CYA missing

[novca]

Hello,

I'm a new pool/spa owner in Novato, CA. The pool/spa are separately plumbed and do not share water. I have clear water in both and have no trouble maintaining it that way thanks to the wonderful advice here.

At the end of June a drained and refilled the spa. It's 600 gallons (I measured during the filling). I treated the water in accordance with the advice here. 34 ppm dichlor (to give 30 ppm CYA), 50 ppm borates (Gentle Spa). After the 34 ppm dichlor I switched to bleach. Since then I've been using only bleach, liquid acid, and 1oz of MPS every 10 days or so. This spa has a low bather load and is not heated when we're not using it. Unheated water is 80F, heated water is 104F. The spa stays covered when not in use and gets about 7 hours of direct sunlight per day. I periodically add make-up water for evaporation.

Anyway... Two months later it seems that the chlorine usage is a little high even when we're NOT using the spa (about 2ppm per day). So I finally broke down and bought a good test kit (up until now I just haven't needed one because I don't have any water problems). The CYA test showed less than 30 ppm (that's as low as my Taylor test kit goes). Hmm. How can this be? I've read here that evaporation doesn't affect CYA. I haven't removed any water...

So for the next two chlorine applications I used dichlor instead of bleach. Another 10 ppm dichlor should have resulted in an increase of 9 ppm CYA. Test again. CYA still less than 30.

If evaporation doesn't affect CYA, how can a total of 44 ppm dichlor result in less than 30 ppm CYA?

I have no problems with water quality, but I think I could lower the chlorine consumption by getting the CYA up to 30. I just can't figure out why it isn't there already...

FC 5
pH 7.6
TA 100
CH 130
CYA <30

Thanks!

 

[duraleigh]

How much dichlor (in weight) did you put in the spa?

 

[chem geek]

There is a slow oxidation of Cyanuric Acid (CYA) by chlorine and this probably happens faster at higher spa temperatures so it's possible for the CYA level to have dropped somewhat after a few months in a hot spa. What I've told people before is that they can retest after a couple of months and if it's low they can use Dichlor again briefly and then switch back to bleach. It sounds like you do get a cloudy result in your CYA test but not cloudy enough to obscure the dot at 30 ppm -- it's probably around 20 ppm or so -- try adding 10 ppm FC of Dichlor. It's not a big deal -- you don't have to be exact about this.

 

[novca]

How much dichlor (in weight) did you put in the spa?

Initially I put in a total of 4.6 oz by volume (obviously not all at one time) in accordance with the pool calculator to reach 34 ppm. Later on I added another 1.4 oz by volume for an additional 10 ppm.

It sounds like you do get a cloudy result in your CYA test but not cloudy enough to obscure the dot at 30 ppm -- it's probably around 20 ppm or so -- try adding 10 ppm FC of Dichlor. It's not a big deal -- you don't have to be exact about this.

Yes, exactly right. It gets a little cloudy but doesn't obscure the dot. The pool water obscures the dot at 55 ppm, so I'm pretty sure I'm doing the test correctly.

While we're here... I don't have a scale that measures weight in ounces. Do you happen to know the weight of a 1" and a 3" TRIchlor tablet? I have to use these every once in a while when I'm out of town for more than a few days, and I want to know how much chlorine I'm actually putting in. The "instructions" on the side of the trichlor bucket are generic and leave much to be desired if you're looking for useful information.

Thanks again!

 

[frustratedpoolmom]

I think the 3" are either 7 or 8 ounces each.

 

[chem geek]

The 3" tabs/pucks are 6, 7 or 8 ounces depending on manufacturer (the most common is 8 ounces). The 1" tablets are typically 3 ounces while the 1/2" tablets are typically 1 ounce.