MA effects on TA; Can CYA drop/evaporate?

[bbrock]

Hi You All,
I tested today (Mon.) and got these latest results:
FC 6
pH 7.5 (maybe 7.6)
CC 0
TA 70
CH 875
CYA 40

Prior to this, my CYA was at 50, as I had completed a partial drain/fill in mid June to bring it down from 100+. Additionally, the CH was very high in the 1000s, and after the parital drain/fill it was at about 700 or so. And, I lowered the TA from 180 to 80. Water temp is perhaps 85 degrees. I use a TF100 kit. FWIW, right now the chlorine stash I am using is 12.5%. When I finish the last 2.5 gal., I will hit my stash of 10% chlorine (Kem-Tek brand from OSH).

Couple of questions:

1. How can the CYA drop like this? Is it affected by the normal water replacement due to evaporation? It seems that every 1-2 weeks or so I am topping off the pool. When I tested the CYA today, I dumped the sample back in the mixing bottle so that I could try it again to see if I was accurate. After 4 times, I kept getting roughly the same results maybe varying by 2-5. But, per the instructions with the TF100, it says to use the number you are closest to, which for me would be 40. On a different note, could the CYA drop from the addition of MA?
2. Should I bother raising the CYA back to 50, or just keep it at 40? Any problems with a CYA of 40? I had read some posts that having a lower CYA may lead to the possibility of greater chlorine burn-off. Would I really see that, or any difference in a level of 40 vs. 50, besides having different goal and shock amounts?
3. I was going to add 49 oz of 14.5% MA, per Pool Math based on my results, to lower the pH from 7.5 - 7.2. I generally see my pH rise over a week up to around 7.5+. In order to have a balanced CSI, I bring down the pH to 7.2 and let it rise again over the subsequent week to 7.5 or so. The problem is that today, out of curiosity, I plugged in 49 oz MA in the "Effects of Adding Chemicals" section to see if it would have any effect on my lower TA of 70. As there was not an option for 14.5% MA, I chose the closet which was 15.7% MA. Low & behold, it showed that 49 oz of MA would lower my ph by 0.31 and lower my TA by 5. I did not add the MA yet, b/c I wanted some input from here. Do you normally add baking soda to counter the expected drop in TA by the MA addition? FYI, I have been adding MA approx. weekly to keep my pH in line and in my tight parameter of 7.2-7.5. Guess perhaps this may have been the culprit that lowered my TA from 80.

Tx for any help you can shed here.

 

[aussieta]

cya test is plus or minus 15 from memory

 

[Abigail K]

1. CYA is not affected by evaporation. It does not drop due to addition of MA either.
2. Someone else would be able to advise on recommendations for your CYA. CYA levels are a balance between chlorine burn-off due to sun, and higher FC levels for maintenance and SLAM.
3. Someone else would be able to advise you on pH levels. The combination of your CH and temperature may push CSI above 0.6 if you maintain about pH 7.6-7.8. Note that baking soda would raise pH slightly, in addition to raising TA.

 

[Smykowski]

2. You'll have to see. Yes, a small change in CYA can end up having a big change in FC burnoff. The relationship seems to be nonlinear, and once you find the CYA tipping point for your pool, your FC consumption by sunlight goes way down. You'll have to just see.
3. Yes, the acid additions are dropping your TA. As to adding the baking soda, I'd just do what's necessary to keep it balanced, and it seems that you have a good understanding of what it will take.

 

[zea3]

How much water are you having to add to top off the pool? It is possible you have a water leak. That would account for the CYA level dropping. You can check for a leak by doing the bucket test. Fill a bucket 1/2 way with pool water and set it on the top step. Use a sharpie and mark the water line inside the bucket and mark the pool water line on the outside of the bucket. Let it sit undisturbed for 24 hours and mark the levels again. If the rate of loss was greater outside the bucket than inside you have a leak.

 

[wayner]

Unless you have a leaky bucket

 

[JamesW]

Unless you have a leaky bucket

To test for a leaky bucket, you could put a smaller bucket inside the bucket to do a bucket bucket test. Lol

 

[bbrock]

Tx for the feedback you all. Anyone else have info to share?

Zea3, wouldn't the bucket itself skew things since the height of the plastic will interfere with the sun's exposure to the water inside vs. outside?

 

[zea3]

Tx for the feedback you all. Anyone else have info to share?

Zea3, wouldn't the bucket itself skew things since the height of the plastic will interfere with the sun's exposure to the water inside vs. outside?

