TA drops, but rebounds

[red-beard]

Still working to get my TA down.

I started by reducing my TA using the Muriatic Acid method. I started at 90 and was able to reduce it to 65-70. The procedure in Pool School says to aerate. But later wisdom from the Forum says that aerating is not generally needed. I pulled my aeration method from the pool and continued. The TA was down to 65-70 (I have a diluted reagent to allow me to do 1 drop = 5 ppm TA).

Today, my water was back up to 7.9 pH. I checked TA; it rebounded to 80. I might have made some water additions, but our tap water is about 60 ppm TA. I've added maybe 2 inches of water, or about 750 gallons. That should only have raised my TA by about 2 ppm.

My pool: FC 11 ppm, CC 0.0 ppm, pH 7.9, TA 80 ppm, CYA 80 ppm, Salt 2900 ppm, BOR 25 ppm, temp 90F, CSI 0.18

I added a gallon of MA to get the pH and TA down. Readings are now pH 7.1 and TA 65. I've now added 1 qt of MA to get it down to pH 7.0 and TA60.

Should I be aerating like the Pool School article or not? My goal is a TA of 55-60, and then I'll add 25 ppm of borates using boric acid.

 

[Texas Splash]

Should I be aerating like the Pool School article or not?

I don't think you have to. I experience the same slow TA rise even from a relatively low TA fill water. Focus on the pH. When it hits 8.0 knock it down to 7.2 and you should see the TA fall each time. Of course with this heat the top-offs will try to increase it again, but that's a TX summer and you're doing fine.

 

[pjt]

our tap water is about 60 ppm TA

Wow, I'm jealous. My fill water TA in The Woodlands is 330.

 

[red-beard]

You must have well water

 

[mgtfp]

Chem geek used to mention that with a SWG it is theoretically possible to have an additional pH-rise (additional to the pH-rise due to CO2-outgassing) should not all of the generated Cl2 gas get dissolved in the water. In that case not all of the pH-rise during the electrolysis gets compensated by dissolution of Cl2 in the water and the subsequent use of HOCl. And any pH-rise not due to CO2 outgassing comes with a rise in TA.

Usually, TA rising due to fill water additions is dominant, and that other potential path is not really discussed anymore at TFP.

But I was thinking for a while that this could be happening in my pool. My fill water has a TA of 30. And even at times when I don't have to top up at all due to lower temperatures and/or sufficient rain, my TA doesn't want to drop below about 70 with regular MA additions about every 3 weeks which I need for pH-maintenance.

I once took the effort to force TA down to 60 with an MA-aeration cycle, but it was back at 70 in no-time.

Curious about experience from other members with low-TA fill water and similar observations. And whether there could be something about the Cl2 outgassing theory.

The issue I have with this theory is that in my pool I needed up to about 20% of the generated Cl2 gas to outgass to compensate the TA-drop by my 3-weekly MA-additions (calculated with Chem Geek's spreadsheet). Which sounds a lot, and I don't seem to miss any of the theoretically produced chlorine to cover my FC-losses. Unless a 10-20% loss has been considered in the specified chlorine output by the SWG-manufacturers. Which I haven't heard of...

It's not really an issue for me. TA 70-80 seems to work well in my pool, and adding some MA every three weeks is not a problem. But I'm still curious to understand what's going on.

 

[red-beard]

I'm going to re-check my fill water. I have my fill water numbers in my record book cover.

 

[pjt]

You must have well water

I'm in MUD 386 and evidently we get water from an aquafer.

 

[red-beard]

Well, that is peculiar. I know they stopped sending us Houston surface water. I did not realize MUD370 on well water was so different:
FC 0.0, CC 0.0 (used to be 2.0ppm Chloramines), pH 7.9, TA 230, CH 30.
Used to be: FC 0.0, CC 2.0, pH 7.4, TA 60, CH 150.
So, this can explain some of the long-term TA rise, but not the rebound.

 

[mgtfp]

So, this can explain some of the long-term TA rise, but not the rebound.

Doesn't it? You initially estimated that your TA might haven have risen by about 2ppm due to replacement of 750 gal of evaporated water with 60ppm fill water. Now you retested and found your fill water TA to be about 4 times higher. That means the TA rise by fill water should be about 4 times higher than initially estimated. That should put you into the 10ppm ballpark.

 

[red-beard]

If my TA is 230, each inch of water is about 360 gallons and raises my TA 3.33 ppm. It will take 3 inches of water to raise my TA 10 points.

Hmmmm. 13 days ago, my TA was 90. I added 1 gal of HCL, which should knock my TA down by 20 points. I saw a ~15 ppm reduction. 10 days ago, I added 64 oz and my TA was reduced to 65 (this was measured using 50% diluted reagent, so 5 ppm per drop). If my TA is now 80, it went up by 15 ppm. This would imply 4.5 inches of water added. I don't think I've added that much water. But again, it has been a hot dry summer with no significant rain.

