Fast pH Rise in hot tub - with measurement spreadsheet!

[HydroxicAcid]

Targets: Bromine 4ppm, pH 7.5, TA 100ppm, CH 170ppm, Borate 50ppm, Temp 100F
Using: bromine tablet floater, dry acid, calcium chloride, borax, bleach.
The hot tub has an ozone unit, but I'm not convinced it works. It's very old and the tubing looks sealed at the kinks.

Tests: Taylor drop kit for everything except borate (strips).


I am finding it impossible to keep the pH steady at all. It seems to like to be at pH 7.8-8.0. If my TA is at 100 or higher I can adjust the pH to 7.4 and it will be 7.8 the next day. If my TA is at 50 or below I can go a few days before the pH gets up to 7.8, but not a full week. Usage is 2 people x 60min per week or less. Tub is a 320 Gal Sundance Rio that is about 20 years old. It has a small continuous pump and four times a day it runs the air bubbles for 1 minute and the main pump for 20 minutes. I need to add about 20 gallons of water a week but there doesn’t seem to be any leaking. Probably the lid is just bad. Is pH important as long as CSI and LSI are okay? Why would I have such trouble with ANSI-recommended values? Shouldn't I be able to set up a buffer system that accommodates my usage scenario? I have attached a text file (csv) that imports easily into a spreadsheet, which contains all my measurements over the past months. If you can see what's going on from there I'd really appreciate an explanation. I do have a background in chemistry but I have only been running a hot tub since October.

 

[cowboycasey]

Welcome to TFP

Can you stop the air bubbles, no need for them to run so much, they are raising your PH every time they come on..

With your TA around 50/60 you may get away with 1 week not adding muriatic acid to drop PH, with the air bubbles off until use you will probably be able to get 2 or 3 weeks before needing MA..

Ph is very important, it can do some major damage if too high or low for an extended amount of time..

the other issue is the amount of water you have, it is under 350 gallons and just does not have the ability to stay balanced like a spa with more water..

Mine stays a little more stable but I am dealing with almost 700 gallons, your trying to find that sweet spot in water balance and that sweet spot is very narrow...

my hot tub also had a 20 minute timer that ran once a day, I turned it off because I also use mine as much as you do, almost every night if not every night

I hope this helps

 

[HydroxicAcid]

Can you stop the air bubbles?

I can't seem to stop the air bubbles completely, but I may be able to reduce the number of times the cycle runs.

With your TA around 50/60 you may get away with 1 week not adding muriatic acid to drop PH, with the air bubbles off until use you will probably be able to get 2 or 3 weeks before needing MA..

That's a huge difference. I didn't think that 1 minute * 4 times a day would bring up the pH that much.

Ph is very important, it can do some major damage if too high or low for an extended amount of time..

Is "too high" and "too low" absolute or is it relative to other factors? pH 7.8 would cause scaling with a TA of 200 but corrosive at a TA of 10 (According to TSI/CSI calculations). That said I probably wouldn't want to sit in a hot tub of pH 9 for too long whether or not the tub could handle it.

my hot tub also had a 20 minute timer that ran once a day, I turned it off because I also use mine as much as you do, almost every night if not every night

We're only using the hot tub once a week, or maybe even once every two weeks. I hope to get to use it more in the near future but for now I just can't get to it.

 

[wjr75]

When I only keep my hot tub open at the end of the season I add 50ppm of boric acid "pellets" from Duda diesel. This helps slow down the ph rise.

 

[pooldv]

Correct. Lower TA to 50ppm and add 50ppm of borates.

More here in the TA/PH section, How do I use Chlorine in my Spa (or pool)?

 

[cowboycasey]

That's a huge difference. I didn't think that 1 minute * 4 times a day would bring up the pH that much.

I know it sounds like very little, but that combined with the pumps running 1.2 hours a day creating turbulence on the surface of the water, that is exactly how we raise PH in a pool, it just takes longer because of more water..

I have not gone to borates yet, I believe because I have more water than most and I have my TA lower I have found a sweet spot for my tub

 

[HydroxicAcid]

When I only keep my hot tub open at the end of the season I add 50ppm of boric acid "pellets" from Duda diesel. This helps slow down the ph rise.

Correct. Lower TA to 50ppm and add 50ppm of borates.

Right, for this reason I currently have 50ppm borates in my spa from Borax which I monitor with test strips.

I did have more success with a 50ppm TA than with 100+ppm but that's pretty far off most recommendations. 100-120ppm is a fairly common recommended range. What do you make of this discrepancy? Could there be unanticipated side effects?