When cycling to lower Hot Tub new water refill from 370 TA, how long to wait after adding MA to start 30 min aeration, or to test for the lowered PH?

Davekro · Mar 12, 2024

I am looking to shorten the time it takes to lower my Hot Tub's TA from 370 to 50–70 ppm, since I need to do so many cycles when I refill the tub. I have a Bullfrog A6L (330 gals.)

When cycling to lower new water refill from 370 TA, how long do I need to wait after adding muriatic acid (to lower PH to ≈7.0), before starting both pumps on high to aerate to raise the PH (to ≈7.8–8)? I have read recommendations from 30 mins, 3–6 hour, to overnight, before testing PH after adding acid.
Since in my case at drain/refill, drop my city water's TA from 370 down to 50–70 pp takes 5–7 cycles of raising PH (both pump on high, aeration 30 mins), add MA to lower PH (both pumps on low "X?" minutes for water to fully absorb/bond and achieve the full PH lowering).

1) At the end of 30 minute 'PH lowering', both pumps on high, aeration cycle, can you just add the predetermined amount of MA while the aeration continues and the acid will mix in (to lower PH) at the same time the aeration is raising PH?
2) Or, at the end of a 30 min aeration/ PH raising cycle, add MA, mix in w/ one?, both?, pumps on low for some amount of time LESS THAN 30 mins before initiating the next aeration/PH raise cycle?
3) Other

ajw22 · Mar 12, 2024

I would say to add MA, wait 15-30 minutes with normal circulation, test to confirm pH is in the low 7's, then turn aeration on until pH in the 8's, repeat as necessary.

Let's see if @JoyfulNoise has a faster process.

JoyfulNoise · Mar 12, 2024

In a small hot tub, the effects of MA on TA should take just a few minutes if the water is circulating. The chemical reactions are nearly instantaneous and you're simply moving the water to homogenize everything. I say turn on the circulation pump, add the acid and then test about 5 minutes after adding to make sure you hit your target TA and pH.

Whenever I need to reduce TA, I add enough acid to lower the TA by an exact amount, usually 10ppm, and I don't focus on pH. I know, I know ... but the Pool School article says .... yeah, I know what it says, it's just not practical for large TA changes. When you lower TA, pH goes down as well but it's temporary. Outgassing of CO2 starts almost immediately especially in a hot tub where the water is warm and the aeration is extreme. You can easily drop the TA in 10ppm increments and the pH will recover very fast. There's nothing magical about an excursion in pH outside normal ranges because it's not going to stay there for very long. It's only when people leave the pH low or crash the pH and don't pay attention ... that's when the trouble begins.

But you do what you're comfortable with. I'm a chemistry nerd, so stuff like this doesn't bother me at all.

phonedave · Mar 13, 2024

I have an A6 and my fill water is about the same TA you have.

I add acid with the jets running. I add enough acid to get the pH to around 7 (or less). Then I test again when the spirit moves me. Sometimes later that day, sometimes the next morning, whatever. I then add enough acid to get the pH back down. After about 7 cycles, things get in balance naturally. Sometimes those 7 cycles take 2 days, sometimes it takes me a week.

My fairly complete pool math logs are available. - PoolMath Logs Make sure you are looking at the hot tub, my pool is the first set of logs.

Last water change (in December) it was 4 doses of acid over about two weeks - which is a bit of an anomaly, usually it balances faster

Davekro · Mar 16, 2024

JoyfulNoise said:
In a small hot tub, the effects of MA on TA should take just a few minutes if the water is circulating. The chemical reactions are nearly instantaneous and you're simply moving the water to homogenize everything. I say turn on the circulation pump, add the acid and then test about 5 minutes after adding to make sure you hit your target TA and pH.

Whenever I need to reduce TA, I add enough acid to lower the TA by an exact amount, usually 10ppm, and I don't focus on pH. I know, I know ... but the Pool School article says .... yeah, I know what it says, it's just not practical for large TA changes. When you lower TA, pH goes down as well but it's temporary. Outgassing of CO2 starts almost immediately especially in a hot tub where the water is warm and the aeration is extreme. You can easily drop the TA in 10ppm increments and the pH will recover very fast. There's nothing magical about an excursion in pH outside normal ranges because it's not going to stay there for very long. It's only when people leave the pH low or crash the pH and don't pay attention ... that's when the trouble begins.

But you do what you're comfortable with. I'm a chemistry nerd, so stuff like this doesn't bother me at all.

@JoyfulNoise ,
Thank you for this info. I just completed a more significant AHH!some purge than my initial one when filling the new 310 gal tub last month (details below). I am just starting my TA reducing cycles now. I will use Larger dose of MA than last time. I won't know a PH number below 7. The only PH color scales I find to see what colors (Taylor reagent testing) below 7 and above eight show green for PH between 7-8, so that is some other scale. Below PH 7 my Taylor K-2006 kit shows yellow, which I assume is in the 6-7 range. I wish I knew what colors were associated with PH 5, 4, 3 , 2. With my higher acid doses today, maybe I'll see the color change from light pink (7), to yellow to ???

What TA should I stop at? I had read before 70-80, but waterbear recommended 50-70 in his Bromine process. I think last time mine had gotten down to 30 ppm. I don't know if 30 is bad low, or very low TA is OK??. Anyway, what is a desirable TA for my newly filled, not heated yet water? I think I had read to do the multiple 'drop PH, Raise Ph' cycles before heating the tub. THEN heat the tub, THEN start adding your sanitization chemicals of choice.

If it does not short circuit either the 'TA dropping cycles' or the 'add initial sanitization' processes, can I start heating now during the TA dropping process? Hmmn, I guess when both pumps are aerating on high, I might not be raising water temp above the ambient air temp, just wasting kW's running the heater??

Yesterday's purge: In prepping to drain/refill to change from Bromine to chlorine sanitization, due to my wife's skin sensitivity I did: 1) 30 + 15 min pumps on high purge w/ one AHH!some gel pkt. 2) W/same water added 4.5 oz of Aqua Clarity, 30 min pumps (1/2 purge dose); 3) W/same water added 3 oz of Aqua Clarity, 30 min pumps ( 1/3 purge dose). With the 3rd 30 min purge cycle, there was very little, but none-the-less still some green biofilm on the shell in the corner the foam moved towards. Even after scooping away the bit of green foam and wiping the small shell area with scum, I continued scooping out foam for another 1o minutes, even though it was white foam that would dissipate quickly unlike the earlier larger bubbled foam that would tend to dissipate more slowly when jets were turned off. The white foam dissipated in ≈ 15 secs or less.

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When cycling to lower Hot Tub new water refill from 370 TA, how long to wait after adding MA to start 30 min aeration, or to test for the lowered PH?

Davekro

ajw22

JoyfulNoise

phonedave

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Davekro