Cya

[hd714]

First off, I would like to thank everyone on this board for the great information here. This board has made my first year of spa ownership a joy.

Now my question.....

I have added 15 teaspoons of dichlor over the course of a two weeks (fresh water). This should be 31ppm of chlorine. And should also be a CYA of 27 or 28.

But my tests show a CYA of 20 with the TFT100 kit, and 22 with the ColorQ Pro7.

Tests are conducted with a water temp of 70 deg. F

Any idea why the discrepancy?

Thanks, HD714

[chem geek]

Some of the CYA may bind to some spa surfaces so that the bulk water shows a somewhat lower reading, but mostly it's probably just test error since the CYA tests are only within around 10 ppm. It could also be some variation in the density of the Dichlor -- I assumed that the density was roughly that 1 ounce volume was 1 ounce weight so if your Dichlor measurements were less dense than that, the resulting chlorine and CYA levels would be somewhat lower.

For the CYA turbidity test, you should view the tube outside with your back to the sun so that you shade the tube looking down in front of you. If you view the tube in less intense indirect light, you will report a lower reading.

[hd714]

Thanks for the reply, Chem Geek.

My chlorine level responded as expected, so I don't think it is a density issue.

Is all dichlor created equal? Can age affect cya and not Cl? Am I over analyzing this?

Should I just go with the calculated level and forget the test results?

Thanks, HD714

[chem geek]

Dichlor is a chemical that has both chlorine and CYA in a single compound as shown here. CYA is shown here so you can see how the CYA is a core ring and Dichlor simply substitutes two of the hydrogen attached to nitrogen with chlorine instead (and the other hydrogen is replaced with sodium).

So the CYA measurement is either off a bit or the CYA in the bulk water isn't evenly distributed -- some attaching to surfaces as I had mentioend. Theoretically having 27-28 but measuring 20-22 isn't a big deal, though a bit more than I would have expected. As for which number to use, again it's not a huge deal since anything in the 20-40 ppm CYA range is going to be good enough and even that's conservative. We didn't start seeing problems with Dichlor-only users on this forum until roughly after the first month which was usually around 100 ppm CYA.

[hd714]

Thanks for the links, good info.

I will go with the calculated level then, and just enjoy the tub.

I appreciate you taking the time to answer my questions, Chem Geek.

HD714

[quantumchromodynamics]

I think that your numbers are within a reasonable level of precision based on all of the ways inaccuracies can add up.

Here are some of the measurements that are not exactly known and can introduce errors that can add up.

1) Water volume of the spa.

2) Total amount of dichlor added.

3) Test accuracy.

4) Water loss and refill.

The exact level of cyanuric acid is not too critical. I think that anywhere in the 20 to 40 range will work just fine.

[chem geek]

Just remember that we found that the CYA will tend to drop by around 5 ppm per month (at usual FC and CYA levels and hot spa temps) so you'll want to have a make-up day using Dichlor once a month if you want to maintain the CYA level. Not a huge deal over a month or two, but if you don't change the water for many months, then the drop in CYA towards zero will have the chlorine be stronger than you'd normally want. So keeping the CYA up has more consistent oxidation/sanitation for as long as you want -- usually 6 months unless your bather load is high (this assumes Dichlor-then-bleach method).

[NewB@Spa]

First off, I would like to thank everyone on this board for the great information here. This board has made my first year of spa ownership a joy.

Now my question.....

I have added 15 teaspoons of dichlor over the course of a two weeks (fresh water). This should be 31ppm of chlorine. And should also be a CYA of 27 or 28.

But my tests show a CYA of 20 with the TFT100 kit, and 22 with the ColorQ Pro7.

Tests are conducted with a water temp of 70 deg. F

Any idea why the discrepancy?

Thanks, HD714

Was this 15 teaspoons counting the initial shock up to 10ppm? I want to make sure I am doing this right. I performed a new fill yesterday and added ~1oz of Dichlor to bring tub to initial 10ppm. Will test again today to determine CD and next add amount. I assume I should be targeting ~4-5ppm? And add everyday to keep it that level until I add ~34ppm of Dichlor?

[hd714]

quantumchromodynamics - Good points. I will aim for a CYA of 35.

chem geek - I had no idea CYA broke down over time. I thought that once it was in, it was in for good.

Thanks.

NewB@Spa - The 15 teaspoons was used in the following manner: 6 tsp for initial shock. 3 tsp to

maintain a FC of 2ppm. The last 6 tsp were used as another shock. I use Nature2

and MSP, so my chlorine levels are lower than most. Hope this helps.

HD714

[chem geek]

chem geek - I had no idea CYA broke down over time. I thought that once it was in, it was in for good.

This is something that though theoretically known to occur very slowly in pools, it is temperature dependent so I figured it might be more noticeable in spas. Some careful tracking of CYA on this forum by several members got us to the 5 ppm per month loss rate as described in this thread.

[NewB@Spa]

quantumchromodynamics - Good points. I will aim for a CYA of 35.

chem geek - I had no idea CYA broke down over time. I thought that once it was in, it was in for good.

Thanks.

NewB@Spa - The 15 teaspoons was used in the following manner: 6 tsp for initial shock. 3 tsp to

maintain a FC of 2ppm. The last 6 tsp were used as another shock. I use Nature2

and MSP, so my chlorine levels are lower than most. Hope this helps.

HD714

Thanks because I just did my first refill (my first fill did not go well...) and I shocked to 10ppm. Just calculated CD and it is about 20% (no use) and I am not sure what, if any, chlorine I should put in today