CYA tested low

[ant 83]

Tested with the t-100 test kit. The black dot was clear with the tube filled all the way.

We were having people over that night, so in the morning, I opened up the cover, tested FC, brought it up to the high twenties, then opened up the air valve to let it aerate, and get some sunlight during its two hour filter cycle.

When I came back and tested FC three hours later, it registered 3-FC.

That didn't seem right, so after some testing, I discovered that my Menards bleach had degraded down from 12 to something like 8, and the aforementioned CYA level.

I use the dichlor-then bleach method in my tub, and it had been about 2 months since the last water change. I had been adding dichlor weekly, about 5 ppm FC to keep levels up, but this apparently wasn't enough. Our tub sees heavy use, and is shocked up into the 20's FC weekly, at least.

I've read the CYA degradation thread, and although a lot of it is too technical for me, I've concluded that my CYA is being oxidized by chlorine.

After the low CYA test, I measured out one third cup of dichlor (around 30ppm CYA/FC) and added it in. Since then (about 3 weeks now), the chlorine demand has been much much lower. Half perhaps.

Also, PH rise has behaved differently. It just stays between 7, and 7.2. I haven't tested alk at all, although I did add 2 tablespoons of baking soda last week. Previously, PH always was on the rise, requiring about 1 TBSP dry acid around every 10 days or so, to keep it out of the red. (7.8-8)

 

[Texas Splash]

It sounds like you've adapted and are managing your water quite well now. Did you have a specific question from here?

 

[ant 83]

So I thought I'd give an update after 10 weeks.

I've been using the chlorine powder as an acid source. If the PH is above 7.5, I use the powder, under 7.5, I use bleach.


CYA was at 60 ten weeks ago, and it's still at 60 as of yesterday.

I also tested alkalinity (with speedstir and damp cloth) of both tap and tub water.

Tap was 50.
Tub was 70.

The CYA chart indicates minimum FC 5, target FC 7, and shock 24.

Perhaps I should change my formula to 50/50 dry acid/dichloro to maintain a lower CYA level. What do ya'll think?

 

[pooldv]

That is fine to manage your tub that way. This is exactly why we advocate a good test kit and an understanding of water chemistry. With the right tools and knowledge there are many options.

It is interesting that TA in the tub is higher than the tap. That is backwards, usually acid additions will lower the TA over tim.. But, the TA in the tap water here goes down as it rains more and up during droughts. So, maybe your tap water had higher TA during the summer. Lowering TA to 50 will reduce pH rise. Adding 50 ppm borates will also help.

CYA will typically degrade around 5 ppm per month in a hot tub. Sounds like your current water chemistry has conveniently settled in where you can lower PH and raise FC and CYA all at the same time. Nice job managing your tub.

Be sure to chlorinate the tub based on the [FC/CYA][/FC/CYA], keep it above minimum. Keeping FC too low will allow the opportunity for bacteria to take hold quickly if FC dips. It is easy to lose control of hot tub in just a day or two if FC drops because bacteria colonies can double in size in less than hour in the hot water. And once they start doubling they will very quickly consume the remaining chlorine.

I assume you have seen this article, but just in case, How do I use Chlorine in my Spa (or pool)?

 

[ant 83]

It is interesting that TA in the tub is higher than the tap. That is backwards, usually acid additions will lower the TA over time

My guess is that TA goes up from tap water being distilled from evaporation. Also, bleach has lye in it. Wouldn't that also put upward pressure on TA?

Splash out counteracts this though, and I do tend to encourage splash out to lower sodium, as I am worried about corrosion from the salt water.

I do not know how to test for salt.

 

[pooldv]

You can get aquachek salt test strips which is what I use or a Taylor K-1766 salt test from TFTestkits.net. Acid and bleach definitely add some salt to the water and lots of tap water also has some salt. But I doubt there is any reason for concern of corrosion. It isn't saltwater like ocean water. A saltwater pool is 3500 ppm, your tub is probably between 1000 and 2000 ppm salt, tears are 9000 ppm and ocean water is 35,000 ppm salt.

 

[ant 83]

I thank you for your replying to my questions.

I looked up the tests kits you mentioned, the test strips may cost more per test, but would be more reasonable for how often I would need to test.

However, perhaps I could just track how much salt gets added instead?

Chem geek states:

For every 10 ppm FC added by ANY source of chlorine, it will result in 8.2 ppm salt from the consumed chlorine that turns into chloride salt. With bleach, chlorinating liquid, and lithium hypochlorite, there is an additional 8.2 ppm salt upon addition so the net total result is 16.5 ppm.

Using pool math, it is easy to calculate. For example, in my 340 gallon tub, one gallon of 6% bleach will raise salt by 300 points, or there abouts.

When researching SWG's for use in spa's on another website, a spa repairman said that he'd seen a lot of heater corrosion failures from the salt water. I can't help but wonder if these failures were because salt got too high.

[(edit) Perhaps these people didn't have any/enough CYA, so the chlorine corroded their heater.]

How high is too high?

Also, I read that not all spa heaters are created equally. Perhaps some are just not designed for relatively higher salt levels.

If I were to strictly follow the instructions that came with my spa (no LC, FC no higher than 5, change water more often) salt would remain a good deal lower than it does.

 

[pooldv]

Low pH is the most corrosive component of water to metals. PH below 7.2 will corrode copper and dissolve it into the water. But, if your heater is cupro-nickel or titanium even that won't corrode it. You also don't know what the salt level is in your tap water. I believe the highest reading I've seen is 1200 ppm. Dumping your water and refilling 2 or 3 times a year will also make it a non-issue. I wouldn't worry about it at all. Wait, I don't.

 

[ant 83]

our water comes from lake Winnebago. That is a very large shallow lake. Originally formed by a glacier, but is fed by rivers from rainfall.

I don't know about its salt content.

I do keep a close eye on PH in my spa. It likes to slowly rise over time.

It definitely stays above 7.2 with my current methods, so that's good I guess.