Saturation Index on the edge despite reasonable targets?

[PhillipH]

Using the Pool Calc, I've typed in the following target values. The targets all seem reasonable to me, and most of them are based on direct feedback I've gotten here. Why does the Pool Calc show a scary CSI level of -0.6? This makes me nervous. I have a gunite/plaster pool and don't want to be near the corrosive side. I know that the TA level is low compared to more typical levels, but this is in the normal range for SWG's and has been recommended to me by TFP posters.

FC = 6
pH = 7.5
TA = 70
CH = 250
CYA = 80
Salt = 3200
Temp = 74 (my current temp)

 

[257WbyMag]

Hi Phillip,

Have you thought about bumping your CH up some? By taking it to 350 or even a little higher, you can buy yourself some room to breathe a little easier there. Also keep in mind that your temp will continue to rise over time and this will help too. You can also let your pH climb a little bit. Just some thoughts.

 

[chem geek]

High salt levels make the water more likely to dissolve plaster and SWG pools need a lower TA and high CYA to minimize pH rise and prolong cell life and both of these also make the water more likely to dissolve plaster. So to compensate, one can target a pH of 7.7 instead of 7.5, especially if this makes the pH more stable (lower pH has more carbon dioxide outgassing so causes the pH to rise faster and requires more acid addition) and one can also set a higher CH level. It's OK to have the saturation index by slightly negative, say at -0.2, since that helps reduce scaling in the cell, but you are right that -0.6 is too low a target level. Finally, your lower temperature is more likely to dissolve plaster -- if your temperature will normally be in the 80's, then that will help; if it's usually lower, then again adjust the pH and CH higher to compensate.

 

[PhillipH]

Thanks guys. I do plan to increase my CH to around 300ppm, so that should help.

I guess I'm confused about TA. I've read that a higher TA supposedly acts to buffer against changes in pH, but I've also read that a higher TA results in pH drift/rise. A TA target of 60-80 seems quite low considering that the CSI calc is at -0.6. Would it not make sense to target a higher TA such as 80-100 to help get me back to a more-neutral CSI reading?

I can target pH of 7.7, and that makes sense. The problem is, with my phenol red pH test, anything b/w 7.6 and 8.0 on the colormetric scale is only a slightly different shade of purple. I was halfway wanting to target 7.5 just because the color change is more obvious. I struggle to detect the difference between 7.6 and 7.8 and 8.0, especially with bright sun and different backgrounds, etc, etc. (I do use a white notecard as a background.) Any help on this?

My temp is currently low-to-mid 70's in North AL. It will be in the 80's up to near 90 all summer and through the end of September.

 

[JasonLion]

High TA does two different things. First, it does slow down/buffer PH changes. But secondly, it places upward pressure on the PH. The net effect is that at even slightly high TA levels with a SWG you get the PH drifting up and requiring constant lowering. What usually happens is that you go to a higher TA level, the PH starts rising, eventually you forget to lower the PH at the right time, the PH gets wildly way to high and you have problems. With a SWG, by keeping TA low, you can have your PH be much more stable. Thus, it is much better to adjust the CSI using other factors.

 

[dschlic1]

TA measures how much effort is needed to lower the pH. It says nothing about how hard it is to raise it. So for SWCG where the pH tends to go up, TA does not measure how rapidily the pH will go up. It does affect how quickly your pH will go DOWN when adding acid.

 

[JasonLion]

TA measures how much effort is needed to lower the pH. It says nothing about how hard it is to raise it.

While it is totally true that TA is defined as "how much effort is needed to lower the pH", the additional knowledge that we are talking about swimming pools, and therefore know which specific chemicals are going to be contributing to the TA measurement, means that we do in fact know what is going to happen when you are trying to raise the PH based on a TA measurement.