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Why not add cyanuric acid + bleach at tub refill?


EugeneJ
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Hello there!

New tub owner (about two months now), and goodness has this forum been illuminating. I've learned a ton from reading all the great thoughtful content here.  THANK YOU!!!

We purchased a new HotSpring Grandee with the Freshwater Salt System (I'm not sure if I'm going to hang onto the salt system, but that's for a future post; TLDR, seems expensive over the long term?). I think maybe I had from-factory-biofilm, because it didn't seem like the salt system was keeping up.  Sure, I was adding dichlor (no bleach) on every use, but FC would just be so low.  After several weeks, I got myself a Taylor K-2006, and after another week I measured the amount of this mysterious chemical cyanuric acid, and wow, I was probably 120 ppm or so (yes, off the charts). Hrm, I better understand what this stuff is.

Fast forward tons of reading later (mostly this forum, thank you!!), plus a half drain/refill and then later an ahhsome purge (gross green stuff!) and full drain/refill, and I think have a decent (novice) sense of hot tub water chemistry (bonus: salt system now appears to actually be generating a decent amount of FC, so seems like the ahhsome purge worked?).

My question is (and I'm 95% sure I saw the answer on this forum somewhere, but for the life of me I cannot find it now):

A major recommendation on this forum is to add dichlor until your cyanuric acid hits 30-40 ppm; originally people were recommending 20 ppm); then switch to bleach.  But then I have to either a) measure the cyanuric acid regularly, or b) calculate how much I should expect to add based upon volume.  Why not just add one single dose (or maybe one almost-enough dose followed by a measurement, followed by a few tiny adjustment doses) of cyanuric acid followed by enough bleach to adjust the FC to the proper ppm? This seems like it might be easier? Or is there something I'm missing?

The Taylor kit reagent R-0013 for the cyanuric test comes in two 22mL bottles, and each test consumes 7mL, so that's basically 6 tests of cyanuric acid before needing to order more.  Not a lot of tests to work with if you're trying to dial it in.

Am I doing it wrong?  (I wouldn't be surprised if I was.)

Thanks again for all your help (past and possibly future)!

PS - I started keeping a hot tub lab notebook starting at my ahhsome purge, in case weights are needed.

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Well, that's exactly what you do in a pool.

A better question is: If you don't have UV exposure in your spa with the cover on, and cya is to stabilize vs UV dissipation, why have it at all? Oh, yeah, they say it buffers the chlorine so it isn't harsh. Still not sure I understand that, but ok.

But I do know that having cya in an indoor public pool or spa will get your pool closed by the health department in many states. Again, not sure why. 

Maybe @waterbearcan shed some light on the subject.

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On 12/11/2020 at 7:19 PM, EugeneJ said:

A major recommendation on this forum is to add dichlor until your cyanuric acid hits 30-40 ppm; originally people were recommending 20 ppm); then switch to bleach.  But then I have to either a) measure the cyanuric acid regularly, or b) calculate how much I should expect to add based upon volume.  Why not just add one single dose (or maybe one almost-enough dose followed by a measurement, followed by a few tiny adjustment doses) of cyanuric acid followed by enough bleach to adjust the FC to the proper ppm? This seems like it might be easier? Or is there something I'm missing?

Very simply because cyanuric acid takes several days to dissolve and show up in the water while dichor dissolves instantly and adds the cynauric acid instantly. Dichlir will add 10 ppm cyanruic acid for every 11 ppm chlorine you have added cumulatively. This makes it easy to get the CYA at the proper level with minimal testing. If you initlaly shock to 10 ppm with dichlor and then, as you use the spa you keep track of how many ppm chlorine you are adding (for example, after using the spa you test the water and find it at 2 ppm and you add enough dichlor to bring it to 5 ppm you would now have added  13 ppm of chlorine. When you hit about 33 ppm test your CYA and you should find it's pretty much at 30 ppm.  Adding CYA and then using bleach is really harder.

On 12/11/2020 at 7:19 PM, EugeneJ said:

The Taylor kit reagent R-0013 for the cyanuric test comes in two 22mL bottles, and each test consumes 7mL, so that's basically 6 tests of cyanuric acid before needing to order more.  Not a lot of tests to work with if you're trying to dial it in.

If you follow my instructions above you only need to test once or twice. When you need to buy more reagents either buy the 2 oz bottle (R-0013-C) or the 16oz (R-0013-E) bottle (most economical) and refill the little bottles in your kit. The droppers just snap out and you will $ave a lot of money this way! If you keep it tightly capped the bigger bottles have a long shelf life!

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10 hours ago, RDspaguy said:

A better question is: If you don't have UV exposure in your spa with the cover on, and cya is to stabilize vs UV dissipation, why have it at all? Oh, yeah, they say it buffers the chlorine so it isn't harsh. Still not sure I understand that, but ok.

It does buffer the chlorine which means it's not as hard on swimsuits and it also means that the FC will not dissipate as quickly because you will be running a slightly higher FC level (4-6ppm at around 30 ppm CYA) as opposed to 1-3 ppm for a tub with no CYA and the extra chlorine is held "in reserve" and ready for action if needed. It's a bit more complicated than that but I won't go into such things as equilibrium points and reversible chemical equations and such.

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On 12/12/2020 at 6:11 PM, waterbear said:

Very simply because cyanuric acid takes several days to dissolve and show up in the water while dichor dissolves instantly and adds the cynauric acid instantly. Dichlir will add 10 ppm cyanruic acid for every 11 ppm chlorine you have added cumulatively. This makes it easy to get the CYA at the proper level with minimal testing. If you initlaly shock to 10 ppm with dichlor and then, as you use the spa you keep track of how many ppm chlorine you are adding (for example, after using the spa you test the water and find it at 2 ppm and you add enough dichlor to bring it to 5 ppm you would now have added  13 ppm of chlorine. When you hit about 33 ppm test your CYA and you should find it's pretty much at 30 ppm.  Adding CYA and then using bleach is really harder.

(Sorry for the delay in replying, grades were due!)

Ahhh!  So that's the reason! Ok that makes perfect sense.  And *thank you* for rule of thumb of 10 ppm CYA for every 11 ppm chlorine added; knowing that will make it *much* easier to dial in the CYA. In fact, I went to my mini lab notebook and added up all the ppm's of dichlor I'd added, and I was at 28ppm. Perfect timing!

 

On 12/12/2020 at 6:11 PM, waterbear said:

If you follow my instructions above you only need to test once or twice. When you need to buy more reagents either buy the 2 oz bottle (R-0013-C) or the 16oz (R-0013-E) bottle (most economical) and refill the little bottles in your kit. The droppers just snap out and you will $ave a lot of money this way! If you keep it tightly capped the bigger bottles have a long shelf life!

Ah, I had already ordered the 2 oz bottle, I didn't even realize there was as 16 oz bottle!  I'll grab that one next time.  Wow, just looked at the prices, and you're right, that's a huge savings!  Thanks! 

Regarding shelf life, that's great to know.  I had read Taylor's recommendation that you replace your K-2006 every year.  But you're right, it's more nuanced than that.  Now you've got me googling and finding all sorts of stuff about stretching out your reagents, and which reagents are ok to keep for longer than a year.  I had no idea!

Thanks again for all your help, this is great!!

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