PoolMath calculator for a hot tub?

Acmfl

Bronze Supporter
Apr 7, 2014
170
Tampa area, Florida
#1
Stupid question maybe. Can I use the pool math calculator to determine amounts of MA for my hot tub, or is just for pools? I have a hot springs spa that I want to start using the TFP method on. My bromine is always good but my ph and TA are always out of whack and I want to start addressing it since I bought the test kit and I have been working on the new pool. I use bromine tabs with the spa time shock but I have not been shocking it regularly, that is changing as of today! My husband primarily uses the hot tub, I get in occasionally. We rarely have guests in it but always shock after heavy loads. We replace the water about every 3-4 months and use granules after refill. My question has to do with how much MA to put in to lower the ph and TA.

I just used the last of the shock, so my bromine is at 10. Here are the rest of the numbers. My HT is 525 gallons.
PH 8.2
TA 140
CH 275
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
14,689
Tucson, AZ
#2
See the PoolMath Link above at the top of the page. Simply change the volume of the water to 525 gallons and it will calculate everything you need.

Also, see these stickies for how to operate a hot tub -

How do I use Bromine in my spa (or pool)?

How do I use Chlorine in my Spa (or pool)?

If you're happy with bromine, there's no reason to change. But many here use the dichlor-then-bleach method which is a lot cheaper than bromine. Up to you.

- - - Updated - - -

I will say this, based on your numbers, your TA is quite high and it's one of the reasons why your pH is always so high. TA needs to be much, much lower in a hot tub because of the aeration from the jets releasing CO2 from the water. The outgassing of CO2 is what causes pH to rise. With your bromine tub and if you are using tablets, I would drop the TA to 80ppm in order to keep the pH control better. You can also use 50ppm borates (by adding boric acid granules) in the water to help suppress the pH rise (borates are extra buffers in the water that work well against pH rise).
 
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Acmfl

Bronze Supporter
Apr 7, 2014
170
Tampa area, Florida
#3
Ok great. Pool math is saying to add 3 oz of MA to get the ph down to 7. I know from the pool high TA struggle it may take a while to get it just right. The problem is our fill water has pretty high TA to start with. Thanks for the help!
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
14,689
Tucson, AZ
#4
The only thing you could do to change the fill water is to use an RO system to generate the water. That would be the super expensive way to do it as 550 gallons of RO'd water would take nearly 800 gallons of tap water and a lot of energy to generate it. The other option is the acid/aeration method to lower TA. TFP's Pool School has an article on how to do that process.


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Acmfl

Bronze Supporter
Apr 7, 2014
170
Tampa area, Florida
#5
Well the quest for perfect hot tub water has begun officially as of today! I'm taking over from my hubby who basically puts bromine tabs in when the floaty tips over...lol! Well he also drains, refills and replaces components as they break (like a recent new heater) so I can't complain too much! Plus the freaking thing is 16 years old, we got it for free from a friend in 2012 because it wasn't heating. My husband ended up replacing some $180 part - I think a heater sensor or something. He loves it. I will love it more when the water is perfectly balanced. Thanks again for the help.
 
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Acmfl

Bronze Supporter
Apr 7, 2014
170
Tampa area, Florida
#10
I tested early this morning and added another 3oz of MA, then aerated for an hour. I just tested again and I'm at 50 TA and 7.5 ph, so I'm happy for now. I'll keep an eye on the ph daily. I ordered gentle spa, and more ahhhsome (sp?) for our next water change next month. Thanks for all the help!