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.