I'm pretty comfortable maintaining my spa chemistry with generic chemicals and my Taylor Tech. K-2000 . I am a bit frustrated maintaining the balance between TA and pH but kind of expect it in 211 gallons.

A kilogram of MPS costs $25 here. It may be my most expensive chemical by mass, per dose and per time. My dealer's guidance was "a cap full after every soak". A cap full is ~2oz, ~$1.50.

Is there an objective test for MPS addition rate, please?

Welcome to TFP!

A rough rule-of-thumb when there is no ozonator is that every person-hour of soaking in a hot (104ºF) tub needs around 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% bleach or 7 teaspoons of non-chlorine shock (43% MPS). With an ozonator, roughly half of these amounts are nedded. Note that some MPS products are weaker than the standard of 43% so check the label.

If you are using bromine tabs in a floater, then that is usually set to a level to keep the bromine level low between soaks. If you use the spa regularly, you may have the feeder set higher in which case you don't need to add as much oxidizer manually as noted above. If you are not using a floater with tabs and are instead just oxidizing bromide salt to bromine, then the above amounts are reasonable.

However, the bottom line rule is to use enough oxidizer so that you always measure some residual disinfectant by the next time you soak. You don't want the sanitizer level to get to zero.

Thanks! I add NaBr once to 10 ppm and test to verify but not again, hence my concern about MPS rate.

The spa is used a bit less than 1 person-soak per day that provides a regular if not consistent dose of organics. The regular MPS addition should make the average [Br]~5 ppm.

Just note that your sig says you have an ozonator and in theory that should be creating more bromine. If it doesn't seem to be creating enough, you could add more sodium bromide to the water which should somewhat increase the bromine level created by the ozone. That can help reduce the amount of MPS you need. With an ozonator, you shouldn't need to add the 7 teaspoons (a little over 1 ounce) and certainly shouldn't need to be adding 2 ounces that your dealer prescribed.

So try increasing your bromide bank by adding more sodium bromide to the water to see if that lets you cut down on the MPS. Bromine does outgas somewhat so your initial 10 ppm of bromide isn't very much. You generally only start off with a low bromide bank if you are using bromine tabs in a feeder since they add more bromine directly. If you are just depending on your ozone and an oxidizer (MPS) to create bromine, then you need a larger bromide bank.

Sorry for this belated acknowledgment and thanks.