Poolmath pH suggestion doesn't match prediction

wysocki

Bronze Supporter
I'm using the old Poolmath calculator online and although the numbers look pretty good (except for the salt), I'm having difficulty dumping enough MA into the water to get pH below 8. And I'm confused by the difference in the recommendation next to the pH row and the Effects of Adding section at the bottom of the page. Here's my current numbers:
FC: 4
CC: 0
pH: 8.0
TA: 70
CH: 300
CYA: 70
SALT:4000

The pH line has 8.0 current and 7.6 target and it recommends 18oz of 31% MA.
But the Effects calculator predicts 18oz would drop pH by 0.18 (and would take 40oz to drop it by 0.4).

It appears that the Effects calculator is more correct since I'm putting in about a quart per day to keep the pH from skyrocketing over 8. Why is the pH row calculation so inaccurate? And can I expect to keep dumping all this acid in forever?

Oly

Gold Supporter
Read the fine print of the pH, now and target section of the on line Pool Math. Your TA is not 50ppm. You are correct to use the "effects of adding" calculation after entering all your water levels in Pool Math.
Test your fill water for TA. I'm guessing it's higher than 70ppm.
High TA will raise pH and adding acid will lower pH and TA.
What test kit do you use?

wysocki

Bronze Supporter
Yeah, TA is 70 according to the readings 1 hour ago from my TF-100 kit. AND it's interesting that the poolmath even accounts for this in its pH calculation there. But it's way off!?! Why are the two projections so different then?

Oly

Gold Supporter
Reread the fine print......the lower recommendation of the "Now and Target" calculation is assuming your TA is 50ppm. ----edit---- sorry just realized the TA number is updated in the pH calc
Test the TA of your fill water relative to your pool water.

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Gold Supporter

JoyfulNoise

TFP Expert
Platinum Supporter
One other thing to keep in mind is that no pH calculator, not even the most advanced ones used by chemists, can not accurately calculate pH changes over large values well (and especially not in a solution with multiple buffers and high ionic strength). The PoolMath calculations are, at best, approximations and if you are trying to change the pH value larger than 0.4 units, you need to do it in stages, not with one huge addition. When you add muriatic acid to your pool , you are changing both pH and TA at the same time. So you need to recheck your TA to make sure the value you are using is correct.

Make smaller corrections to pH.

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PoolStored

wysocki

Bronze Supporter
@PoolStored "...upper portion takes in the affects of everything else.. Use the top area..." Yeah, that's why it's weird that the bottom area is much more accurate.

@JoyfulNoise I don't notice a significant swing in TA when I acid down. I'll check it all again tomorrow in addition to my fill water.

wysocki

Bronze Supporter
So this morning I did a test of pool water, fill water, (and for kicks, the softened house water). Here's some results:
 POOL FILL SOFTENED PH 8.1 8.2 8.5 TA 60 80 90 CH 325 100 <=25
Interestingly, my TA has dropped from 70 and CH has risen from 300. The pH still tries to climb. I don't think the fill does a lot since I estimated I filled about 1000 gal (of 24000) since initial reading. Also interesting is that my water softener kicks butt with calcium (but with strange effect on pH and TA). I think I'll put a little baking soda in to raise the TA, then some MA to bring pH down and see what happens.

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