Originally Posted by
jereece
Thanks for the additional info. I have a few comments / questions.
1. I just purchased enough bleach (found it on sale) to last me the rest of this season. I normally close early to mid September. And the only local pool store I have had never heard of using bleach let alone selling it. When I asked from Sodium Hypochlorite, they looked at me like I was an alien. When I told them it was bleach, they still looked confused. So I can't find the higher strength stuff, but I have used at least 3 different brand names of bleach this year and seem to get similar results.
If they sell it at all, it's called liquid chlorine or chlorinating liquid.
2. I forgot to mention this in my past post, but my CA is in fact 10ppm. I have very, very soft water. Using my Taylor test kit, it only takes one drop to get a color change. I have asked if this is a problem on this board before and even Ben has said for vinyl pools low CA is not a concern. I thought about using some Calcium Hypochlorite for a while to increase it, but I can get bleach cheaper so I have just stuck with bleach.
OK, you've got a vinyl pool and that's true you don't need extra calcium, but it does change my calculations in subtle ways, but not too much.
3. I have not really noticed needing to add acid any less frequent at 120 TA than at 80. However I have not really been keeping records either. So it may be but I just have not taken notice. I still add acid about once a week.
What I was asking was the total amount of acid you add per week, but it sounds like that hasn't changed very much so that means that the outgassing of carbon dioxide is probably not very high and not the source of this rising pH problem.
4. Now for a radical question. If having high TA causes an imbalance in the water and if having low TA causes the pH to want to increase less, why not just go with little or no TA? I understand that pH would swing more, but from what I hear you saying it will tend to reach equilibrium and maybe the pH would increase 0.1 units per week. That would not be so bad. So why would I not just have very little or no TA so my pool would be at equilibrium?
If you had a plaster/grout pool then you would need the TA along with CH to prevent corrosion, but since you have a vinyl pool you don't need TA nor CH for that purpose. However, if you had little or no TA, then your pool's pH would swing HUGELY and you'd have to be adding acid every day if not more frequently!
If we assume, based on your numbers, that the additional "hidden" source of increasing pH is equivalent to roughly 1 ounce weight of caustic soda or lye per day, then every day you add chlorine to your pool your pH goes up 0.15 and the chlorine then gets used up during the day and your pH drops back down resulting in a net increase of 0.04 per day. This is at a TA of 120. At a TA of 80, adding chlorine has your pH go up by 0.23 and then the chlorine gets used up and your pH has a net gain of 0.06. At a TA of 10, adding chlorine would make your pH would go up almost 1.1 units! And yes, as the chlorine got used up your pH would drop back down for a net of 0.26, but that's a huge swing and is why you don't want very low TA in your system. Even adding acid every day wouldn't help the situation with a TA of 10. You'd have to add a very small amount of add acid almost every hour to maintain a reasonable pH! If you were willing to add acid every day along with your chlorine (not mixing them, of course), then you could go with a TA as low as 60 which would give a 0.30 increase when adding chlorine and a net gain of 0.07 which you would neutralize with about 3.5 fluid ounces of Muriatic acid per day. I wouldn't go any lower than this 60 TA, but I really don't see much benefit of that since it's more convenient to have the higher TA (say, 100) and add acid less frequently, but it's really up to you.
Thanks again for the interesting posts.
Jim
Bookmarks