Optimal pH for closing?

[Bart]

I have a gunite pool with a SWG that's just gone through its second season. When it was opened in the spring I had a massive scale problem. I got rid of it all with a couple of no-drain acid washes but I want to ensure I don't have a repeat performance.

What pH should I shoot for before they turn off the pump and close it down? 7.2 or should I go lower?

Latest numbers:
FC 5
CC 0
pH 7.3
TA 80 (working on lowering it a bit)
CH 550
CYA 85

Thanks!

 

[poolsupplysales]

You do not want to go below 7.2 because you water can be carosive and that is hard on the plaster and equipment

 

[Bart]

Thanks!

 

[UnderWaterVanya]

I have a gunite pool with a SWG that's just gone through its second season. When it was opened in the spring I had a massive scale problem. I got rid of it all with a couple of no-drain acid washes but I want to ensure I don't have a repeat performance.

What pH should I shoot for before they turn off the pump and close it down? 7.2 or should I go lower?

Latest numbers:
FC 5
CC 0
pH 7.3
TA 80 (working on lowering it a bit)
CH 550
CYA 85

I plugged those numbers into the Poolcalculator and it looks like you would be OK at 7.3; both result in slightly negative CSI index values that aren't that aggressive until the water gets to about 40 degrees. Since you have had scaling problems I assume you want a negative CSI - however since you do get to -0.61 at 40 degrees with a 7.2 pH that might be more aggressive than you want. However I'm not very experienced in this area so I would ask around a bit more before making up my mind.

 

[chem geek]

As the water cools down, the pH will rise even if there is no carbon dioxide outgassing. From 85ºF to 45ºF, if there is 50 ppm CYA in the water then the pH goes from 7.5 to 7.7 while with no CYA it goes from 7.5 to 7.8. So the saturation index tends not to move very much with temperature because of this effect.

 

[UnderWaterVanya]

As the water cools down, the pH will rise even if there is no carbon dioxide outgassing. From 85ºF to 45ºF, if there is 50 ppm CYA in the water then the pH goes from 7.5 to 7.7 while with no CYA it goes from 7.5 to 7.8. So the saturation index tends not to move very much with temperature because of this effect.

Sticky this info! That's very helpful to know.

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[y_not]

As the water cools down, the pH will rise even if there is no carbon dioxide outgassing. From 85ºF to 45ºF, if there is 50 ppm CYA in the water then the pH goes from 7.5 to 7.7 while with no CYA it goes from 7.5 to 7.8. So the saturation index tends not to move very much with temperature because of this effect.

Sticky this info! That's very helpful to know.

Indeed it is, especially since it seems pool calc doesn't compensate for this.
Correct?
I can do just what I'm sure UWV did and that's to enter all your test results, then enter a temp. As you change that temp by entering different values, you notice the CSI numbers fluctuate. Hrrrmmmm

 

[UnderWaterVanya]

[quote="chem geek":17jyzexy]As the water cools down, the pH will rise even if there is no carbon dioxide outgassing. From 85ºF to 45ºF, if there is 50 ppm CYA in the water then the pH goes from 7.5 to 7.7 while with no CYA it goes from 7.5 to 7.8. So the saturation index tends not to move very much with temperature because of this effect.

Sticky this info! That's very helpful to know.

Indeed it is, especially since it seems pool calc doesn't compensate for this.
Correct?
I can do just what I'm sure UWV did and that's to enter all your test results, then enter a temp. As you change that temp by entering different values, you notice the CSI numbers fluctuate. Hrrrmmmm[/quote:17jyzexy]
If you think about it, the calculator did what it should - it told you that at a particular temp with that pH the CSI would be "x". What it does not do is give you an option to say that temps are falling what happens...in which case pH should rise and help the index stay similar to before.
It is obvious now that Richard has said it. As I recall gas solubility in water goes down as temps fall.

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[y_not]

If you think about it, the calculator did what it should - it told you that at a particular temp with that pH the CSI would be "x". What it does not do is give you an option to say that temps are falling what happens...in which case pH should rise and help the index stay similar to before.
It is obvious now that Richard has said it. As I recall gas solubility in water goes down as temps fall.

Aaah, you are correct indeed. I missed the part where he said that pH goes up as the temp goes down. I thought it all remained constant because of a temperature offset.. I was wondering why nothing else would change.... N/M. LOL
BRAIN PPPPBBBTTTT!!

So if we set it at X temp and CSI is where we want her. Then she stays the same as the temp falls throughout the winter and into the nights. Yeah?
What about when temp goes up? What happens then?

 

[chem geek]

So if we set it at X temp and CSI is where we want her. Then she stays the same as the temp falls throughout the winter and into the nights. Yeah?
What about when temp goes up? What happens then?

The CSI doesn't stay exactly the same. As the temperature goes down, the CSI goes down a little, but less than if the pH didn't change. When the temperature goes up, the pH drops exactly as much as it rose when the temperature went down and the CSI goes up a little back to where it started. The reason for the pH change is a change in the equilibrium constant of water (in equilibrium with hydrogen, really hydronium, and hydroxyl ions) as a function of temperature.

Since the temperature changes are slow and since there is usually carbon dioxide outgassing going on, one may not notice this effect. I do notice it in my pool since I have an automatic electric cover on the pool so when not in use it doesn't get much outgassing and the temperature can drop a lot in a week after I stop using gas at the end of the season for heating it. Same thing with spring startup. So I notice this effect, but most people wouldn't. During the months of winter, however, there tends to be a slow pH rise in my pool that I need to adjust for now and then, but I keep the pH on the higher side when the water is cold as one way to keep the CSI more in balance.