what I was thinking.
They did send me a 1lb bag of "multi-clear shock" dichloro-s-triazinetrione with no instruction about when to use it.
The only instruction is to add a half cup of bleach each day after swimming. Not real helpful.

Is the dichloro that I have the same dichlor that you discuss that will produce CYA? No wonder people get confused about what to use for chlorination if they don't mention CYA anywhere on the package! Add the general misunderstanding people seem to have about pool chemistry and things really get messed up!

Scott

Very true. Some pool store employees don't even know that dichlor and trichlor add CYA to pools. The Pool Math link at the top of the site is very useful and I was simply using its "Effects of adding chemicals" section at the bottom of the page. Put your pool volume in the top of the page and then using that "Effects" drop down, changing the chemical and amount added can be a very good way to see what each chemical adds to your pool and how it affects chemistry. All of this is covered in Pool School as well in the ABCs of Pool Water Chemistry and Pool School - Recommended Pool Chemicals

 

[EndlessScott]

Update:
Now after a few weeks of using dichlor to chlorinate and add CYA, my FC readings have been much more stable. I read a consistent 2-4ppm FC which drops roughly 1ppm daily, with none to 0.5ppm CC. Victory!!

In addition, as predicted by the group, the minimal chlorine smell has dissipated and the water seems to be less drying to skin, likely owing to the very low but stable buffered active chlorine levels, estimated by chem geek to be equivalent to 0.2ppm. Water is clear and sparkly, and the wife is happily swimming away.

I am still not quite up to 30ppm with CYA, should be about 22-24 given the dichlor used so far, but it's tough to measure so low a concentration in the Taylor kit I have. I will stop at about 30ppm expected CYA, and leave the dichlor on the shelf for future drain/fill operations.

The well considered and highly articulate help you all provide is balm for the confusion wrought by pool industry marketers and owners without a basic understanding of chemistry.
Thank you so very much.
Scott

 

[chem geek]

Note that what you were experiencing without any CYA in the water is what happens in many commercial/public pools especially indoors where they usually don't use CYA. They are essentially over-chlorinating with more chlorine smell and faster oxidation of swimsuits, skin, and hair, and also faster production of disinfection by-products.