Shock And Aeration

[Vermonter]

So now you guys have me thinking about aeration... well, sort of. This is (probably) the last question on the topic, so here goes: when shocking a spa, specifically with MPS, is it more effective to do so with the aeration on, off, or does it make no difference? I'm talking about the hour after you add it when the cover is off for oxidization. Thanks in advance (once again)

Tony

[Hillbilly Hot Tub]

So now you guys have me thinking about aeration... well, sort of. This is (probably) the last question on the topic, so here goes: when shocking a spa, specifically with MPS, is it more effective to do so with the aeration on, off, or does it make no difference? I'm talking about the hour after you add it when the cover is off for oxidization. Thanks in advance (once again)

Tony

Although I don't think it matters much, although chemically adding MPS with a low PH and turing on the air which can increase the PH you would think they would wash each other out a bit.....but, IMO I turn on the air to mix the chemicals really well into the water then turn off the air to reduce cooling of the water and debris such as pollen and dust being pulled into the tub via air lines. Someone may disagree with me on that one.

[Vermonter]

Well, I like your logic... That's pretty much what I would think, too. Thanks!

[chem geek]

If you added 1 tablespoon of MPS in 350 gallons which is roughly equivalent in oxidizing power to 4 ppm FC, this would only lower the pH from 7.5 to 7.46 (at a TA of 100 ppm) so is not a big deal at all. I think it would be just fine to add it with the circulation pump running but the aeration jets off. Your pH will likely rise on its own from normal outgassing or from when you next use the spa anyway. Of course, this analysis depends entirely on how much MPS you use.

Richard

[Vermonter]

If you added 1 tablespoon of MPS in 350 gallons which is roughly equivalent in oxidizing power to 4 ppm FC, this would only lower the pH from 7.5 to 7.46 (at a TA of 100 ppm) so is not a big deal at all. I think it would be just fine to add it with the circulation pump running but the aeration jets off. Your pH will likely rise on its own from normal outgassing or from when you next use the spa anyway. Of course, this analysis depends entirely on how much MPS you use.

Richard

For whatever reason, I use A LOT of MPS... 2 oz. after every use, typically 2 people for 40 minutes each session, sometimes more, sometimes less. I've tried skipping a dose, and it almost alway comes back to bite me in the bum (cloudy, smelly water), so I figure better safe than sorry.

Probably largely due to this, my pH actually tends to drop throughout the week; I have been starting the week with a pH of 7.8 and TA of around 70, and by the next week I'm usually at pH of 7.2 and TA of 50. Right now (today being my chemistry day ), I'm actually starting the week off with a pH of 7.5 and TA of 100. We'll see how it goes.

Interestingly, it's been used once in the last 2 weeks (it's just too hot!), and got the usual 2 oz. dose on that day, and yet the pH and TA still dropped, although it seems they dropped the same amount they usually drop in a one week period over two weeks.

Tony

[chem geek]

Tony,

Your required MPS usage makes sense assuming your hot tub is around 350 gallons because it takes around 7 ppm FC per person-hour of soaking to oxidize the ammonia/urea from sweat/urine. 40 minutes * 2 people = 1.33 person-hour so a demand of 9 ppm FC so a little over 2 tablespoons or one ounce volume (1.35 ounces weight) of MPS. For whatever reason, you need even more than that (about double the dose).

2 ounces volume of MPS in 350 gallons over one week starting at a pH of 7.8 and TA of 70 would result in a pH of 6.9 and TA of 60 so you've probably got some pH rise from aeration to end up at a pH of 7.2. I think that if you use a higher TA level then you can find a balance between the MPS and outgassing of carbon dioxide so that the pH remains pretty stable. Then you'll only need to add baking soda as the TA goes down each week.

I can't explain the larger 1-week-sized drop from one dose.

Richard

[Vermonter]

Tony,

Your required MPS usage makes sense assuming your hot tub is around 350 gallons because it takes around 7 ppm FC per person-hour of soaking to oxidize the ammonia/urea from sweat/urine. 40 minutes * 2 people = 1.33 person-hour so a demand of 9 ppm FC so a little over 2 tablespoons or one ounce volume (1.35 ounces weight) of MPS. For whatever reason, you need even more than that (about double the dose).

2 ounces volume of MPS in 350 gallons over one week starting at a pH of 7.8 and TA of 70 would result in a pH of 6.9 and TA of 60 so you've probably got some pH rise from aeration to end up at a pH of 7.2. I think that if you use a higher TA level then you can find a balance between the MPS and outgassing of carbon dioxide so that the pH remains pretty stable. Then you'll only need to add baking soda as the TA goes down each week.

I can't explain the larger 1-week-sized drop from one dose.

Richard

Hmm, well perhaps I've just been over-doing it with the MPS, although I'm fairly certain I've tried using 1 oz doses and still ended with the stinky water, but that could have been a result of something else (not enough Bromine being released from the feeder, for example). I'd love to be able to cut my MPS use in half, so perhaps I'll give it a go and do a little experimenting (which, like I said, always gets me in trouble, but what the hell). Yes, the tub is 350 gallons by the way. And I've been using 2 oz by weight, so perhaps I'll just cut it to 1.35 oz and start from there.

Thanks again Richard.

Tony