Calcium level in the winter

[orthofish]

Hey y'all. Hope everyone's doing o.k. Been off the forum for awhile.
Last summer I had a small issue with cobalt staining. Nothing too severe and only I noticed it. After posting pics and getting advice here, attribibuted it to low Ca. Since I don't fully understand all the the chemistry details, is it important to keep the calcium level up around 250-300 during the winter months? Does this leaching of cobalt from my fiberglass still occur in very cold weather?? I've had to drain water from my pool on 2-3 occasions due to rain and I'm sure the calcium has gotten a little low.
Thanks for any answer, and I hope to hang around here more often :-D
BTW, my water temp is in the upper 30's. Pump only runs when freeze protecter kicks it on, which has been quite a bit lately

 

[glassguy1]

Orthofish, Low calcium can harm a fiberglass pool or fiberglass lined gunite pool in every season. We recommend to our more than 5,000 commercial and residential pool and hot tub customers that a calcium level of 300-350 be maintained year-round. Low alkalinity can also cause damage to gel coat. -Bill

 

[orthofish]

Thanks a bunch Bill Guess I better check it.

 

[frustratedpoolmom]

Thanks a bunch Bill Guess I better check it.

I agree

 

[orthofish]

Hey Ann

Just ran a full set of numbers. Dang, that water was cold I let it warm up some before testing. Here they are:
pH 7.5
TA 70
CH 250
FC 2.5
CC 0
CYA 70-80
Temp 38
They look pretty good to me. Didn't check the salt as my SWCG isn't going to work in this cold water anyway.
What do you experts think???

 

[chem geek]

Since you let the water warm up some before testing, I'll use 55ºF as the water temp at the time of testing since the pH would have risen a little as the water warmed up. You generally want to do the pH test using the original water temp and also record that temp. You can warm up the water for the other tests, some of which really need it to be reliable (especially CYA, but it also helps with CH; TA not so affected). I'll also assume 3000 ppm salt given your SWG, though it may be less due to water dilution.

The calcite saturation index is around -0.8 so is still pretty low. With an SWG that requires higher salt levels and also prefers a lower TA to reduce the rate of pH rise, you need to have your CH be higher as well as a higher pH target. For now, if you raise the pH to 7.7 or so, that will help. Eventually, you'll want a higher CH level since at 85ºF with a pH of 7.7 you'd need a CH of 350 ppm to have a saturation index of -0.2 which is quite reasonable.

 

[orthofish]

Well I did something right :-D I checked my pH as soon as the water came out of the pool cause I was afraid it might change. I guess having fish tanks helped me on that
My water temp is only 38, so I'm still not sure what I need to do chem geek. My SWCG won't work below about 50 degrees.
Maybe I'm just not reading your post right

 

[chem geek]

So if I use a temperature of 38ºF, then the saturation index is even lower at -0.9 so I would definitely get the pH up to 7.7 or 7.8 and also consider increasing the CH though if you wanted to wait until spring opening for raising the CH then that's OK. Because the water is so cold, any reactions dissolving plaster are going to be much slower so this isn't anything to panic about, but I wouldn't keep this situation like this all season long so should address it early next year.

Normally when water gets colder, the pH will rise, so have you been adding any acid as the water got colder in order to keep the pH near 7.5? If so, don't do that. Let it rise on its own. If you started with 7.5 at 85ºF, then the pH would have risen on its own to around 7.7 as it got to 38ºF.

 

[orthofish]

I've not added anything to the pool. If it will get above freezing the next few days, I might add some Ca to it and bump the pH up some also.
Pool is fiberglass.