Strange issue with Alkalinity and pH

TA is mostly a measure of the carbonates (especially bicarbonate) in the water and these not only buffer pH but are a SOURCE of rising pH in their own right due to carbon dioxide outgassing. Pools are intentionally over-carbonated to provide carbonate to protect plaster surfaces (calcium is added for this purpose as well since the water should be saturated with calcium carbonate). As shown in this chart, there is more outgassing at higher TA and at lower pH. The outgassing of carbon dioxide raises the pH with no change in TA. Adding acid lowers both pH and TA.

The extra aeration from an SWCG increases the rate of carbon dioxide and pH rise. If you have short pipe runs, then undissolved chlorine gas can also outgas and cause further pH rise. For an SWCG pool, the TA should be lower, usually closer to 70 ppm and certainly not as high as 100 ppm. The Borates provide additional pH buffering and are NOT a source of rising pH since they do not outgas. The borates do not change the amount of acid you add over time, but they slow the rate of pH rise down so you don't need to add acid as frequently. They also help to prevent calcium carbonate scaling in the SWCG cell since they roughly cut down the pH rise in half at the hydrogen gas generation plate.

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Originally Posted by chem geek

TA is mostly a measure of the carbonates (especially bicarbonate) in the water and these not only buffer pH but are a SOURCE of rising pH in their own right due to carbon dioxide outgassing. Pools are intentionally over-carbonated to provide carbonate to protect plaster surfaces (calcium is added for this purpose as well since the water should be saturated with calcium carbonate). As shown in this chart, there is more outgassing at higher TA and at lower pH. The outgassing of carbon dioxide raises the pH with no change in TA. Adding acid lowers both pH and TA.

The extra aeration from an SWCG increases the rate of carbon dioxide and pH rise. If you have short pipe runs, then undissolved chlorine gas can also outgas and cause further pH rise. For an SWCG pool, the TA should be lower, usually closer to 70 ppm and certainly not as high as 100 ppm. The Borates provide additional pH buffering and are NOT a source of rising pH since they do not outgas. The borates do not change the amount of acid you add over time, but they slow the rate of pH rise down so you don't need to add acid as frequently. They also help to prevent calcium carbonate scaling in the SWCG cell since they roughly cut down the pH rise in half at the hydrogen gas generation plate.

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Better post!

It's strange in the pool industry that they don't differentiate between pool types, whilst the figures that are adopted are widespread they don't really seem to be needed for vinyl liners as the surface doesn't require calcium. Everyones water is different but I don't have fountains or water falls and borates aren't allowed in EU so I run my vinyl pool around a TA of 40 and that requires barely any acid additions and it's only the sodium hydroxide used to stabilise the liquid chlorine that has any effect. On another vinyl pool TA is around 22 (no fountains or water features) the calcium hardness is also very low at 50-60ppm and there is no pH bounce, just the normal behavior.
As Chemgeek says TA is also a source of out gassing. Using a lower TA allows me to run a slightly lower pH at 7.2-7.3 getting a bit more killing power from the chlorine therefore using less chlorine. Well that's the theory.

Thanks everyone for all the detailed information. I checked the pool this afternoon, and got the following results using a Taylor K-2006:
Water temp: 71 F
FC: 7.0
CC: 0
pH: 8+
TA: 90
CH: 425
CYA: 80
Phosphate: 0
Salt: 2200
As you can see, the pH is back up to 8. Without adding borates, I'm going to work on keeping the TA at a lower level. chem geek suggested 70, so I'm going to shoot in that direction. It's probably going to take several weeks before the TA begins to drop due to adding liquid acid.

I'm also going to check it mid-week, say Wednesday, and report results again.

Checked it today - Saturday.
pH: 7.8. I checked it on Monday, and it was 7.0. Climbs up pretty quick in a week. If I waited and checked it tomorrow Sunday, it may have been at 8 again.