different bromine levels with different tests

[Schmath]

I can't for the life of me keep up with my hot tub. I probably could if I test every day, but we sometimes go weeks without using the tub, and I forget to add bleach, and then the tub sits at 0 chlorine for long periods of time. I've decided to try bromine, so I can put a floater in there and hopefully not have to add sanitizer as often when it's not in use. I added the bromine reserve yesterday. There was some MPS in there already, so I think some of it activated the bromine, bringing it up to 8ppm bromine. (according to DPD test. 16.25 according to FAS-DPD maybe there was still mps in the water?) Then 4 teenage boys used the tub, completely depleting the bromine, (and I'm assuming depleting any mps as well.) It took 2 cups of bleach to get the bromine back up to 10 afterwards. Can I assume the mps is gone? I'm wondering because my bromine readings are different depending on if I use the FAS-DPD test or the DPD test. The FAS-DPD test is giving me much higher readings. Is there anything else that would cause this? Wouldn't the MPS have all turned to bromine by now? I just tested again and my DPD test said 4, while my FAS-DPD test said 9. What is going on? I'm using the same tests I used for chlorine, just multiplying the chlorine measurement by 2.25. Maybe my reagents are too old? Some of them I bought in June 2022 and some are from June 2021. It's going to be painful to replace them all!

 

[Mdragger88]

The dpd test has its limitations- use the fas dpd test for accuracy & testing higher sanitizer levels.
I see you have a swg for your pool - you should try one for the spa to help with the daily standby chlorination when using chlorine in the tub.
I have a saltron mini & wouldn’t be without one. It takes care of all my standby fc & even short soaks.
Incase you haven’t already you should do an Ahhsome purge to rid your plumbing of biofilms so you can start off fresh. This is something that should be done every 3-6 months as a part of regular maintenance.
Here’s the guide for bromine

How do I use Bromine in my Spa or Hot Tub?

Originally Posted by waterbear There is a lot of misinformation on the proper way to use Bromine so I hope this primer is helpful. First, you need to establish a bromide reserve in the water. Bromine tablets can do it by themselves but it can take literally weeks until enough dissolves. Some...

www.troublefreepool.com

& the one for chlorine

How do I use Chlorine in my Spa or Hot Tub?

If you have a question about any information within this Sticky, please start a new thread for yourself where you will receive very quick responses to your specific questions. Thank you! Introduction There are two main things to consider when maintaining your hot tub water, Water Balance and...

www.troublefreepool.com

There’s not a ton of bromine users here so there’s a bit more support for chlorine users.
Either method requires frequent testing or you end up with a sess pool because hot tubs are literally people soup.