If my measurements are off by up to 5ppm, the range would be 3-4.5 inches of water. I could see 3 inches of water over 10 days. Hmmmmm.

 

[proavia]

What is this diluted reagent you mentioned?
That could increase the testing tolerances quite a bit and add more error into the result.

 

[duraleigh]

But later wisdom from the Forum says that aerating is not generally needed

Not sure where you saw that but it is essentially incorrect. If you REALLY want to lower TA it is done by the method taught in pool school which is......

1. Add acid to lower your PH to between 7.0 and 7.2 (this also lowers TA)
2. Aerate until PH rises to around 7.6 (the only way to raise PH without also raising TA)
3. Repeat steps 1 and 2 until you reach the desired TA.

Now, do you really need to lower TA? That's sorta' your choice although many, many pools on this forum are reporting a TA of around 60 seems to be ideal

I would suggest you lower yours but you will need to aerate.

 

[red-beard]

Not sure where you saw that but it is essentially incorrect. If you REALLY want to lower TA it is done by the method taught in pool school which is......

1. Add acid to lower your PH to between 7.0 and 7.2 (this also lowers TA)
2. Aerate until PH rises to around 7.6 (the only way to raise PH without also raising TA)
3. Repeat steps 1 and 2 until you reach the desired TA.

Now, do you really need to lower TA? That's sorta' your choice although many, many pools on this forum are reporting a TA of around 60 seems to be ideal

I would suggest you lower yours but you will need to aerate.

The Pool School says one thing, but some of the forum leaders state that aeration is not needed.

I'll go back to the Aeration method.

 

[red-beard]

What is this diluted reagent you mentioned?
That could increase the testing tolerances quite a bit and add more error into the result.

Taylor R-0009, I have a second bottle that is diluted 50% with distilled water (very carefully measured) used to make more exact measurements. Each drop is 5 ppm TA indicated. I have tested these drops against the original.

 

[red-beard]

Doesn't it? You initially estimated that your TA might haven have risen by about 2ppm due to replacement of 750 gal of evaporated water with 60ppm fill water. Now you retested and found your fill water TA to be about 4 times higher. That means the TA rise by fill water should be about 4 times higher than initially estimated. That should put you into the 10ppm ballpark.

I have to think this is probably correct.

Maybe we need some advanced features in Poolmath. You put in an analysis of your tap water. Measure how long it takes to fill a gallon jug from your water supply. Add a time circuit, this will calculate how many gallons added then make an addition to CH and TA based on the water analysis.
The same could be done for rain water, since it is nearly distilled water. For every inch of rain received, beyond the pool drain, everything is diluted.

It is interesting to see how regular things like water additions can mess with the chemistry. And to that point, that we need to test our water supply more often.

 

[Toxophilite]

The Pool School says one thing, but some of the forum leaders state that aeration is not needed.

I'll go back to the Aeration method.

First season with pool, but finding there's a lot to this pH/TA equation. With fill water of 260 TA, I had aggressively worked with MA and aeration to bring my TA down to 80. For long while now, months, it seemed to stabilize there, so I went to just maintenance doses and keeping my pH in the mid-7's. Last test a week ago, I noticed my TA had crept to 90. Last 2 days, I'm finding that maintenance level of MA is doing squat. I can dose in the morning, have 7.4-7.6 pH, and by afternoon is almost 8.0! Yesterday's eve dose is gone to 8.0 just this morning. I'm going to run the battery test this afternoon, but can bet I'm back on the deep cycle of pH program again with my old large doses of MA.

 

[red-beard]

New data, my best measurement is a 1 gallon jug. It takes my pool fill about 5 seconds to fill it. I re-tested several times. Difficult with the fill in the edge of the tile. This is 20 GPM. My typical fill is about 10-15 minutes, so 200-300 gallons each time I fill. I am filling about every 2-3 days depending on the climate conditions and whether I'm in town. I'd say I am probably putting in on average 100 GPD. Water is about 90 degrees and dewpoint is 70F with Air temp near 100F. Not as bad as Pheonix, but still hotter and dryer than normal for Houston.
And I just added 300 more gallons....

 

[proavia]

I really think you are over-thinking this. Evaporation rates constantly change and the TA of the fill water may not be a constant either.
Using diluted reagent is NOT making the TA test any more accurate - actually, you have zero data to support that this should even be done, let alone that it's more accurate.

Measuring the TA to multiples of 10 is accurate enough.
Once the TA is down to the level you want it (in multiples of 10), just test the TA weekly or monthly and take the appropriate action.
You are making this way harder than it really is.

We really don't want users to think that diluting the reagents is a good thing. Taylor is the only entity that can make this determination - and I don't see that happening anytime soon.

Leave your test kit inside, grab an adult beverage and go swimming. Enjoy your pool and don't be a slave to the perfect numbers.

 

[duraleigh]

+1 what proavia said.