pH dropped MUCH lower than Pool Math predicted?

[Stoopalini]

This is the 1st time this has happened to me in the 2 years I've been managing the pool. I've always done the TFP method, and it's been great! While my neighbors have pool service companies coming once (sometimes twice) a week, and they still experience algae blooms ... I've never had any troubles for two years

Today my pH was reading 8.3, TA 100, and Borates 50 ... so I dosed what Pool Math told me to (as usual) to hit 7.5 pH, which was 117oz of 31.45%. About 1.5 hours later, I retested and the pH fell to 6.8 and TA dropped to 60? I'm fine with the TA being so low, and will run the waterfall and just wait for the pH to come back up (which should only take a day or 2 given our evaporation rates right now and the high TA fill water I have), but this makes me uncomfortable to have it overshoot so much.

The only thing different today from the past 2 years is I used Klean Strip 31.45% muriatic acid instead of the KemTek 31.45% I've always used in the past. When I went to Lowes to get some KemTek last weekend, they were completely out of KemTek everything in the pool/garden section ... so I went inside and grabbed the Klean Strip. This isn't the "low fumes" version, but the blue label one which has 31.5% | 20* Baume stamped right on it.

I used the Klean Strip for the 1st time last weekend, and it hit the target exactly as Pool Math predicted ... but today, it significantly overshot the projection.

My initial thought was my pH buffer (Borates) was gone somehow, but I retested today and verified it's reading 50.

When I dosed the 117oz today, I used 60oz from the same bottle I used last weekend, and then 57oz from a 2nd bottle which I had to open today. Maybe it's possible this 2nd bottle is much stronger than 31.45% due to some error at the factory? Is there a way to test the consentration of it with my TF-100 kit?

I can't figure out what else could cause this, and I'd really like to know what happened here so I don't accidentally crash my pH in the future.

 

[ajw22]

You should not lower your pH by more than about 0.3 in one dose. PoolMath calculations get more approximate the larger the pH change is.

For a 0.8 reduction do it in two steps and test in between.

 

[Stoopalini]

You should not lower your pH by more than about 0.3 in one dose. PoolMath calculations get more approximate the larger the pH change is.

For a 0.8 reduction do it in two steps and test in between.

Hmm, gotcha. I can understand that, although I've always done a single dose in the past no matter how far the drop. It's not often I reach 8.3 though. It's usually around 8 or 8.1 or so. But due to the crazy temps, dry weather, and wind this week ... I'm sure my evaporation is sky high. So the 120 - 130 TA fill water is coming in and driving things up.

Just to confirm, I plugged the numbers into the old PoolMath webpage, and it gave me 117oz as well. I suppose in the future, I'll never go over 0.5 pH points in one dose ... thanks for that tip

 

[Lake Placid]

I just setup your scenario in the web version of pool calc. It recommended 117 oz as well, but then in the effects of adding projector in the bottom the same 117 oz projects a ph drop of 2. Working backwards with that calculation, 47 oz would have gotten a ph drop of .8. Not sure why it would estimate that far off, but when additions get that large I double check them in effects of adding as a sanity check.

 

[Jimrahbe]

Stoop,

When I enter a 14K pool into pool math and a current pH of 8.3, it shows adding 117 oz of 31% MA will drop your ph by 2.0.. Which seems to be exactly what happened???

Thanks,

Jim R.

 

[Stoopalini]

Jim ... It appears the "Effects of Adding" calculations are right, but the Pool Math pH reduction projection is not.

See Lake Placid's response above. What he observed is exactly what I'm seeing too. Good tip to always double check against the effects of adding I suppose ... but in 2 years, I've never had an issue like this.

I wonder why PoolMath got it so wrong for this particular dataset?

 

[mknauss]

Which seems to be exactly what happened???

Be sure you have 50 ppm borates and 70 ppm CYA in Poolmath. Makes a big difference.

 

[mknauss]

I wonder why PoolMath got is so wrong for this particular dataset?

Effects of adding do not account for Borates. See the note at the bottom of the page.

 

[Stoopalini]

Effects of adding do not account for Borates. See the note at the bottom of the page.

I do have 50ppm Borates and 70ppm CYA measured in PoolMath.

See my PoolMath logs for details ... but even in the web version of PoolMath, it's suggests 117oz to drop from 8.3 to 7.5, with 50ppm Borates and 70 CYA.

But adding 117oz dropped the pH all the way to 6.8

 

[mknauss]

But adding 117oz dropped the pH all the way to 6.8

Only chemistry can make that happen. Borates are not really 50 ppm, CYA is not really 70 ppm, TA is not really 100. Or by some chance the muriatic acid concentrations were different.

I do have 50ppm Borates and 70ppm CYA measured in PoolMath.

I realize that. But those using Effects Of Adding Chemicals in Poolmath have to realize that does NOT account for Borates. It is only accurate for a very tight range of parameters. As the Note on the page states.

