trouble increasing pH with pool covered

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TimTucker

TimTucker

Gold Supporter
LifeTime Supporter
Oct 14, 2014
67
Kingston, Tennessee
Pool Size
23500
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
CircuPool RJ-60 Plus
Hi guys. This is my first post. I've been doing some searching and reading on TFP and can tell there is a wealth of knowledge and information here. Wish I had come 14 years ago. I usually have very little trouble with my pool. The possible exception being the pH going low. This year was a bit better after I learned that pointing my jets up would help rid the pool of CO2.

I covered (solid cover) the pool 5 days ago (10-10-2014) and had the water analyzed (TA:120; pH:6.8; TC:2.0; FC: 0.8). The pool store had me add 6 lbs of sodium carbonate and then 15 pounds of shock after six hours. I pull the cover back, pre-mix everything, and dump it into the pool. Everytime.

On the 11th they re-analyzed (TA:170; pH:7.0; TC:4.0; FC: 4.0). The pool store had me add 6 lbs of sodium carbonate (“pH increaser”).
On the 13th they re-analyzed (TA:185; pH:7.2; TC:3.0; FC: 3.0). The pool store had me add 6 lbs of sodium carbonate.
On the 14th they re-analyzed (TA:215; pH:7.3; TC:2.8; FC: 2.8). The pool store instructed me to keeping adding sodium carbonate.

18# of sodium carbonate moved my 30k pool from 6.8 to 7.3 and the plan is to add more…?

I suspect covering my pool while it had a chlorine demand and low pH before shocking has caused this problem. Comments? This weekend appears clear (no rain). I plan on peeling the cover back and letting the pool breath/ outgas .

Thanks for any feedback. Regardless of how brutal it may be.
Tim
 
Welcome to TFP!

What kind of "shock" are you using? Several of the obvious possibilities are very acidic.

Also, as a general rule it is not a good idea to trust pool store water test results. There are some great pool stores out there, but the number of wildly wrong test results from pool stores reported here is staggering.
 
I have used Poolife Turbo Shock (as you know Calcium hypochlorite) as the shock and Poolife 3” tablets (Trichloro) in my “offline” Hayward chlorinator since it was new. I have 2 of the cheaper test kits and have basically gotten the same numbers. I can’t total chlorine, just free.

It did surprise me that they were unaware sodium carbonate raises TA. They thought I was adding baking soda.

If I understand correctly, the addition of sodium carbonate produces a bicarbonate ion. Which eventually combines produces carbon dioxide gas. If this is not allowed to escape would it go back into solution and drive my pH down?

I’ll be spending this winter studying this site to learn cheaper more effective ways of maintaining my pool.
Thanks
Tim
 
Sodium carbonate is what we call soda ash. It raises both PH and TA, though it raises PH a lot more than it raises TA. It has the same net effect if the pool is covered or uncovered.

Dissolved CO2 is part of the carbonate buffer. Swimming pools are normally super-saturated with CO2, just like soda. Since there is fairly little surface compared to the total volume this can be maintained fairly easily, i.e. the CO2 doesn't all outgas instantly.

When the pool is uncovered the CO2 will outgas slowly, which raises the PH. How quickly this happens depends on several factors, mostly low PH, high TA, and aeration raise the rate, while high PH, low TA, and lack of aeration lower the rate.

Trichlor tablets are net acidic. You normally need to add soda ash regularly when using trichlor. This effect can be mitigated somewhat by maintaining a high TA, which will cause gradual CO2 outgassing, which will compensate for at least some of the acidity. When the pool is covered the CO2 outgassing does not occur, so you need more soda ash than you would need when the pool was not covered, and the TA will tend to go up relative to what happens when the pool is not covered (assuming you are maintaining a constant PH in both cases).
 
Thank you for such an excellent, detailed reply and description of what’s going on.

I hate to put you on the spot, but would you close the pool with these numbers? Would this “trapped” CO2 eventually drop the pH? Or, is my plan of uncovering and getting the pH up my best plan of attack at this point?
Again, Thanks!
 
I would recommend getting your own test kit so that you can properly manage your chemistry. The 40 cya seems unlikely if you've been using tabs for a while. The high calcium and TA put you at risk for scaling if the pH were to rise.