Not really. You can set the bucket on the steps so the top of the bucket is approximately the same height as the coping on the pool, and fill the bucket so it is close to the same level inside and outside of the bucket to start the test if you want, but the rate of evaporation is going to be about the same.

 

[bbrock]

Not sure if people had seen this question, but I did not want it to get buried. Just asking it again, as I dumped in 60 oz of 14.5% MA today to lower my pH from 7.6 to 7.2. Do you normally add baking soda to counter the expected drop in TA by the MA addition? Chem Geek, you ought there? I have a feeling you would know the answer. From Pool Math "Effects of Adding Chemicals," I determined I could add 25 oz of baking soda, which would raised the TA by 5.9. Per this calculator, it shows 60 oz of MA, but at 15.7% strength, will drop my TA by 6.2. Too bad it doesn't have a choice for 14.5% MA. Can you add that in Jason Lion?

My FC today was 6. Looked like it burned off 2 ppm of chlorine from the Mon., when I had added 38 oz of 12.5% chlorine to raise FC from 6 to 8. I added this amount because I had assumed my CYA was 50, until I tested it later yesterday. We did use the pool yesterday and were splashing around with our twins. We used it for about 30 min. from 6:45 pm to 7:15 or so. I am really surprised it burned off 2 ppm of chlorine. The weather was just overcast and air temp was probably 89 degrees. My usual chlorine is 1 - 2 ppm, but this was at a CYA of 50. Now with it at 40, I would not have thought I would lose 2 ppm just like that. I think I am going to test with the FAS-DPD more days this week to check for any trends.

 

[timerguy]

Gosh, I'd love to have only 1-2 ppm daily FC loss here in SC. mine is 2.5- 3.

 

[zea3]

Usually we don't recommend adjusting the TA unless the pH is too unstable (rapid drops and rises out of range). I would lower the pH and then check it the next couple of days. If it is rising or falling too rapidly then you can raise the TA back up. If you already know that 70 ppm is where your pH is most stable then after you adjust pH check and adjust the TA.

 

[bbrock]

Tx Zea3.

I guess I could wait and see if over a couple more weeks or so the TA drops further. I thought and swore I read here that generally speaking, pH will rise in pools d/t the effects of the chlorine. With my TA now at 70, I am curious to see how this will play out on my pH. Even though I thought pH generally rises, can it ever get to a point that if my TA gets too low, I will actually start seeing the pH decrease?

And, you state that " If you already know that 70 ppm is where your pH is most stable then after you adjust pH check and adjust the TA." Would I would want to wait at least 30 mins. for the water to mix after adjusting pH before testing TA? Also, I am not sure if 70 is my "sweet spot," more so that 70 works out well according to Pool Math. I thought the other time to adjust TA is when your CSI would be affected. Please comment.

Tx again for the help.

 

[Abigail K]

Chlorine can cause a short term pH increase when it is added, however, the pH will reduce back to a steady state as the chlorine is consumed.
Even if TA reduces, it does not cause a pH decrease in and of itself.

The main reason to adjust TA is to reduce pH swing, the other reason is CSI.
I would normally suggest allowing the pH to rise to 7.6-7.8 and seeing if it holds there, however that may push your CSI towards 0.6, as I have stated above.

 

[chem geek]

Even though I thought pH generally rises, can it ever get to a point that if my TA gets too low, I will actually start seeing the pH decrease?

No, you usually won't see the pH decrease because you aren't adding any net acidic chemicals. The pH will typically become stable or only rise slowly, but if the TA gets low then this affects the saturation index and if you don't compensate with a higher pH target or a higher CH then you can risk plaster deterioration depending on how low the saturation index goes.

 

[bbrock]

Zea3, I think I'm adding about 1 in. of water every week, approximately. While I haven't done the bucket test yet, from what I understand this may be normal. What r others water addition trends?

 

[bbrock]

Just found a great thread by Chem Geek re: CYA Degradation by Oxidation from Chlorine. http://www.troublefreepool.com/threads/8498-Degradation-of-Cyanuric-Acid-(CYA)

While a bit technical, the point is understood. This makes sense as to how my CYA dropped from 50 to 40 over approx 2 mo.

 

[chem geek]

I suggest you keep track of your CYA level over multiple months since the CYA test isn't that precise. I calculated 2-3 ppm CYA loss per month by measuring over a 6 month period. It's easier to see this in hot spas where they typically lose around 5 ppm CYA per month.