I am getting incorrect results with PoolMath

[IsaacFL]

My levels are:
FC 7
pH 8.0
TA 70
CH 1075
CYA 80
Salt 3400
Borate 0

When I enter a pH target of 7.6 the PoolMath says to:
Add 5.1 oz of 31.45% muriatic acid

However, if I go to bottom, Effects of adding chemicals:
it says that Adding 5.1 oz of 31.45% muriatic acid will
lower pH by 0.18 and lower TA by 3. which would only take pH to 7.82

if I enter 11 oz instead, it says it will:
lower pH by 0.39 and lower TA by 6.4.

I know from prior times adding acid that the 11 oz number will take my pH to 7.6 which is where I want it.

Or am I not using this right?

 

[jblizzle]

The top takes your TA and borate level into account. The bottom does not. Do you have your TA set at the top?

Any chance your acid is not 31% and is instead the half strength stuff that a lot of places are now selling?

 

[IsaacFL]

My TA is set at the top. All my numbers above are plugged in.

The jug says it is 31.45%.

 

[duraleigh]

Check your input, Isaac, the error is not in poolmath.........it's been around a long time

 

[IsaacFL]

I have attached a screen shot. Here you can see it is recommending to add 5.1 oz of acid. But at bottom effects of adding 5.1 oz only lowers pH by 0.18.


- - - Updated - - -

Here is the screenshot where I change it at bottom to 11 oz. That says it will lower by 0.39 which should take me about to the 7.6 target I want.

 

[jtaves85mcss]

Although there is not enough resolution for me to enlarge the attachments and read, I get the same numbers as Issac, top versus bottom (5.1 oz and 11 oz) using his general parameters.

I will say the top calculation has always given me the measure I need to get the desired Ph.

 

[Bevostein]

I have noticed the same discrepancy between the top and bottom

 

[jblizzle]

The top should be more accurate ... the bottom makes some assumptions for the pH calculations (as is noted in that section).

 

[chem geek]

Assuming you put your 6700 gallon pool size into PoolMath, the 5.1 fluid ounces of 31.45% hydrochloric acid is approximately correct. The precise answer using the Pool Equations spreadsheet is 6.6 fluid ounces. PoolMath uses tables to approximate the calculation since pH calculations are complex.

The amount of acid indicated needed in the "Effects of adding chemicals" section does NOT use your entered TA number and instead assumes an 80-120 ppm TA range. In this case, it assumes it takes 11.1 fluid ounces to drop the pH from 8.0 to 7.6 (i.t. by 0.4 units) and you would get this if the TA were around 120 ppm. This particular section could probably be improved somewhat.