What CH level would you recommend for closing a pool with new plaster?

[three4rd]

Hi,

I was playing around with the SI index calculator and discovered that as the water temperature drops significantly (like down into the 40s or lower) the SI goes into the corrosive zone - around -0.42. I did these calculations with a CH of between 180-200. However, if the CH is boosted up to around 300, the SI comes back up to around -0.25. Is this why my pool store always told me that for winter balancing the CH should be increased? They have always suggested that it be in a range of 200-275.

My pool plasterer has suggested that I leave the CH on a low side being that curing plaster usually drives the SI towards the basic side and that if the new plaster (done in July) is ever going to scale it is typically during the first winter.

Any suggestions as to what my water balancing levels should be? Right now I'm at the following:

TA = 85
pH = 7.6
CH = 185

Unless I am advised differently here, I'll probably plan on keeping these levels much the same when I close, and then will recheck sometime in October and, if needed, rebalance by adding dissolved chemical into the pool with the cover partially pulled back.

Thanks.

 

[duraleigh]

I am on your side. Leave your CH alone. It will continue to rise somewhat over the winter anyway.

What is the CH of your fill water?

As an aside, getting caught in the middle between pool store, plasterer, TFP and your own education is a recipe for disaster.

Help your self and learn what makes sense for you pool. CH is not a hugely critical factor as long as it stays close to suggested range and you try not to tinker with it too much.

 

[onBalance]

Ditto Dave's comments, and your plasterer is correct.
The pH will also rise after you close it moving the SI closer towards zero.

 

[three4rd]

onBalance,

Sorry but I don't understand why you feel the SI will move closer towards zero as the pH rises. Every scenario I've run on the SI calculator shows that the SI runs into the scaling zone as the pH increases above 8.5. Below that level, the SI should be ok. Certainly the colder the water temp the lower the SI should be also. I'll try to rebalance things sometime in October depending on where the pH goes.

 

[onBalance]

Thanks for clarifying.
I was only saying that if the pH goes from 7.6 to 8.0 (which is likely the case) during the winter that the LSI would become fairly close to zero or still slightly negative in your particular situation.
Yes, you are right that a pH going above 8.5 would be scaling conditions.
However, it is unlikely that the pH would go above 8.3 over the winter (covered and very cold conditions) if the TA is about 75 to 80 ppm and contains some CYA.

 

[three4rd]

Hope you're right! I'd hate to open up to scale next spring!.