Ta Keeps Dropping

[Jankjo]

I am a new hot tub user, and my TA keeps going down below 40 even though the only thing I am adding is MPS.

At first my pH was rising alot, so I was adding a bit of acid. I started with a TA of 120, and my TA went down under 40 in about a week. I assume that the TA was dropping since I was adding acid.

Then I figured out I could turn off the aeration when I wasn't using the tub, and the pH has no longer been rising, and I have not needed to add any more acid.

I am still having trouble with the TA dropping below 40 and the only thing I am adding to the tub is MPS.

In the last two weeks the TA has dropped below 40 twice. (once to 20 and once to 30). I have been raising it back up to 60 because of the original problem I was having with high pH.

I am using a 3 part bromine system with a floater in a small tub 185-200 gallons (240 max capacity)

The CH is 200

I have been using mps as an oxidizer.

yesterday the TA was at 30. The tub needed water (which tends to raise my TA). I added water and a half tsp of baking soda. I tested the tub again today.

Currently the Bromine is at 5 and the pH is at 7.8. The TA is at 40

I have a high bather load probably 1 person hours a day in a small tub.

I understand that adding borates could stablize things for me, and I plan to do that. But I'd like to understand what is causing the TA to keep dropping. Does the MPS cause this?

Thanks

[chem geek]

Both bromine tabs and MPS are net acidic so they will lower the TA over time. As far as pH goes, there is a TA level and aeration amount that will balance the pH drop from the bromine and MPS vs. the pH rise from carbon dioxide outgassing. However, even at this point of pH stability, the TA will still drop from the net acidity, but at least at that point you only need to add baking soda to restore the TA and it usually doesn't take very much.

All the borates will do is have the pH move more slowly, but it won't change how fast the TA moves nor how much baking soda you would need. Borates are more helpful in spas using hypochlorite sources of chlorine (like bleach) where the TA is fairly stable but the pH can rise unless the TA is low so borates help to provide additional pH buffering in that situation.

I would suggest you find a TA level where the pH is fairly stable and then just add baking soda on occasion to maintain that magic TA level. 1.45 ounces weight (about 1 ounce volume, so 2 tablespoons) of MPS will lower the TA by 10 ppm in your 200 gallon spa. The bromine tabs also lower the TA, but probably not by very much since most of the bather load is handled by the bromine created from the MPS oxidizer.

Another alternative that would be much less expensive would be for you to use bleach as your oxidizer instead of MPS. If you only add it after your soak, there won't be any chlorine the next time you soak if you were worried about that (in fact, the chlorine will get used up creating bromine from the bromide bank probably well within an hour after your soak). If you use bleach, then your TA won't drop very much since only the bromine tabs will contribute to the lowering of TA. Your TA target for pH stability, however, will be low -- in the 40-50 ppm range -- and this is a scenario where the 50 ppm borates might be helpful.

[Jankjo]

This is super helpful! Thank you Chem Geek. Based on what you said, I think I might try the chlorine and the borates.

[chem geek]

Do you have an ozonator? If you didn't, then every person-hour of soaking would need roughly 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach or 7 teaspoons of MPS. It doesn't sound like you are adding nearly 2 tablespoons of MPS every day because if you did then the TA would be dropping by nearly 10 ppm each day. I am not accounting for the bromine from your tabs and perhaps you have the feeder opened up all the way.

[Jankjo]

I do have an ozonator. I have found that 1.5-2 tsp of MPS person hour in my tub brings the bromine back up. When I shock on the weekends, 1TBS brings it way over 10ppm. I have wondered about this because I had read about the 7 tsp. The feeder on my floater is barely opened up. I think I have it set to 1.5.

[arches2]

This is super helpful! Thank you Chem Geek. Based on what you said, I think I might try the chlorine and the borates.

This is what i do and it works great and is very stable. I have a bigger tub (350 gal) but i think it works great. My load is a little smaller, maybe only a couple hours total a week (20-30 minute 3-4 days a week) and my floater pretty much takes care of everything but the bi-weekly shocking. I add BS maybe twice a month.

[chem geek]

I do have an ozonator. I have found that 1.5-2 tsp of MPS person hour in my tub brings the bromine back up. When I shock on the weekends, 1TBS brings it way over 10ppm. I have wondered about this because I had read about the 7 tsp. The feeder on my floater is barely opened up. I think I have it set to 1.5.

OK, then that makes more sense. An ozonator usually cuts in half the required amount of oxidizer. A strong ozonator that is running more frequently can cut down the required oxidizer even more. Between that and your tabs, that explains why you are only needing 1.5-2 teaspoons instead of 7 teaspoons per person-hour. The bottom line is to add whatever is needed so that you measure a residual at the start of your next soak. The guidelines per person-hour are just that, guidelines not absolute rules, and they are only applicable to soaking in hot (104ºF) spas with no ozonator or other supplemental oxidation.

[justintime]

I have been having a similar problem in that my TA is also low - in the 20-30 range, but my Ph has been stable in the 7.4-7.6 range. I use a bromine floater and do have an ozonator. If I use baking powder to raise the TA, this also raises the Ph, and then when I try to lower Ph again by adding acid, this obviously lowers my TA again. It seems like a circular problem I can't work my way out of.

Is there a way to raise the TA without raising the Ph also? And what level is appropriate for TA? I thought around 100 was right, but reading the above makes it sound like ~50 is okay.

Is there any risk with getting in a tub with low TA but correct Ph/bromine/CH?

[chem geek]

If you add baking soda to raise the TA then the pH will only rise some unless your pH is very low. Add the baking soda slowly and without aeration. If you then add acid to adjust the pH, the TA will still be net higher. Acid doesn't lower the TA by very much unless you add a lot and you only do that cumulatively when combined with aeration in order to net lower the TA.

The problem is that if you are at a higher TA level, then there is more carbon dioxide outgassing that causes the pH to rise. So you can certainly get to a higher TA and normal pH in the short run. The problem is what happens after that. At a higher TA level, the pH will rise. Note that your bromine tabs are net acidic so over time will lower the TA so you pretty much have to keep adding baking soda to maintain that, but you do not need to raise the TA to 100 ppm. You shouldn't need to go much above 50 ppm if your pH is stable-to-rising. You only need a higher TA (for a non-plaster situation) if your pH tends to drop over time.

See how you do at 50 ppm. If that has some pH rise, you can consider using 50 ppm Borates as an additional pH buffer.

[justintime]

^ Okay, thanks. I will try to add some baking soda without aeration and see if that gets the TA up without raising Ph too much.

Is it harmful to get in the tub with a low TA?

[chem geek]

No, there is no harm getting in the tub with low TA. If it were very, very low, such as 20 ppm or less, and if there were no borates or other pH buffers in the water, then the pH may swing a lot depending on the chemicals you are adding to the tub. It won't be harmful in terms of soaking, however. The main reason higher TA is recommended by the industry is that they presume you will be using a net acidic source of disinfection/oxidation such as Dichlor, MPS or bromine tabs. Any of these net acidic chemicals will lower the TA over time and if the TA then gets very low or to 0, the pH can crash down and harm equipment via metal corrosion from the low pH.