Difference in MA Addition Recommendation?

[ajw22]

@Leebo Why are these PM recommendations so different when calculating it with TA or from Effects of Adding?

This arose in this thread where 17 oz is not getting close to lowering pH by 0.5.

Chasing PH

We just took over our pool from the startup company this month. I am having trouble getting my PH down. Wet Edge refers to LSI maintenance for the warranty, so I log in Pool Math and get chemical dosage suggestions there, but also play with the sliders in the Orenda app to make sure I'm in the...

www.troublefreepool.com

 

[Texas Splash]

Lee, FWIW, I see the same thing on the old Poolmath web page. It shows 2 cups (16 oz) of MA to lower the pH from 8.3 to 7.7, but if you go to the bottom "Effects of Adding", and enter the info, it says 16 oz of MA (31.5%) only lowers the pH by 0.23. There must be some TFP secret squirrel logic to that difference.

 

[Newdude]

The current Ph matters and so does the TA and efects of adding is a WAG.

For example, let's lower Ph by 0.4 six times.

Mess with start Ph:

7.8 / 80 would need 18 oz
7.6 / 80 would need 26 oz
7.4 / 80 would need 37 oz

Mess with TA

8.0 / 80 would need 14 oz
8.0 / 70 would need 12 oz
8.0 / 60 would need 11 oz.

Telling effects of adding only that you added 24 oz of MA doesn't tell it enough.

 

[Newdude]

Seeing it so spelled out above, that 11oz to 37 oz may all lower Ph by 0.4 in 16k gallons with mostly similar start points, this feature should be disabled IMO.

 

[JoyfulNoise]

I’m confused … what’s the issue?

Pool Math has ALWAYS had issues calculating pH changes because there is no exact formula for it. An aqueous solution of high ionic strength with multiple buffering chemicals like pool water requires the simultaneous solution of around 11 equations with 11 unknowns. One can make certain assumptions that drops that down to possibly 5 equations to solve simultaneously but it’s still a very difficult problem. Even in the most advanced chemistry courses taught barely scrape the subject of pH calculations.

So, for PoolMath, you basically have to take the pH values with a grain of salt. I often ignore it because the predictions are bad. I usually add enough acid to drop my TA by a fixed amount (typically 10ppm) and then I don’t care about the pH. It’ll go down, it’ll come up, it’ll hang out at 7.8 most of the time. It’s pretty much irrelevant to my pool care routine.

 

[ajw22]

So, for PoolMath, you basically have to take the pH values with a grain of salt. I often ignore it because the predictions are bad.

Got it.

Can they get Pool Math to be consistently bad?

 

[JoyfulNoise]

Got it.

Can they get Pool Math to be consistently bad?

I would prefer that TFP teach differently .. pH isn’t really that important. It’s more of an indicator of when the chemistry is not right, not something you try to control. Sadly, that’s not going to happen because people have been misled to believe that pH is some kind of determinative chemical parameter … it’s not. But it’s something that’s easy to measure and easy to explain … “your pool is basically vinegar!!!” … “your tears have a pH of 7.4 so that’s why your eyes burn!!!” … and easy to scare people with.

Honestly, I would do away with the entire “Effects of Adding” section to PM because it’s not very useful. But that’s me … you do you …

 

[AUSpool]

Going out of its ideal range really confuses it.

For 20kL at a TA of 70.
Going from 8.3 to 7.9 it says 13oz @ 29%.
From 7.7 to 7.3 it says 27oz @ 29%.
From 7.1 to 6.7 it says 72oz @29%.

I thought with more hydroxide ions around in the upper end it would take more acid to move the same four points then at the lower end.

I was thinking of this yesterday. For @Glitterbub its probably better to calculate a 0.4 point change within the ideal range, say 7.4-7.8, where for a 16kL pool it will give a dose of 18oz of 29%. As apposed to 8.4 - 8.0 which gives a 11oz dose. Add the first dose, retest, and calculate a second. I would want to move it back into the sevens a bit sooner.

It does say that calculations are the closest within its ideal range. Although I didn’t think the variance would get that great at only 0.5 point out of range.

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.

 

[AUSpool]

Seeing it so spelled out above, that 11oz to 37 oz may all lower Ph by 0.4 in 16k gallons with mostly similar start points, this feature should be disabled IMO.

No. I need it. To calculate how much acid I need to lower my TA.

 

[JoyfulNoise]

It’s not going to predict pH correctly because it uses a fixed TA. As you add acid to the pool, you consume TA and lower pH. In the real world, this is happening as a smooth, continuous process - the concentration of alkaline components (mostly carbonate alkalinity) is decreasing and therefore the pH is changing. The equilibrium chemical reactions are unfolding as they should. In the simulated-reality that is PoolMath, the TA is remaining fixed and the pH is being calculated using data that was linearly interpolated. The assumptions in all of that “math” are so far away from reality that it’s only a good approximation over a very small range. So the pH calculations are, at best, just a guideline. You add X amount of acid and the pH should decrease by Y +/- 10% as long as your pH change isn’t too big.

TA change calculations are exact.