TA adjustment with Muriatic acid - diminishing returns?

[JCarsten]

I'm just getting a new pool up and running (new construction, filled less than a week ago). Based on the well water we used, I knew TA and pH were going to be issues until I got them under control. I'm new to the whole pool care experience. But it's surprising me that I seem to be using a lot of Muriatic acid with very little results. Here's my first full set of test results, followed by the results of my pH and TA tests over subsequent days as I began adding acid:

As of Friday (7/11 - evening):
Chlorine was in the 5-10 range on the simple test. Did not do the FC/CC test Friday night
TA - 200
pH - 7.8
CH - 400
CYA - 0 (this was a new fill with well water and a small amount of CYA added 2 days before the first test. Still slowly building up the CYA level)

My pool contractor both shocked the pool on Friday, as well as left the SWG running in "shock" mode Friday afternoon.

Friday night I added 32 ounces of 20 baum Muriatic acid (left the pump running all night with the SWG set on 65% - I turned off the "shock mode" around 10 pm when I realized the contractor had left it set that way). I was doing this to mix the acid. I have since realized I don't need to let it run this long just to mix the chemicals.

Saturday (7/12 -morning)
FC - 10 (I'm now tinkering with pump run time and SWG % to bring this level down)
CC - .5
TA - 180
pH - 7.8+ (it was darker than the lower amount but not as dark as the 8.2 - could this increase be because I had run the pump all night with the SWG going 65%?)
CH - did not test
CYA - did not test

So Saturday night, I added 64 more ounces of muriatic acid.

Sunday (7/13 - midday)
FC - 7 (sun had been on the pool about 2+ hrs when I tested)
CC - .5
TA - 160 (I found this odd since I had added twice the acid)
pH - 7.5+ (it was darker than 7.5 but lighter than 7.8)

So Sunday night, I added another 64 ounces of acid. This time, I only ran the pump for 2 hrs after adding the acid. Pump/SWG would not come on again until 8 am Monday

Today (7/14 - 7:30 am)
Chlorine (it was in the 5-10 block on the simple chlorine test. did not use the FC/CC test)
pH - 7.2 (that's at least consistent with the kind of drop I would expect for another 64 ounces of acid)
TA - 150 (actually, tested twice. Got 150 on one test and 160 on the other)

So the TA is going down, but I am a bit surprised at how much acid it is taking. And the shift seems to be getting smaller each time. With the pH down to 7.2, I am probably going to skip adding acid tonight and allow the pump/SWG to bring it back up before adding acid to impact TA again.

But I'm a little concerned that I am going to have to keep feeding this thing MA to keep my TA in check. Or is it something that once I get it below 100, it should stay there barring any major water replacement. And at that point, I will just be using smaller amounts of MA to keep the pH in range?

 

[JohnT]

Disregard your pH readings when the chlorine was high. Chlorine interferes with the pH test at around 10ppm FC.

Simply adding acid doesn't lower the TA, at least not permanently. You need to drop your pH to the 7.0 range and then aerate the pool. Lowering Total Alkalinity

But at 200ppm, there is no need to get in a hurry about it as long as you can keep your pH down without it being a burden on your time. The acid/aeration method works faster, but just staying on top of your pH will work eventually.

 

[JCarsten]

Does the SWG come into play at all as far as pH is concerned? I know that the aeration provided by the SWG will increase pH. But does the chlorine generated by the SWG also cause the pH to rise?

I guess what you're saying is to just stock up on MA. And follow the method in that link. Drop pH, aerate to raise pH, wash-rinse-repeat until my TA is where I want it?

 

[Richard320]

Does the SWG come into play at all as far as pH is concerned? I know that the aeration provided by the SWG will increase pH. But does the chlorine generated by the SWG also cause the pH to rise?

Yes. The SWG will cause pH to rise. And high FC levels will also cause the pH tester to read falsely high. Which is why the recommendation was made to "Disregard your pH readings when the chlorine was high."

I guess what you're saying is to just stock up on MA. And follow the method in that link. Drop pH, aerate to raise pH, wash-rinse-repeat until my TA is where I want it?

There's a straight correlation between the amount of acid and the amount of TA you eliminate. Doing the aggressive aeration technique doesn't change that, it just speeds it up.

 

[JCarsten]

There's a straight correlation between the amount of acid and the amount of TA you eliminate. Doing the aggressive aeration technique doesn't change that, it just speeds it up.

Will the SWG (or any other kind of chlorination, i.e., bleach) also cause the TA to rise, or does it only affect pH? Is it safe to assume that once I get my TA under control, that barring a chemical addition to raise TA, or significant pool water replacement with my source (high TA) water, my TA should largely remain under control and I will mostly be using the acid to keep the pH in balance?

 

[JasonLion]

If you can get the TA down to around 60 to 70 and there isn't anything raising it back up, then you can stop using acid and bother PH and TA will be fairly stable. If your fill water has high TA levels and you need to top off the pool frequently, then you will be adding acid regularly more or less forever.

PH goes up mostly when TA is high and/or PH is low and there is aeration. The SWG provides aeration even if nothing else does.