First, thank you to this Forum for teaching me how to use bromine. Chlorine was a pain and didn't smell good. The combination of sodium bromide, a brom-tab floater, and regular doses of non-chlorine shock has worked very well.

my questions:
1. initial loading with NaBr: for a 300 gallon spa, how much should I put in?
2. How do I know when the NaBr level is getting low; i.e. how often should I re-load?
3. My store guy told me to throw in a capful of potassium peroxymonosulfate shock after every spa use. Is this over-kill?

Thanks in advance.

happy 1995 Hot Springs Sovereign spa owner

A cap full of MPS after each use was also recommended to me. That's about 2 ounces (I measured).

Without delving into the NaBr formula-weight math, my package says 1 ounce per 100 gallons equals 60 parts per million. I load to 10 ppm = 1/3 ounce NaBr on fill. 10 ppm bromine is the highest maximum concentration that I have seen, so I'm sure I won't exceed it. Even when you can't measure your bromine concentration, because it's present as bromamine, bromamine is a good sanitizer itself.

The Bromine never goes away. The NaBr disassociates in water to Na and Br ions, the Br ions forming the "Br ion bank." The Br ions are oxidized by the MPS, in our case, to hypobromous acid HOBr that degrades the bacteria and organic contaminants in your spa, forming Bromamines that disassociate in water to make the Bromine ions available to the next dose of MPS/oxidizer. (From Pool & Spa Water Chemistry, Taylor #2004B)

When you measure your Bromine concentration low, that means you need a dose of oxidizer - presuming a proper Bromine ion bank. The bromine test that I use (Taylor R-0001 & R-0002 compared to their color standard) as I recall is interfered by high pH.

Roughly speaking, every person-hour in a hot (104ºF) tub without an ozonator and no automated feeder requires around 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach or 7 teaspoons of non-chlorine shock (43% MPS) to oxidize the bather waste. 7 teaspoons is almost 2 ounces so that sounds about right for one person-hour of soaking. There will be some bromine level being maintained by the bromine tabs in the floating feeder so you shouldn't need as much as I just indicated, but it's at least a rough starting point if you have your feeder set low because you don't use the spa frequently.

The basic rule is just to use whatever amount of oxidizer is necessary to maintain a bromine level in the spa, probably at least 2-4 ppm bromine. If you soak every day, you may be able to turn up your feeder to handle the bather waste. If you don't soak as often, then you'll probably need to add an oxidizer after your soak.

Thanks to Doug and ChemGeek.
I am still not clear on the proper amount of NaBr to add on the initial load, when the water is replaced.
For 60 ppm I would need 3 ounces (by volume of dry granules, I assume) but for 10 ppm only 1/6 of that amount?
Which is correct?

As noted in Using Bromine in a Spa, follow the manufacturer's directions on dosing which usually results in the equivalent of 30 ppm bromide. However, the exact amount isn't that important since the 3-step bromine process uses bromine tabs that will be adding more bromine (which becomes bromide) on a regular basis. You just need enough bromide to get started so that the chlorine in the tabs or in the chlorine or MPS that you add after a soak has something to oxidize into bromine. Anything over 10 ppm will be fine so long as you are using tabs.

If you were using a 2-step bromine process where you are only using sodium bromide and manually shocking with chlorine or MPS without any use of bromine tabs, then a higher amount would be better so that you don't run out of bromine (some will outgas or be carried out in swimsuits or filtered out with organics).