High CH.....how does calcium get in a pool anyway?

[hawkeyes]

I have already had to drain and acid wash my 6 mo old pool due to scaling so I'm paranoid about it now. I keep very close tabs on my pH and try to target 7.2 even though I rarely get to that (7.4 is a very good day!). My fill water has a CH of 300-----so how can my pool water test higher than that? Since my fill water is obviously on the high side to start with, is there anything else I can do to help prevent scaling? Sequesterant? I know at re-start (about 2 mo ago) they added 2 bottles of JM Purple Stuff. Will it help to add that at a maintenance dose level?

Here are my numbers:

pH: 7.8 (added acid tonight...it will go down to about 7.4 tomorrow and be right back up to 7.8 the next day....it's a vicious circle)
FC: 3.5
CH: 400 (usually tests at around 320-350----why would it jump to 400 now????)
CYA: not sure b/c I'm waiting on a refill for the test kit but has been right around 70
TA: 80

 

[JasonLion]

When you add fill water to the pool, there is CH in the fill water that goes into the pool. But when water evaporates, the CH stays in the pool. The more water that evaporates and gets refilled, the higher your CH level goes.

Sequestrant can help, but isn't needed when your CH level is still plausible. You need to compensate for your high CH level by lowering your TA and PH levels. This is fairly easy to do when CH is 600 or lower, but gets more challenging as CH goes up above that. CH levels up to about 1,000 or perhaps 1,200 can be managed by careful adjustments and control over your PH and TA levels. You can use the CSI calculation in the Pool Calculator to help you figure out where to keep PH and TA. Try to keep CSI slightly negative, and give yourself a little head room in case PH goes up when you were not planning for it to go up.

You can help your PH stay stable you letting your TA level come down some more, say to 60.

 

[hawkeyes]

I am confused as to why the pool calculator "target" number, when I enter in pH of 7.2/7.4 and TA of 60/70 goes to "corrosion likely". If I want to target lower pH and TA numbers, my CSI numbers go too far negative. With my current numbers, it says balanced...but the lower TA and pH numbers change that. What am I not comprehending?

 

[JasonLion]

If the CSI goes a bit too negative for a day or two it really doesn't matter. But if it is below -0.6 for a long time there can be problems. So you can take the PH down to 7.2 as part of lowering the TA, but you don't want to leave the PH that low long term unless your CH level goes much higher.

 

[PaulR]

I played around with the numbers in the pool calculator for a bit.
pH 7.8, TA 80, CH 400, CYA 70*, Salt 3000*, Temp 85*, => CSI +0.1.
* = assumed

Lowering the pH and TA targets some...
pH 7.5, TA 60, CH 400, CYA 70, Salt 3000, Temp 85, => CSI -0.37

So, lowering TA will give you some headroom on pH, CH and temp, which will all tend to rise due to the SWG, hard fill water, and summer sun, respectively. CSI in the -.3 range should be okay.
--paulr

 

[hawkeyes]

Thank you both very much!!