You can't really tell. You have to estimate based on water replacement. You have to add more sodium bromide to make up any lost to splash out, backwashing etc. It doesn't hurt to have too much sodium bromide, so you can add extra just to be sure.

The test kits do not directly distinguish between chlorine and bromine, but when bromine combines with ammonia to form monobromamine (NH₂Br), it still shows up in a Free Chlorine (FC) test rather than as Combined Chlorine (CC) which happens when chlorine combines with ammonia. This is probably because monobromamine is in equilibrium with monobromammonium ion (NH₃Br⁺) that may release a positively charged bromine atom (Br⁺) that reacts with the dye in the FC test (see this paper for technical details).

Unfortunately, no one makes a test kit where you can add some very diluted ammonia to a water sample to make this distinction (where CC reading would mean chlorine while FC reading would mean bromine) and we haven't tried this out to see if it would really work anyway.

You could regularly test for CCs, especially after heavy pool use, to get an idea if there is any chlorine remaining. Even if the bromine bank is completely gone, it really shouldn't matter that much, because chlorine works fine, too. You could manage the chemistry such that the numbers work either way.

Watch for any chloramine smell or reports of swimmer complaints.

Remember that MPS will register on the CC test unless you use the interference remover.

So to implement this in a pool that's already been using bromine could one just add sodium bromide periodically while constantly feeding liquid chlorine with a peri pump at lower rates? While it's only happened once or twice, I have had complaints of minor skin irritation (only in the bromine pool/spa) and now I'm led to believe eliminating DMH would be a step toward eliminating them entirely.

Yes, I think that that should give better results than using tabs.Below is a reference to the method.


There are several possibilities that could cause swimmer irritation. It could be an adverse reaction to the DMH. It might be that excessive DMH is preventing the bromine from properly sanitizing the water, or it might be something else. I agree that minimizing DMH is a good idea.

What kind of a dosing schedule would I be looking at with the sodium bromide? My usual dosage for bromine is 2 bags (roughly 60 1" tabs each at 0.5 oz./tab = 60 oz. = 3.75 lbs) every 10-14 days, depending on bather loads, and I like to keep it at around 6ppm. As far as water replacement, I usually refill about ¾-1" every week (~381-510ish gallons), not taking backwashing into account which is usually done about once or twice a month, as needed.

Any ballpark estimates? Also, with those numbers, would you have a rough estimate of the amount of DMH currently in the pool? Thanks a lot, this is great info!

2.4 ounces of bromine tabs added to 24,000 gallons will add 1 ppm of bromine, 0.4 ppm of DMH and 0.25 ppm of bromide.

60 ounces of BCDMH tabs will add 25 ppm of bromine, 10 ppm of DMH and 6.25 ppm of bromide. If we use a 12.5 day period, then you are adding 2 ppm bromine, 0.8 ppm DMH and 0.5 ppm bromide per day.

If we assume 90 days since the last fill, then your DMH should be 72 and your bromide should 45 ppm.

If your bromide is at 45 ppm, then you won't need to add the initial 8 pounds of sodium bromide. You would just need to add 5.5 ounces of sodium bromide per 1,000 gallons of makeup water.

11 ounces of 12 % liquid chlorine will raise your bromine level by 1 ppm. If you need to add 2 ppm of bromine per day, then you would need 22 ounces of 12 % liquid chlorine per day.

Note: Edited to correct the amount of bromide.

Thanks a lot James, I really appreciate you calculating all that!

You say with ~90 ppm bromide I have enough of a bromide bank to skip the initial 8 lbs. What is the target size of the bromide bank? I know there's no upper limit, but how about a lower limit? I've seen some products intended for small-volume home spas that mention having a 30ppm bromide reserve. Does this figure apply with larger pools as well?

And, just so I have a better understanding, how did you come up with 5.5 oz. sodium bromide/1,000gallons? The CPO handbook has a handy chemical adjustment guide for a lot of chemicals (x amount chemical = xppm change in x amount of gallons) but it doesn't list sodium bromide and I can't find that info anywhere else.

Thanks again.

5.5 ounces per 1,000 gallons will give you a sodium bromide level of 40 ppm. Sodium bromide is 22.4 % sodium and 77.6 % bromide. 5.5 ounces of sodium bromide per 1,000 gallons will give you 31 ppm of bromide ions, which can be converted into up to 62 ppm of bromine.

The minimum bromide should always be at least half of the maximum bromine level you will want to achieve.

Here is an example: If your bromide level were 5 ppm and you added 220 ounces of 12 % liquid chlorine, then 110 ounces of the liquid chlorine would convert all of the bromide into 10 ppm bromine and the rest of the 110 ounces of the liquid chlorine would create 4.44 ppm of chlorine.

Thanks a lot James, I think I've got a slightly better handle on it. I'll give you a shout if I have any more questions on this. I can't say I've read much of anything about DHM before you brought it up.

Just another thing not taught in CPO training

I haven't thought through this completely, but is there no way to determine what the bromide bank is with this (bolded) type of information by doing a DPD test before and after adding the chlorine?

Nevermind....I thought it through now.

Unfortunately, you cannot easily distinguish between bromine and chlorine since they both show up in the Free Chlorine (FC) test. However, there is a way to tell if you have bromine vs. chlorine by adding ammonia to the water since monobromamine will continue to register as FC (because of its equilibrium with the monobromammonium ion which reacts with DPD dye) while monochloramine will not but will register as Combined Chlorine (CC) instead. As for the bromide bank, unfortunately the chloride salt tests do not readily distinguish between chloride and bromide.