Trouble calculating amount of MA to add

[Ezun]

Need a little help here from the experts on chemical additions. Bottom line question is that I don’t understand why the PoolMath app is miscalculating what to add to my pool.

Here are my most recent numbers...
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Pool
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Build Type: Plaster
Volume: 28000 gallons
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Latest Test Result Summary:
FC: 5.0 (12 minutes ago)
CC: 0.0 (12 minutes ago)
pH: 7.6 (12 minutes ago)
TA: 70 (8 days ago)
CH: 325 (23 days ago)
CYA: 30 (3 days ago)
SALT: 3400 (one month ago)
BORATES: 80 (one month ago)
TEMPERATURE: 84° (3 days ago)
CSI: -0.28 (12 minutes ago)
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Here’s my story:

Following everything on TFP my pool is well balanced and stable. I’m still working on keeping my FC low (like 4) and not letting it drop too much to go below 3. slow and steady wins the race for me on this, last time I moved too fast I had an algae bloom start. ANYWAY, over time, because of aeration and use of the pool, my PH rises. It naturally wants to sit around 7.8-8, and when I get to 8.0 I try to drop it to 7.6.

Today was the day it hit 8.0, so I used pool math to calculate how much MA to add to drop it. I’ve been very skittish on doing this because most recently when I added what the pool math app said I needed to add to drop PH to 7.6, and it dropped to 7.0. Which is not good...

So, I created this thread to get help calculating the size of my pool thinking that was the trouble with the calculation. I was told my pool was 33,000 gallons, but based on the TA method, I’m closer to 28,000.

So today I leveraged the new size of 28,000 in the PoolMath app to calculate what I needed to drop PH from 8.0 to 7.6. What it came back with was 140 oz (1 Gallon, 2 Cups). Given my shyness from before, I decided to add less than 1/2 of what PoolMath said and I did 64oz (8 Cups).

The result was PERFECT, I landed right at a PH of 7.6

Anyway, back to my question: What I don’t understand is why the PoolMath app is miscalculating what to add to my pool. Only thing I can think of is Borates being off, but all I have to determine that is test strips (hate them).

Please help!

 

[mknauss]

I entered you data into Poolmath I get the 140 oz (31.45% MA). That number seems high because of the borates at 80 ppm.

If the borates are actually 30, then you need 66oz.

 

[Ezun]

So I just tested again today and my borates appear to be closer to 50ppm. I struggle with the test strips ... at exactly 15 seconds, the strip appears to match the 50ppm color, about 5-10 seconds later it matches the 80ppm color.

So, based on the instructions on the strips, I'm going to stick with the 15 second mark and say it's 50. I just retested everything today and will post that too. We'll see what PoolMath recommends next time based on the 50ppm number.

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Latest Test Result Summary:
FC: 5.5 (an hour ago)
CC: 0.0 (an hour ago)
pH: 7.6 (an hour ago)
TA: 70 (an hour ago)
CH: 300 (an hour ago)
CYA: 30 (3 days ago)
SALT: 3200 (an hour ago)
BORATES: 50 (an hour ago)
TEMPERATURE: 83° (an hour ago)
CSI: -0.27 (an hour ago)
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