Rising ph and ch?

[texasdad]

My ph has been climbing from about 7.2 to 8.O in a weeks time. I've been adding muratic once a week to bring it back down. Over the summer the fountains ran daily and I assumed the aeration was causing this. The fountains have not run for several weeks now and it still uses the same amount of muratic.

Also. It has been a while (aug?) since last time I checked my calcium. Last time I checked it was 290. Today it 490. I don't know why?

I only chlorinate with bleach and do not add any shock or etc.......

Full test numbers:

Cl. 3.5
pH 7.2
TA 60
Ch 490
Cya 60
Borate 50

 

[JasonLion]

If your plaster is less than a year old then that is completely normal. The PH, TA, and CH levels all go up as the plaster cures, often for as long as a year. If your plaster is older than that, could you tell us what other chemicals, if any, you use regularly.

 

[chem geek]

Based on this post, the pool was newly filled a few weeks before 6/10/11 so it's now still only 5 months old. The curing of plaster produces calcium hydroxide that increases CH, pH and TA. For every 10 ppm CH that is produced, it increases the TA by 10 ppm and greatly increases the pH and requires 25-1/2 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid) per 10,000 gallons to bring the pH and TA back down.

You can easily distinguish between carbon dioxide outgassing vs. plaster curing because your acid addition to keep the pH down will end up lowering the TA if the source is carbon dioxide outgassing while if the TA doesn't go down over time then the acid is just balancing plaster curing. With your TA down at 60 ppm, even the fountains were probably not causing enough outgassing to be noticeable relative to the strong effect from the plaster curing. You will probably notice the curing rate slowing down as the water gets colder and should see the rate lower next year.

Nevertheless, a 200 ppm CH increase in 2-3 months does seem to be on the high side. Perhaps you also have evaporation and refill with water that is high in CH. In 13,000 gallons, if that rise was only from plaster curing, then it would be over 5 gallons of acid. Is that how much acid you have added since the 290 ppm CH reading? If that was 10 weeks ago, then that's around 8 cups of acid per week which seems high. If the pH rise was solely from plaster curing, then with your numbers and based on the pH rise from 7.2 to 8.0 the CH would rise by 16 ppm per week and require 53 fluid ounces (over 6-1/2 cups) of acid (to get from 8.0 to 7.2 pH). That still seems high though the rest of the CH rise may indeed be from evaporation/refill (or test results may not be completely accurate). You might see if you use less acid just lowering the pH to 7.5 or 7.4 instead of 7.2 since at 7.2 the saturation index is lower than -0.4 and would encourage more dissolving of plaster.

 

[texasdad]

Thank you
Yes, the pool was built this spring do is lstill new. I didn't realize the plaster would still be curing, pool builder said it would require extra acid for the first few weeks and he added it then.

I've been adding about 4 cups of acid each week.

I'll try lowering the ph to 7.5 instead of 7.2 and see if it comes up slower.