Calcium Hardness - 500 ppm

[mkubiniec]

To remove some discoloring/staining of my white plaster, I recently did an acid wash. Added muriatic acid to the pool, and brushed pool daily for 4 days. Then added baking soda to bring pH and TA back into spec. CH was then at about 650 ppm, so I drained some water out of the pool and refilled. CH is now at 500 ppm. Bringing CH back down to 400 (or 350) would require even more draining.

Question - can I reasonable live with this (500 ppm) for now? Will it gradually go down with rain water dilution, etc? Are there any major downsides to this - other than some possible scaling to the tile at the water line (which may require scrubbing)? Or do I really need to bite the bullet and drain even more water out of the pool?

Thanks

 

[JoyfulNoise]

My pool water has 750ppm CH and it is crystal clear with no scale on my tiles.

The main cause of scaling is loss of water balance (pH, TA & temp too high). Once you go above the recommended CH ranges, then you need to be much more careful with keeping your water in balance.

If you get enough water from winter rains, you can drain and let the rain water refill the pool.


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[JoyfulNoise]

Can you provide your most recent water test values? If so, I may be able to give you more targeted advice.


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[chem geek]

Welcome to TFP!

The main issue will be scaling in your SWG cell since the higher pH at the hydrogen gas generation plate promotes scaling there. If you use 50 ppm Borates, that will minimize that problem since it roughly cuts the pH rise in half in the cell. You should be able to maintain reasonable pH and TA to prevent scaling elsewhere. We usually don't see scaling in pools until the saturation index gets to 0.7 or more, though as you point out at the water line with evaporation one can get calcium carbonate scale show up even at normal or slightly negative saturation index values -- it's just less at a lower index value.

 

[mkubiniec]

Test results from yesterday:
pH - 7.5
CH - 500
TC/FC - 5
CYA - 65
TA - 70
NaCl - 3900 (added about 25# to bring up to 4100-4200)

Note 1: CH no doubt increased dramatically due to the acid wash and brushing of the plaster.
Note 2: Being a new pool (Sep 2014), pH continues to go up, so I regularly add Muriatic acid - about a quart a week.

 

[JoyfulNoise]

Test results from yesterday:
pH - 7.5
CH - 500
TC/FC - 5
CYA - 65
TA - 70
NaCl - 3900 (added about 25# to bring up to 4100-4200)

Note 1: CH no doubt increased dramatically due to the acid wash and brushing of the plaster.
Note 2: Being a new pool (Sep 2014), pH continues to go up, so I regularly add Muriatic acid - about a quart a week.

Based on those numbers and assuming a water temperature of 86F, PoolMath calculates your CSI as -0.19. A CSI of -0.3 to -0.6 indicates a possibility to be corrosive to plaster while anything lower than -0.6 indicates water that is corrosive to plaster. If you let you pH rise to 7.6 and kept it between 7.6-7.8 (CSI between -0.1 and +0.1), you would be absolutely fine. Adding borates to your water at 50ppm would also help to keep the pH rise from the SWG down and it would shift your CSI to always less than 0 for pH ranging between 7.6-7.8 assuming all other water parameters remain the same.

Note that borates reduce the rate of rise in pH but not the amount of acid needed to reduce pH. So while the interval between acid additions increases, you need to use more acid to get the pH back down. Thus the total amount of acid needed doesn't really change.