CSI and cold water..

[mart242]

I've noticed that on a pool with SWG, if we follow the guidelines from troublefreepool pool-school/water_balance_saltwater_generator the CSI will be quite low and to make it manageable, the PH has to be higher and do does the hardness. Temperature also makes a big difference, if you leave it at 70 - 80 for the summer, you can achieve the recommended -0.3 or higher but as soon as you drop the temperature to 60 or so, BAM, you're back to -0.6 to -0.7.. drop it down to 35F and you're reaching close to a CSI of -1.0..

Does that mean that the recommended formula is for zones that have fairly high temperatures and no frost? Or do I need to balance my water according to different recommendations?

 

[chem geek]

First off, the pH of water will naturally rise as the temperature drops. This helps to counteract the temperature effect on CSI so the CSI doesn't drop as much as you describe so long as you don't do something like add acid to prevent this rise in pH. The only real downside to the pH rise is if one has metals in the water as the chance for metal staining increases at higher pH (this happened in my own pool over this winter).

As an example with a pool that starts out with a pH of 7.7, TA 70, CYA 80, Salt 3000 ppm (TDS 3160 ppm), and Temp 80ºF with a CSI of -0.2, if the temperature were to drop to 50ºF, then if the pH didn't change then the CSI would drop to nearly -0.5, but in practice the pH rises to 7.8 so the CSI drops to between -0.3 and -0.4.

The easiest way to counteract the drop in CSI over the winter is to raise the TA level since that's the easiest thing to lower when the water warms up in spring. Also, with the SWG off during the winter and with the cooler temperatures there is very little carbon dioxide outgassing so not much pH rise. So in the above example, raising the TA to 90 ppm would bring the CSI to around -0.2 again.

Also, note that colder water temperatures result in slower chemical reaction rates and that includes the dissolving of plaster due to low CSI. I'm not saying to intentionally have a low CSI over the winter, but that a low CSI over the winter is better than having a low CSI when the water is much warmer.

 

[mart242]

hmm.. with these numbers, an increase of 0.1 for the pH means an increase of .06 for the CSI which doesn't appear to be enough so I guess adding some TA would help at that point as you said.

What I've noticed though is that if I aim to be at the perfect level for a vinyl lined pool, pH = 7.5 (middle of recommended 7.2 - 7.8 for SWG, and lower end according to this site)
TA = 90
CH = 200 (for my SWG, as recommended by hayward)
CYA = 80 (as per the SWG once again)
Salt = 3200
Borates = 50ppm

I end up with a CSI of -0.7 which is too low. Does this simply mean that I shouldn't focus too much on the optimal values and instead just try to be in the range? If I move my pH to 7.8, that gives me a CSI of -0.46 but once temperature will drop near freezing point I'll be at -0.71 and to correct that my pH would have to be above 8 (seems too high) or my TA would have to be higher than recommended..

I guess I was naive last week when I had found a recommended pool shop, had them test my water and bought the chemicals they recommended.. thinking during that time that "hey, for once, I may just go with what they recommended and not spend evenings on the web reading about pool chemistry". I realized that they recommended a CH too low (120) , CYA way too low (30 - 50) even though I had specified that I had a SWG. As they say, if you want a thing done well do it yourself..

Now all I have to do is order my TF100, borates strip, salt test (worth it? my pro logic seems to be a bit off..), magnetic stirrer and spend half a day going across border to pick it up.

 

[chem geek]

If you've got a vinyl lined pool and have no exposed grout with tile or any other plaster exposed to the water, then the CSI does not have to be near zero and you don't have to have a lot of CH in the water. So if that is your situation, then don't worry about this at all.

As for the recommendations of Hayward or other manufacturers or the industry, if they end up with significantly negative CSI then this is a problem for plaster pools -- apparently, Hayward cares more about avoiding scaling in their SWG cell then they do about plaster degradation. This would not be the first time that industry recommendations are a problem. It's similar to some stabilized chlorine manufacturers recommending 1-3 ppm FC or even 1-1.5 ppm FC regardless of CYA level.

 

[mart242]

If I had my test kit I could monitor my pool condition to see what's happening. We've got weather here these days and my pool temperature dropped from 63F down to 47F.. and the weather forecast doesn't look good this week, we'll get below freezing temperatures for another night or two.