TFP Calculator compared to Taylor chart re decreasing pH

[brauh01]

Hello,
Working on reducing pH levels since pool opening. Not a particular problem per se, but just noticed something this morning. Entering the test results into TFP's helpful PoolMath page, I get a recommendation to add muriatic acid in the 29 to 32 ounces range. Using the acid demand test and the Taylor chart, I'm seeing something on the order of 2 quarts which is about double the PoolMath site. So, my question is, does the PoolMath site purposely lower the recommendation, especially given what might be considered a "large" change in pH?

Reading Date pH TA PoolMath Recommendation

4/26 8.0+ 105 32 ounces muriatic acid
5/2 8.0 100 32 ounces muriatic acid
5/3 8.0 90 29 ounces muriatic acid​

Using a Taylor K-2006 test kit. Water temp (not sure this is needed!) is around 60F. Solar cover installed after running uncovered 24/7 for 5 days.
Bill

 

[Defgufman]

I quit using the Taylor chart as it was over shooting my pools needs. I found the calculator to be spot on on all chems.

 

[Richard320]

The difference is that the pH tester reads anything above 8 as 8. So it might be 9. And since you don't know for sure, neither does poolmath, so it doesn't get it right. It also isn't accurate for large changes in pH - there is this disclaimer at the bottom.

Note: pH calculations are not exact. These numbers are only suggestive of the relative magnitude of the pH change you can expect. Small changes, +-0.4, with pH between 7.2-7.8, TA around 80-120, and Borate near zero will be approximately correct. The further you go from those ranges the less these pH changes will correspond to reality..

The acid demand test doesn't care what the pH is now, only what it is after the drops are added.

 

[ewkearns]

I quit using the Taylor chart as it was over shooting my pools needs. I found the calculator to be spot on on all chems.

I had the same experience with the charts.

Poolmath is dead on, depending on how accurate you input your pool's ACTUAL gallon size....

 

[Kiss4aFrog]

+1, Pool Math seems to put me right where I expect to be each time.

 

[brauh01]

Thanks for the replies.
Richard320, thanks for explaining the limits of the testing (and thus what PoolMath must deal with). Sounds like I can reasonably expect, after following poolmath's recommendation today, that I'll have one more round of adding acid. Helpful to know.
Bill