 

[Stoopalini]

Be sure you have 50 ppm borates and 70 ppm CYA in Poolmath. Makes a big difference.

Playing around with the old PoolMath webpage, changing the CYA does not affect the suggested amount of MA to reach target ... Should it?

In either case, the amounts were already in PoolMath when it gave me the recommended amount.

Only chemistry can make that happen. Borates are not really 50 ppm, CYA is not really 70 ppm, TA is not really 100. Or by some chance the muriatic acid concentrations were different.

I realize that. But those using Effects Of Adding Chemicals in Poolmath have to realize that does NOT account for Borates. It is only accurate for a very tight range of parameters. As the Note on the page states.

Yes, I understand it's a chemistry thing, which is why I'm looking to understand what happened.

I verified Borates @50 using the same test strips I've always used (the Pool Check ones recommended by TFP), the TA test is pretty easy and a value of 100 is what I would expect, and I'm using an Apera pH meter for pH testing (due to my color blindness). When I saw it dropped all the way to 6.8, my 1st thought was the calibration of the meter was off and it wasn't 8.3 when I started; even though I re-calibrated the meter last weekend ... so I used the comparator block and had my wife look at it. She said it was reading 6.8 - 7.0 ... so that confirmed for me the meter is reading accurately.

And just for clarity, I did not use "Effects of Adding" for getting recommended dosage amounts. I only use the PoolMath app suggestions when I plug in a desired target.

Like I said, this hasn't happened in the 2 years I've been using PoolMath to manage my pool. It's always been spot on with suggestions for dosing. This is the 1st time it's been so far off.

Do you know if there's a way to confirm the strength of the MA using the TF-100 chemicals?

 

[Stoopalini]

Here it is with CYA of 300 ... recommending 117oz



Here it is with CYA of 10, still recommending 117oz



Should the recommended amount of MA change between the two values of CYA?

 

[mknauss]

Again, all the test information is more than acceptable for normal operations. But say the Borates are off by 10 ppm, never see it on a strip. The TA if off by 10 ppm, easy to do with droplet size, etc. Errors add up.

That is why making pH changes of greater than 0.3 or 0.4 at a time contain risk.

The CYA level can effect the calculation, but at 70 ppm, it is not as great as it is at higher levels.

 

[mknauss]

Should the recommended amount of MA change between the two values of CYA?

Again -- it is not a great effect. So even though it might technically change it, the effect must have been determined by our chemistry experts in the past to not mess with it in the calculation.

 

[mknauss]

Do you know if there's a way to confirm the strength of the MA using the TF-100 chemicals?

Other than add to the pool and check the change, not really. A small change.

 

[mknauss]

Here it is with CYA of 300 ... recommending 117oz

Your TA would be near 200 ppm with that high of CYA.

 

[Stoopalini]

Gotcha. So you’re saying 117oz of 31.45% is correct to drop 8.3pH down to 7.5 with 50ppm borates and 100 TA… and the CYA is inconsequential for my situation. so if I ended up with a 6.8 result, then something in my measurement was off?

Even playing around with borates and TA, and dropping them by 10, still doesn’t seem to have much change on the recommended MA.

I suppose the takeaway is don’t use PoolMath to make pH adjustments of more than 0.4. Maybe it could prompt the user of this risk if they put in a target which is more than 0.4 away from current?

 

[mknauss]

Also the acid could be stronger. Though it being 50% stronger seems like a leap.

Send the suggestion on the Poolmath statement to [email protected]

 

[Stoopalini]

The two bottles do have two different date codes ... I'll wait for the pH to come back up and then next weekend, when I dose again, I'll do a very small change of say 0.2 as a test.

I also sent the email, thanks.

 

[mgtfp]

I just did some calculations with Chem Geek's spreadsheet, which should be more accurate than PoolMath when it gets to larger pH changes.

When entering parameters as in your logs, then the PoolMath calculation is actually not far off, those 117oz should have lowered pH from 8.3 to 7.6.

I suspect that your borates are lower than you assumed. With BOR 20, pH would drop to 6.9.

The TA drop to about 60 makes sense, it matches the 117oz of MA, and the TA reduction is more or less independent from the BOR level.

How are you testing your borates level?

As recommended, make bigger adjustments in smaller steps.

Edit:
Just saw that you are using strips for Borates. They are OK for a rough estimation. How does your colour blindness affect reading these strips?

There is a titration test possible with a few chemicals added (Mannitol sweetener and a pH-Indicator) on top of standard Taylor reagents. But you need to be able to compare blue-shades to get the endpoint right. It might be possible to use your pH-meter instead of the pH-Indicator.

It is described here:

New borate drop test at piscines-apollo vs. test strip

Thanks, SunnyBlues. You have been reading and learning a LONG time to remember all those experts. Thanks for your testimonial.

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