The tabs in the feeder will continue to lower the pH and offset any pH increase.

The pool store results don't add up. 15 lbs of calcium hypochlorite should have raised the fc to about 39. With a solid cover, there would be no loss to sunlight. Therefore, unless there was a lot of chlorine demand, most of the chlorine should have been still in the water. I would suspect that the fc was really closer to 30 + and the pool store probably used dpd where it partly bleached out leading to a false low reading.

Also, 18 lbs of soda ash would have raised the pH more. Most likely, the ph was much lower than 6.8 during the initial test. I would suspect that the high fc during the subsequent pH tests caused false high readings.

I would suspect that the cyanuric acid level is probably very high. How many pounds of tabs have you used since the pool was filled?

I would suggest allowing the system to operate until you can get your own test kit and then decide what to do based on your own test results.
 
James,
The TF-100 with several of the options has been ordered. My guess on the tablets would be 50-75 lbs (or more) per years for 13 years. When I drain, I do so using main drain only. They say cyanuric acid is heavier than water?

I'm anxious to get this kit and see what numbers I come up with.

btw, I now see that my usage of Trichlor must end or be greatly curtailed.
Tim
 

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All,
The kit arrived yesterday and I ran tests today. Real eye opener. I come from a chemistry background and ran the TA and CH tests twice. I ran CYA 3 times. 1st full strength water; 2nd and 3rd diluted 50% with tap water. Numbers:
TC: 2
FC: 2
FAS/DPD: run tomorrow
TA: 260
CH: 700
CYA: >200
Borate: <15 (I've added 4 boxes of 20 mule team over the summer)

I guess some main drain dumping is in order here. How far would be safe to drop with a vinyl liner pool? How long for the water to re-stabilize? God know any suggestions or ideas are welcome. I feel like a complete idiot for letting this go on as long as it has! I am manually adding 8.25% bleach (1/2 - 1 gallon a day). I check FC every day. The trichlor tablet feeder is empty! On the bright side: the water looks great!

Thanks
Tim
 
You should leave at least a foot of water in the shallow end to reduce the likelihood of the liner shifting.
I might suggest that you lower the water to having one foot, circulate for at least an hour, and then do it again. You may even need a third time before the CYA is reasonable.

Not sure what you mean by "re-stabilize". Once you refill, circulate for an hour or so and then you can test again. I would not test before at least 2 dump and refills.

For the CYA test, you do realize you can dump the solution back and forth a few times, right? No need to run the test multiple times unless you had a problem.
 
jblizzle said:
You should leave at least a foot of water in the shallow end to reduce the likelihood of the liner shifting.
I might suggest that you lower the water to having one foot, circulate for at least an hour, and then do it again. You may even need a third time before the CYA is reasonable.

Not sure what you mean by "re-stabilize". Once you refill, circulate for an hour or so and then you can test again. I would not test before at least 2 dump and refills.

For the CYA test, you do realize you can dump the solution back and forth a few times, right? No need to run the test multiple times unless you had a problem.
Click to expand...

Thanks! I just read the procedure by JasonLion for CYA >90. Many thanks for pointing that out!
Tim
 
I see the first note about pouring back into the mixing bottle. My mixture was very cloudy. My pool water is clear. I went back out and re-tested using Jason's techniques. Same result >200.
Tim
 
You can do the cyanuric acid test with 1/3 pool water and 2/3 tap water and multiply by 3 to get a better idea of the real level. That will allow you to test up to about 300 ppm. However, the accuracy won't be really good.

In any case, a partial drain and refill is in order. Now that you have a good test kit and a better understanding of the real chemistry, you should be able to keep the water in excellent condition.
 
Well guys after four partial drains and refills I think we've done it! Here are today's test numbers:
FAS/DPD
FC: 7
CC:0
TA:140
CH:225
CYA: 45 ppm

The water looks great and the pool is clean. The CYA level should drop a bit more as I drop the level below the skimmer for closing and re-fill in the spring. The Liquidator should be here and hopefully installed on Saturday. For closing, bring the FC to 18ppm and follow the great directions on the site here. Correct?

Thanks,
Tim
 
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