Dichlor-Bleach Cya Drift Question

[spidey9]

About 3 months ago I purchased a Jacuzzi J-345. Usage is light but frequent: I soak for 20 min every evening, and my wife joins me about half the time. Based on that and what I've read in this forum, I decided to switch from the Silk Balance system that came with the spa to dichlor-bleach when I changed the water three weeks ago.

After the first week CYA was still undetectable (Taylor K-2006) so I continued to use dichlor. Adding a teaspoon after I soaked alone, and 1.5 teaspoons when my wife joined me kept FC at 3-5 ppm and CC near zero. After a second week of dichlor, CYA had increased to around 33 ppm, so I switched to bleach. After a little experimentation I've found that adding 2 oz after I soak alone and 3 oz after we both soak (6% bleach) keeps FC at 3-5 ppm

I check FC, CC and pH at least once a day, and do a complete set of tests every Saturday. Today everything looked great except that CYA was back to under 20 ppm. The test sample was only slightly cloudy, and I could still make out the black dot with the cylinder completely full. I repeated the test to make sure, with the same result.

Here are complete readings (Taylor K-2006):

pH: 7.6

FC: 3.0

CC: none detected

TA: 70

CH: 170

CYA: less than 20 ppm

Borate: 50 ppm (test strip)

Water temp: 103 F

The J-345 holds 340 gal

pH has been rock steady at 7.6 the entire 3 weeks.

Should I go back to using dichlor for a week to bring CYA back up?

Is it normal for CYA to drift like this?

[chem geek]

The CYA gets slowly oxidized by chlorine at roughly the rate of 5 ppm per month so using Dichlor for a short time once a month will bring the level back up. So what you are seeing is normal and easily handled as you describe, though you probably won't need to use Dichlor for a full week to get 10 ppm. Just remember that for every 10 ppm FC you add with Dichlor, it adds 9 ppm CYA.

[spidey9]

The CYA gets slowly oxidized by chlorine at roughly the rate of 5 ppm per month so using Dichlor for a short time once a month will bring the level back up. So what you are seeing is normal and easily handled as you describe, though you probably won't need to use Dichlor for a full week to get 10 ppm. Just remember that for every 10 ppm FC you add with Dichlor, it adds 9 ppm CYA.

Thanks for the quick reply. I'll go back to Dichlor for a few days . . . and order some more CYA reagent.

[spidey9]

Just remember that for every 10 ppm FC you add with Dichlor, it adds 9 ppm CYA.

I read elsewhere in this forum that a teaspoon of Dichlor will raise FC 6 ppm in 100 gal of water. Playing around with the numbers, it looks like it will take about 5-6 teaspoons of Dichlor to increase my CYA 10 ppm in my 340 gal tub. Does that sound about right? Thanks again for all your help.

[chem geek]

One teaspoon of Dichlor raises the FC by around 7 ppm in 100 gallons so for 340 gallons it would take about 5-1/2 teaspoons to raise the CYA by 10 ppm. You don't have to do it all at once, of course. You can just use Dichlor for your regular chlorination after your soak until you reach your goal, at least approximately.

[spidey9]

You don't have to do it all at once, of course.

Of course.

This will, however, give me an idea of when I should test CYA again.

[spidey9]

Update: It's been a week and I have added 6 teaspoons of Dichlor over the course of the week. When I tested for CYA today, it was still below the detectable level. While the sample had some cloudiness, I could still definitely make out the black dot with the cylinder filled to the top. I have performed the test on some 50 ppm standard solution successfully, so I think I am doing it right. The Dichlor I am using is SpaGuard Chlorinating Concentrate Sanitizer, Sodium dichloro-s-triazinetrione dihydrade 99.0%.

This is baffling. I'm considering getting some CYA from the local pool supply, although it comes in a 5 lb container that should last roughly 100 years.

Any suggestions?

[chem geek]

Well 6 teaspoons would only be around 10 ppm and that might not show up. If you were at 10 or near 0, it might not obscure the dot at the top (with the K-2006 the top is 30 ppm). I'd just keep using the Dichlor for another week at least. Perhaps for some reason the CYA dropped faster in your spa so you were lower than you think.

[spidey9]

Okay, I'll give that a try. After all, it took two weeks of Dichlor before I started seeing the CYA the first time. Although I kind of like the idea of getting some CYA and doing away with the Dichlor entirely.

As always, thanks for all your help.

[waterbear]

You can use CYA but it is very slow dissolving compared to dichlor so your level is not going to rise very fast. This also makes it a bit more problematic to add to your water.YOuwill most likely have undissolved CYA sitting in the bottom of the tub for a day or so.. If you do use it don't even test it for a few days after. I would stick with the dichlor since it does dissolve almost instantly and you need to add chlorine anyway.

This is why it's called the DICHLOR/bleach method and not the CYA/bleach method!

[spidey9]

Being the impatient type, I added some CYA to the tub a few days ago. Fortunately, I had seen another post (probably also from waterbear or chem geek) mentioning the slow dissolving issue, so I proceeded with caution. It has now been a couple of days since I added the CYA (just a couple of teaspoons), my CYA is at an acceptable 35 ppm, I'm sanitizing with bleach, the water looks great, and Life is Good.

When it comes time to drain/refill, would it work if I measured out an appropriate amount of CYA a few days before and added it to a gallon or so of water, which I could then add to the tub as part of my initial balancing (my tap water has a pH of 8.0)?

[chem geek]

It's fine to add pure CYA to the spa. Just keep in mind that it will take a while to dissolve, but it dissolves more quickly in the hot spa water than in cooler pool water. As you experienced, it dissolved within 2 days and I suspect it probably dissolves within a day at hot spa temps. If you wanted to dissolve CYA ahead of time, that's OK, but keep in mind that it could saturate the water so not fully dissolve. At 77ºF the solubility of 0.27 g/100 ml implies 2700 mg/L (or ppm) though in buffered water it'll go higher than that (in distilled water the pH drops that inhibits further dissolving). 30 ppm CYA in 350 gallons is 5250 ppm CYA in 2 gallons so might not all dissolve in that small a volume, but you can let us know if you try this out.

[Spanky]

I've added CYA to my pool by using a small fine mesh bag tied to the return (sort of like a "Tea Bag") and it was completely dissolved within 2 days. If I ever decide to switch my Hot Tub from 3 step bromine I'd certainly use the same technique by placing the "Tea Bag" in the filter box.