How to measure how much bromine to add

tm8747a

Active member
May 17, 2021
31
Ottawa, ON, Canada
So right when everything was great and I had mastered my pool chemistry... I got a hot tub! And now I'm reading and am more confused than ever. I went to the store to get bromine, the person explained I could get either tabs but then I had to buy a second product to activate, or I could just buy granulated product and all I had to do is add that, but I'd probably need to do it more often. I got the granulated product, but now I'm here reading about bromine banks, how you actually would just add bleach, and I'm confused even after reading the sticky thread. What I got says active ingredients are Available Chlorine (present as sodium dichloro-s-triazinetrione) 52.7% and Sodium-bromide 14.6%. Do I need anything else? If not, given I need to keep the hot tub around 4-6ppm bromine levels, how do I calculate how much to put in after testing. I did seem to understand I can test using the Taylor kit and multiply the free chlorine result by 2.25, so am good there, but still don't know what that means as far as what or how much I should add after each test.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
18,826
Tucson, AZ
Pool Size
16000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
The granular chemical you have will work as it is basically dichlor (a source of oxidizing chlorine) and sodium bromide (a source of bromide ions). When you add it to water, the chlorine will react with the bromide to convert it from its inactive state (-1 oxidation state) to its active bromine form (+1 oxidation state). The chlorine simple gets reduced to chloride salt in the process. As is with most spa chemicals, its a costly version of the much cheaper method of buying sodium bromide granules online (it's a cheap, harmless salt) and using regular store bleach to activate it. All you need to do is add enough sodium bromide to create a 30ppm bank of bromide ions and then use bleach to activate it. For all intents and purposes, you can simply define what bromine level you want, then divide it 2.25 and that's the amount of FC you need to add to get there. You can then add that amount of bleach and then retest to see how close you got to the bromine level you wanted. It's a bit of trial error in the beginning to circle in on what you need to add but you'll eventually get to know how your tub reacts and after a while it will be easy.

You can also use dichlor granules in the same way but it does add CYA to your water which won't affect the bromine but it can change the pH/TA balance a little bit. Dichlor is also mildly acidic so it will lower pH with use. Bleach is relatively neutral in terms of pH and doesn't add anything to your tub water.

In order to get 30ppm Br- bank in 420 gallons of water, you need about 61.3 grams or 2.2 ounces of sodium bromide. It's really not super necessary to be that accurate as the bromide bank is just there to act as a reserve of bromide ions so it could be 30, 40, 50ppm or whatever is easiest to add. 30ppm just ensures you have enough bromide in the water so that if you want to shock the tub, you can get up to the 20ppm bromine level easily. Bromide doesn't go away so you should only need to create the bromide bank once per fill and then it's good to go.

Stay away from bromine tablets or reserve them for use only when you'll be away from the tub for significant periods of time. Brominating tablets tend to dissolve too quickly and they are acidic. So there tends to be wilder fluctuations in bromine levels as well as pH issues. They also add DMH (dimethyl hydantoin) to the water which acts a lot like CYA and, at high enough levels, can reduce the effective strength of the bromine leading to sanitation issues.
 
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tm8747a

Active member
May 17, 2021
31
Ottawa, ON, Canada
Thanks a bunch for the detailed response. So I think I'm getting closer to an understanding, still running into some challenges though. So in regards to the above, my one question regards the bromide bank. The product I have has both sodium bromide and dichlor, so can I use it to build up this bank or will that not work? Is it normal that with this product I literally have to put some in every single day, otherwise my FAS-DPD test is always 0 the next day, even though it might be over 20 20 minutes before I add chemicals?

The second thing is, does this product make it hard to maintain TA and PH? Because I find myself constantly having to increase my TA and lowering my PH. If I drop my PH to 7.4, it's almost guaranteed the next day it'll go up to 8. I thought the water would acidify, but the reverse keeps happening. My TA also drops, I had brought it to 120 and a couple days later it was back down to 70.