Pool Math discrepancy?

[AggiEngineer]

I was playing around with Pool Math tonight and noticed a discrepancy that I couldn't figure out. When I go to the "Effects of Adding" section and ask it the effects of adding 32 oz of 31.45% Muriatic Acid to my 18,875 gallon pool, it says pH will be lowered by 0.4. When I go to the main screen where I see my latest test results and click on the latest pH reading, I set my Target pH to 7.6 (0.4 less than my current 8.0). It suggests that I add only 15 oz of 31.45% Muriatic acid. My TA is currently 75. So amount of acid is going to reduce my pH by 0.4, 32 oz or 15 oz? Why the discrepancy? Thanks all.

...I'm wondering if this could explain why I can't seem to get my pH to the value I want using the suggestions from Pool Math.

EDIT: I now see that there are several threads about this same exact issue. My pH of 8 falls outside the range noted in "Effects of Adding". I will use the suggested amount and re-test pH after 1 hour instead of next day.

 

[ajw22]

Effects of Adding does not consider your TA while the main page target calculation uses the TA value.

That said, all the pH adjustment calculations are approximations and the larger the adjustment the more it can be off. Small adjustments of reducing the pH by 0.1 to 0.3 is pretty accurate. Once you get larger adjustments of 0.4 and more the pH calculation gets a wider error range.

What test kits are you using that gives a TA value of 75? The Taylor and TFT tests measure TA in increments of 10.

 

[AggiEngineer]

I’m using the TF Test Kit Pro Salt. I arrived at 75 because the 7th drop was a dull red and the 8th made it bright red. I figured I needed the split the difference. Both were red but one was more red

 

[ajw22]

You don’t split differences,

The test is over when the next drop makes no difference in color. Don’t count that drop that did not make a difference.

Your TA is 80.

 

[Newdude]

My pH of 8 falls outside the range noted in "Effects of Adding

Just for anyone finding *this* thread down the road, the bottom of effects of adding has a disclaimer when looking at PH.

 

[AggiEngineer]

Thank you ajw22 and Newdude. I somehow missed the "test is over when the next drop makes no difference in color" part when reading through all the Pool School articles but that makes perfect sense. I'll heed that advice from now on.

I did confirm my Borates were 0 with some test strips. I've been struggling to get my pH to the desired level using the recommended amounts of muriatic acid. I will say though, that I've been measuring the resultant pH the next day rather than an hour later.

 

[jimbethesda]

Thank you ajw22 and Newdude. I somehow missed the "test is over when the next drop makes no difference in color" part when reading through all the Pool School articles but that makes perfect sense. I'll heed that advice from now on.

I did confirm my Borates were 0 with some test strips. I've been struggling to get my pH to the desired level using the recommended amounts of muriatic acid. I will say though, that I've been measuring the resultant pH the next day rather than an hour later.

When was your pool built? PH rises rapidly in newly built plaster pools. Your acid usage should decrease over time.

 

[Newdude]

test is over when the next drop makes no difference in color

And just because it got us all at some point, when the test is over, it's over. The vial may go pink again after testing FC/CC. That one doesn't count if you didn't immediately dump it when it went clear.

I will say though, that I've been measuring the resultant pH the next day rather than an hour later.

Test 20 mins later with the pump running, once or twice to prove you hit your target. The MA could be half strength, the pool volume off, etc.

I've been struggling to get my pH to the desired level

Are you targeting the high 7s ? Many dose to the low 7s thinking it will take longer to bounce back but thats usually not the case.

 

[AggiEngineer]

Are you targeting the high 7s ? Many dose to the low 7s thinking it will take longer to bounce back but thats usually not the case.

Yeah. When I measure pH at around 8, I try to knock it down to 7.2. I added the recommended amount of MA this morning to bring it down to 7.2 but wasn’t able to test until 6 hours later. Measured level was around 7.5. Would pH have gone up by 0.3 in that time?

@jimbethesda, Pool is new. It was finished in at the end of September this year

 

[PoolStored]

Would pH have gone up by 0.3 in that time?

Sure. The issue you are facing is that when TA is high, and/or pH is low is when pH rises the fastest.

Stop trying to force pH to 7.2. It will likely settle around 7.8 and SLOWLY rise to 8.

pH tests to be fairly stable around 7.8/7.9 with TA between 60-80.

Just drop it to 7.8 next time and see how long that lasts.

 

[Homebrewale]

What test kits are you using that gives a TA value of 75? The Taylor and TFT tests measure TA in increments of 10.

You can run a 10 ml sample with the Taylor kit. 1 drop of R-0007, 3 drops of R-0008 and multiply R-0009 drops by 25. Now this test is for when high TA is anticipated. In this case, the TA is not high.