What causes Bromine (or chlorine) level to drop?

[dooger54]

So with the help of this forum I have been getting my hot tub water levels right. After purging with Ahh..some I refilled and started over. Mine is a bromine tub, so I added the recommended level of sodium bromide to establish my bromine bank. I also added a cup of liquid bleach right away. After about 4 days of adjustments, my numbers today were-

Ph - 8.0. Added dry acid to bring down to 7.2 or so
TA -60. Was low so have been adding Alkalinity Up
Bromine 8.0.

My Bromine level has been hovering between 7.5 and 10. I have not added any bleach since the initial cup four days ago. We have used the tub twice, with a total of about 1.5 person hours of use. I keep the cover on at all times when not in use. The capacity of the tub is 250 gallons.

My “newbie” question is how does the Bromine level go down? I
Understand my target should be around 4-6.

If I don’t use it will the Br level go down naturally with time?

I assume that using the tub will bring down the Br level? That’s why I would normally add a small amount of bleach after every use?

Thanks for the help. Hopefully I’ve made myself clear [emoji6]

 

[Chrispy]

Following!

 

[JamesW]

Bromine goes down with time. Bromide can be reactivated with bleach, ozone or MPS. If you have an ozonator, your levels might seem to stay higher than you might expect.

 

[dooger54]

I do have an ozonator but it is original to the tub and therefore 12 years old. My understanding is these quit working after about 1-2 years. So I can assume it is not working.

 

[JoyfulNoise]

The ozonator is very likely dead.

You recently purged the tub with Ahh-Some so it is squeaky clean, ie, not a lot of gunk for the bromine to oxidize. The bromine will come down with time. If you want it to come down a little faster, open the lid and turn the heater and jets on for a while. The aeration should help by outgassing bromine. The Henry's Law constant for hypobromous acid is about 5 times higher than hypochlorous acid and since there's nothing binding the bromine up (DMH from bromine tablets) then there is a significantly higher outgassing rate of bromine versus chlorine.

As a side note, while liquid muriatic acid is a bit more harsh on the nose than dry acids, you really don't want to use dry acid. It's sodium bisulfate so, like MPS, you're building up sulfates in your tub water which is not good for heater elements. You can get half-strength (~15%) muriatic acid, so called "green" MA, at hardware stores and it has hardly any odor to it. Since you tub volume is on the lower side, the difference in dosing volumes is not all that terrible to deal with.

 

[dooger54]

The ozonator is very likely dead.

You recently purged the tub with Ahh-Some so it is squeaky clean, ie, not a lot of gunk for the bromine to oxidize. The bromine will come down with time. If you want it to come down a little faster, open the lid and turn the heater and jets on for a while. The aeration should help by outgassing bromine. The Henry's Law constant for hypobromous acid is about 5 times higher than hypochlorous acid and since there's nothing binding the bromine up (DMH from bromine tablets) then there is a significantly higher outgassing rate of bromine versus chlorine.

As a side note, while liquid muriatic acid is a bit more harsh on the nose than dry acids, you really don't want to use dry acid. It's sodium bisulfate so, like MPS, you're building up sulfates in your tub water which is not good for heater elements. You can get half-strength (~15%) muriatic acid, so called "green" MA, at hardware stores and it has hardly any odor to it. Since you tub volume is on the lower side, the difference in dosing volumes is not all that terrible to deal with.

So here is where the local pool store messes me up again. I was in recently and they told me I could not, under any condition, use liquid acid in my acrylic spa. That’s why I am using dry acid.

 

[JoyfulNoise]

So here is where the local pool store messes me up again. I was in recently and they told me I could not, under any condition, use liquid acid in my acrylic spa. That’s why I am using dry acid.

Because selling you dry acid costs you 2X as much and is profitable for them. Selling you liquid muriatic acid, and competing with hardware stores, is not.

There is absolutely no reason you can not use muriatic acid aside from being careful with it. If you add it, make sure you add it while the circulation pumps are going and be sure that it mixes well (it's more dense than water and so it can pool in a stagnant tub). As long as you add it in front of a return jet with fast moving water, it will mix just fine. Using muriatic acid increases your tubs chloride content, but over a few months, the increase is negligible. Chloride ion is not nearly as bad for the heater as sulfates are.

You've now discovered why TFP exists - to debunk a majority of the nonsense out there that masquerades as "good advice" ...