Balancing pH and TA in a small Intex pool

[owkaye]

Why would TA fail to go down after adding a cup of Muriatic Acid to a tiny 1200 gallon pool? My TA was 200 ppm when I added the acid yesterday afternoon, but this morning TA is still 200 ppm. My pH dropped a lot because of the acid addition, so why didn't the TA drop along with it?

Background info:

I set up this very small 1200 gallon above ground Intex soft-sided metal frame pool about a week ago. The nearby shade trees prevent direct sunlight from hitting more than 30-40% of the pool surface at any one time. We seldom use the pool so I keep it covered with a large sheet of vinyl (from a billboard) most of the time. The cover rests on 90% of the water surface, dramatically reducing the air-exposed surface area.

I filled the pool with tap water then added regular chlorine bleach for the first few days of use before I got my test kit. I have not added any dichor/trichlor/cyanuric acid so my CYA is probably zero -- unless CYA is a component in my local public water supply?

I currently use the $15 test kit from Home Depot so I also bought this Blue Devil CYA Test Kit from amazon.com which should arrive in a few days.

My overall goal is to bring my TA down to 80-100 ppm, then aerate to bring the pH up to 7.5, then add enough dichlor to bring my CYA level up to about 50 ppm. Then when everything is stable I plan to add Boric Acid as an additional buffer / water conditioner. After all this is done I should have a relatively stable pool that requires only small amounts of bleach to maintain the correct FC levels, or at least this is what some websites suggest.

 

[JVTrain]

Likely testing error or a well-known issue with static electricity on new droppers of TA reagent. Next time you test TA, wipe the tip gently with a damp cloth between each drop to maintain consistent drop size. As you keep the pH in line (drop to 7.0 or 7.2 when it gets to 7.8), the TA will come down.

 

[owkaye]

I don't think there was any testing error. I'm relatively confident that I ran all my tests correctly. I always rinse the test vials thoroughly several times and use the correct water level. I take my water sample at about 20 inches depth in the same place every time. I take my time to measure accurately. I use a white card behind the test vials to insure that I'm seeing the correct color matches. And I always carefully count the drops of the #3 reagent. What else can I do to avoid errors except to wipe the tip of the #3 vial with a damp cloth before creating each droplet?

I added a cup of bleach late this morning after I discovered that my FC was 0.6 ppm. This brought the FC up to 3.0. Since then I've been aerating the pool (not very aggressively) to bring the pH up from its very low early morning value of 6.8 (or lower).

A few minutes ago I re-ran my tests for TA and pH. I took careful precautions to discharge any static electricity from the reagent bottles and the test vials this time. Then in my TA test I carefully wiped the tip of the #3 dropper bottle with a damp cloth before releasing each droplet. The results are a TA of 170, down 20-30 ppm from my early morning reading of 190-200 ppm. Then I ran the pH test and got a value of 7.2, up from this early morning's reading of 6.8.

It looks like pH is gradually increasing while TA is gradually decreasing. Does this make sense?

 

[chem geek]

No, the TA decreased when you added the acid, but perhaps the water was not thoroughly mixed in the pool so the TA was dropped further in the area you added the acid and took longer for that drop to become more evenly distributed throughout the pool including where you measured it later. If your pump turned off shortly after you added the acid (since I assume it was on when you added it since you are supposed to pour it slowly over a return flow with the pump running), then the water would not be mixed overnight. Either that or it's test error as was already suggested.

One cup of full-strength Muriatic Acid in 1200 gallons would lower the TA by 26 ppm so you are now getting the correct reading.

Acid lowers both pH and TA. Carbon dioxide outgassing raises pH with no change in TA.

 

[owkaye]

No, the TA decreased when you added the acid, but perhaps the water was not thoroughly mixed in the pool ...

I'm professionally trained as a geochemist so incomplete mixing seems more likely to me than inaccurate measurements. Thanks.

Here's what I read online that basically outlines my goal for this pool:

With pH and alkalinity at the proper levels they are fairly stable. Cyanuric acid at 30 - 50 ppm will act as a buffer to prevent pH from lowering. Now you can add borates at 30 - 50 ppm to keep pH from rising. Keep the alkalinity about 80 - 90 ppm and the pH about 7.5. This level for alkalinity is a little lower than the level usually maintained and recommended but the pH is very stable at this alkalinity with 30 - 50 ppm of cyanuric acid and 30 - 50 ppm of borates. The borate works better near 50 ppm.

Assuming this info is accurate, is this the correct procedure for me to follow from now on?

1- Add acid every day until my TA eventually gets down to 80-90 ppm.
2- Aerate daily until the pH recovers to 7.5 or higher after each acid addition.
3- When my TA and pH are relatively stable at the above levels, add dichlor to raise my CYA to 40 ppm.
4- Add Boric Acid to bring the borate level up to 50 ppm.
5- Stop using dichlor and start using plain unscented sodium hypochlorite bleach as my sanitizer.

My understanding here (and please correct me if I'm wrong) is that once the above levels have been achieved I will almost never have to add borates or cyanuric acid because these components will never evaporate or be consumed in the pool. The only way they can leave the pool is via draining, splash-out, backwashing of my (future) sand filter, etc.

Regarding CYA: I was given a half-bucket of dichlor from a neighbor, that's why I chose dichlor as my CYA source.

