CYA/pH and calcium hardness questions

[mccato]

I've read here several times that you can keep your pH a little higher if your CYA level is high (mine is 150); but is there a chart somewhere that I could use to determine what my pH should be? I'm thinking a new column added to the best guess CYA chart for pH? My pH is 7.9 now - do I need to lower it with a CYA of 150?

Here are the other numbers:

16,400 gallon vinyl liner
TC 9.2
FC 7.4
Alkalinity - I have a TA of 197 and an adjusted alkalinity of 152. This is an Alex test report, and it's assuming that I'm going to add 3 pounds of pH decreaser.

I've been partially draining and refilling to reduce my CYA. It's gone from 330 to 150, but today my total hardness is 239. Should I try to lower that? I've read that draining and refilling is the way to do that, but apparently it's not working in my case because the hardness has increased since I've been doing that. I'm just not sure if it's high enough to worry about.

[aylad]

I don't know that your CYA level actually has an effect on where you keep your pH level--it's Cl that definitely needs to be higher with high CYA levels. As far as pH goes, 7.9 is on the higher end of "normal", but if that's where your pool likes to stay, then you can leave it. Sounds like you're making huge progress in lowering your CYA--wow--330 was a lot!

Your calcium should lower with the draining/refilling you're doing, unless you're using cal-hypo to chlorinate/shock with. A hardness of 239 is fine in a vinyl pool, but if you get it too much higher, your high pH and high TA may cause some scaling if you have a heater or other equipment.

Janet

[mccato]

Thanks, Janet! That was fast!

I'm definitely not using anymore cal-hypo or dichlor. Bleach is working great for me!

[chem geek]

According to my calculations with your data, to go from 7.9 to 7.4 pH with your numbers you would need to add 5.2 pounds of Sodium Bisulfate solid (93.2% purity), not 3 pounds. If you were to use Muriatic Acid instead, you would need 8.2 cups. I'll bet the recommendations were based on an "assumption" of a TA of 80 (with a CYA of 150 or some other combination).

And yes, as Janet said, you were at a little risk of scaling and/or cloudiness at a pH of 7.9, but at a pH of 7.4 you'll be fine. The higher pH still reduces chlorine's effectiveness, but not as much as if you didn't have CYA. At 7.9 pH you have 0.018 ppm disinfecting chlorine (HOCl) while at 7.4 pH you have 0.021 ppm disinfecting chlorine.

As for your chlorine level with your high 150 CYA, you are a little on the low side and should have closer to around 10 ppm of chlorine to get decent algae prevention (disinfecting chlorine level of 0.029). If your pool is exposed to sunlight, you might lose quite a lot of chlorine each day (even with the CYA protecting it) and have difficulty keeping it this high. If this were earlier in the swimming season, I'd recommend further drain and refill dilution to lower your CYA, but since it's nearing the end of the season you can ride this out with high chlorine and then hope for a CYA reduction over the winter.

Richard

[mccato]

Thanks for your reply, Richard. That is very good information!