How Low Can The Ta Go?

[G-Dub]

I have a 175 gallon hot tub, and I'm using the 3 step bromine method with a bromine floater.

I always seem to struggle with climbing pH. Keeping my TA near 50 or so leads to my pH climbing to 8.0. I have added 50ppm borates as well when the pH was 7.6

Today my settings are

pH 7.8

TA 30

CH 120

FC 4.0 (by Taylor drop test, converting to bromine actually about 9)

My water is crystal clear, last water change about one month ago, filter changed today.

My bromine was a bit high, because I had just shocked with bleach.

Can I leave this TA where it is? My plan was to add some calcium to get the hardness up to 180 or so to get the saturation index closer to 0 (still will be slightly negative)

[waterbear]

I have a 175 gallon hot tub, and I'm using the 3 step bromine method with a bromine floater.

I always seem to struggle with climbing pH. Keeping my TA near 50 or so leads to my pH climbing to 8.0. I have added 50ppm borates as well when the pH was 7.6

Today my settings are

pH 7.8

TA 30

CH 120

FC 4.0 (by Taylor drop test, converting to bromine actually about 9)

My water is crystal clear, last water change about one month ago, filter changed today.

My bromine was a bit high, because I had just shocked with bleach.

Can I leave this TA where it is? My plan was to add some calcium to get the hardness up to 180 or so to get the saturation index closer to 0 (still will be slightly negative)

Bromine is effect in a pH range up to 8.0 so you are fine. When the pH hits 8.0 then drop it a bit but if it stays below you are fine. Personally I would raise the TA a bit since bromine tabs are slightly acidic and can cause the pH to 'crash' when the TA is very low. I ususally keep the TA in a bromine tub arounjd 100 when tabs are used.

pH in a hot tub WILL rise no matter what because of the constant aeration of the water.

[chem geek]

The rise in pH from outgassing of carbon dioxide would only stop when the water and air were at equilibrium and that would only occur at a TA of around 9 ppm (with no CYA) if the pH is 7.5 so this is impractical for the reasons waterbear gave -- you want some carbonate buffering to prevent the pH from crashing too low. The borates are a good buffer against a rise in pH, but they have less capacity against a drop and bromine tabs are acidic.

For reference, the equilibrium amount of carbonate alkalinity (TA adjusted for CYA and borates) is as follows at various pH (and at 104ºF):

pH Carb.Alk.

7.0 ... 2.8

7.1 ... 3.5

7.2 ... 4.4

7.3 ... 5.6

7.4 ... 7.0

7.5 ... 8.8

7.6 .. 11.2

7.7 .. 14.1

7.8 .. 17.8

7.9 .. 22.6

8.0 .. 28.6

So you can see that as the pH rises, the equilibrium TA level goes up a lot which means that you are better off setting a higher target pH of around 7.7 to 7.8 so when it rises to 8.0 you only lower a bit and not down to 7.5 or lower since that would just have the pH rise more quickly.

You could try a TA of 50 ppm (rather than 100 ppm that waterbear mentioned) which should still be fairly ample and is where you were before, but try not to fight the pH quite so much (i.e. let it get to 8.0 before lowering it just to 7.7). It's still a bit surprising that with your use of bromine tabs and with 50 ppm borates that your pH is rising so rapidly. I suspect you were trying to bring the pH down too far to 7.5 or lower. Try letting it settle in higher and adjust from there.

Your smaller tub at 175 gallons means the surface area to volume ratio is higher so outgassing has a greater effect on making the pH rise.