Regarding Borates: Pool Calculator says I need 46 oz. of Boric Acid to go from 0 ppm to 50 ppm so I bought 9 x 5 oz. bottles at Dollar Tree for $1 each. This should bring my borates almost up to the recommended 50 ppm level.

Regarding my tap water: Based on my first TA pool water test I'm guessing that my tap water has a TA of 240 ppm (or possibly more, I will test it later). So I will still have to add acid whenever I top-off the pool with this water, but probably not at any other time, correct?

Regarding my pool cover: It's a big sheet of vinyl that rests on the top of the water thus sealing off at least 90% of the air-water interface. What effect might this have on the stability of my pool water? Should I modify this cover so that it does not rest on the water surface, thus allowing greater exchange of carbon dioxide between water and air?

 

[JohnT]

My advice is to just ignore pH and TA altogether unless your hardness is such that you are at risk of scaling or the pH is low enough to cause eye irritation. It isn't that important in such small pool without metal components.

If your cover allows gas exchange between water and air, it will allow evaporative cooling and reduce the heat conserving benefits of your cover.

You need to get the CYA in there now to avoid requiring frequent chlorine additions.

 

[owkaye]

My advice is to just ignore pH and TA altogether ... it isn't that important in such small pool without metal components.

If I do this I won't be able to use the system described in my previous post, which apparently produces the best stability while minimizing the use of bleach -- and adds the benefits of borates! I'm not trying to dismiss your suggestion but you're the first person I have seen in any pool forum to suggest ignoring pH and TA ... so let's say I take your advice and just ignore these parameters:

What issues am I likely to run into that I could have avoided had I instead used the above-described system?

The reason I ask is because I'm the kind of guy who like to minimize potential future problems, and the system described above seems like it might be a great way to accomplish this. If I can create a very stable system from the start, maybe it will take care of me more than I have to take care of it!?

If your cover allows gas exchange between water and air, it will allow evaporative cooling and reduce the heat conserving benefits of your cover.

Summer temperatures in Biloxi seldom get below 80 degrees at night, and daytime temps are generally in the mid- to high-90's, so a 'cool pool' is ideal for me. I put the pool under the trees precisely to minimize solar heat gain and insure that it remains as cool as possible. Evaporative cooling seems like a 'good thing'.

You need to get the CYA in there now to avoid requiring frequent chlorine additions.

Does tap water have cyanuric acid in it? If not, I don't mind doing this. I just don't want to exceed the recommended CYA levels accidentally and then have to drain the pool before the end of summer.

 

[JVTrain]

I would NOT ignore pH. You can pretty much ignore TA if you keep the pH between 7.2 and 7.8 using acid additions. TA is a number that is very subjective from pool to pool. The best TA level is the one that is somewhere above 50 to 60 but less than 120 and produces the slowest pH rise. The answer is, maintain your pH in that range I mentioned and don't get to carried away with keeping it at EXACTLY 7.4 or EXACTLY 7.5. If your pH is at 7.8 and doesn't change for a week or more, let it be or if you need to for comfort reasons, bring it down to 7.5 or so. The correct TA for your pool will likely be somewhere between 60 ppm and 100 ppm but only you will know based on when your pH rise slows to a crawl.

Tap water will not have CYA in it.

 

[owkaye]

Hi JVTrain, thanks for the suggestion to pay attention to pH even if ignoring TA.

Tap water will not have CYA in it.

Based upon this info and the suggestion of JohnT (thanks John!) I just added 4.0 oz. of dichlor granules. Given the likelihood that my CYA level was zero before this addition, it should be 13 ppm now according to Pool Calculator. I won't add any more dichlor until my CYA test arrives, then I can confirm my actual CYA level before possibly raising it higher.

One thing I have a question about now is my FC level. It tested at 2.0 before the dichlor addition, but now Pool Calculator says it should be 16 ppm -- which I cannot test for using my current test kit -- so I feel the need to ask:

What comfort / health / safety issues should I be aware of when swimming in water with such a high FC level?
Or should we just stay out of the pool until the FC level drops to a much lower level?
Or doesn't FC level have much (or any) effect on swimmer comfort or safety?

 

[JVTrain]

Check out the CYA/Chlorine Chart link in my signature. The safe level for swimming is any FC level at or above the minimum and below the shock level for a given CYA level. For instance, at 30 ppm CYA, you can swim at any FC up to 12 FC.

 

[JohnT]

There are really no significant negative effects to having your pH high unless scaling is a possibility or you have metals in your water. It's primarily an equipment issue, and your equipment isn't sensitive to pH. Even 8.1 is fine. Ben Powell has written about high pH pools and their advantages, and it is known that higher pH is more comfortable for swimmer's eyes.

Let your pH go and see where it settles. Anywhere between 7.2 and 8.2 will work.

 

[domct203]

....... One thing I have a question about now is my FC level. It tested at 2.0 before the dichlor addition, but now Pool Calculator says it should be 16 ppm -- which I cannot test for using my current test kit -- so I feel the need to ask:

What comfort / health / safety issues should I be aware of when swimming in water with such a high FC level?
Or should we just stay out of the pool until the FC level drops to a much lower level?
Or doesn't FC level have much (or any) effect on swimmer comfort or safety?

With 20ppm or less CYA your FC target is 3ppm, minimum 2ppm, shock 10ppm.
http://www.troublefreepool.com/content/128-chlorine-cya-chart-slam-shock

As mentioned it is safe to swim when FC is between min and shock level for your CYA level.


Dom