When To Add Mps - Before Or After Soaking?

[PaulaME]

Just completed a refill. Testing with a Taylor test kit the results are - CH-150, TA-120, PH-7.6, after adding 2 oz. of 100% Sodium Bromide and 1 oz of MPS the bromine is 6. I'll be using 1.3 dibromo-5.5dimethlyhydantoin (brilliance) in a floater. I've gotten conflicting advice on whether to add MPS weekly only or use small quanities daily (the tub isused 5-6 times a week by 1-2 people). Also getting conflicting advice on whether to add the MPS before soaking or after. The tub is 500 gallons.

[Jankjo]

I found with my bromine tub that it worked best to add MPS after soaking. I have a small tub, and the bromine levels would drop after a soak. I didn't need to add that much to bring the levels back up. ~tsp per person per 30 minutes. On the weekends I would shock with Chlorine (bleach).

[chem geek]

Assuming you are measuring some disinfectant level (bromine or chlorine, depending on what you are using -- bromine in your case) just before you soak, then you generally add an oxidizer (when using bromine) or disinfectant (such as chlorine when using chlorine) after your soak. This is because most of the demand will come from your soak and can get oxidized afterwards. You could dose some before, but the higher disinfectant level may be irritating. If you were soaking with strangers and was concerned about person-to-person transmission of disease, then starting the soak with a higher level of disinfectant would be a more conservative thing to do.

Do you have an ozonator? What temperature is your spa? Usually you would need 7 teaspoons of MPS per person-hour of soaking so 3-1/2 teaspoons for each person for 30 minutes. 1 teaspoon normally wouldn't be enough, but you say that you do read bromine the next day? Perhaps your bromine floater is putting out enough to keep up and/or your ozonator is reducing demand (usually it cuts it in half).

[PaulaME]

I do have an ozonator and the temp is 100-101. I was planning on a 1/2 oz. of MPS after each soak (1-2 people for 30 min.). I'm under the impression as long as I have some bromine reading, even a 1 or 2, I'm OK as long as I add the shock after.

[chem geek]

Ahhh. With an ozonator you don't add as much oxidizer. So yes, you are correct that as long as you have a bromine reading you are OK. Since you usually want around 1-2 ppm FC at the start of a soak you should shoot for 2-4 ppm bromine since the units of measure between the two are roughly a factor of 2.

[PaulaME]

Thank you. Now I'm curious... just how high should the bromine go after I put the shock in? Since I found this forum, changed the water and got a real test kit, I think I'm getting some understanding of what I'm doing! I wish the spa sales people knew half as much and didn't just send you home with a bottle of test strips and a bucket of bromine.

[chem geek]

If you are talking about the extra dosing after your soak, the amount that the bromine level goes up is irrelevant. What matters is that the quantity of bromine plus what the ozonator is doing are in combination sufficient to oxidize all of your bather waste. You'll know if it's enough if you still get a reasonable 2-4 ppm bromine level 24 hours after your soak and dosing. If you use too little, then the bromine level will be too low. If you add too much, then it will be too high. You'll quickly get a sense for the right amount and it will be proportional to how long you soak and how many people soak (i.e. person-hours).

In between soaks, the ozonator will likely produce enough bromine from your bromide bank to maintain the bromine level. Ideally you'd be able to control the ozonator on-time to be able to make sure that happens, but that may not be possible (except by modifying circulation pump run time, if that's changeable).