Accuracy of Acid Demand Test vs Pool Calculator

[diasurfer]

My water is close to the recommended balance and I'm about to starting using my SWCG that I just installed. I drained 60% of my pool water and slowly crept up on my goal values to avoid overshoot. That story is outlined in my thread in Just Getting Started. Along the way, one thing I noticed and remarked upon in my thread but might be of wider interest is that I found that the Pool Calculator significantly underestimated the amount of muriatic acid I needed to bring down my pH, whereas the acid demand test in my Taylor K-2006 seemed to estimate it quite well.

My test kit only measures pH up to 8.0 so I disregard those measurements where from hindsight I know it was clearly higher. But two days ago, I measured a pH of 7.8 (my first measurement clearly below 8.0). Acid demand test showed 1 drop to 7.6 and 2 drops to 7.4. The formula for fl oz of 31.45% muriatic acid used to populate Table E of Taylor's Pool & Spa Water Chemistry handbook is MA (fl oz) = 9.165 * 1e-4 * N * V where N is the number of drops required to reach goal pH and V is the volume of the pool in gallons. (See also the first equation in Wojtowicz's paper here: http://jspsi.poolhelp.com/ARTICLES/JSPS ... p39-56.pdf ). [edited to fix notation in equation for clarity]

Since I was aiming for 7.5 (not realizing at the time that 7.8 is in fact considered "ideal") I used 1.5 drops and 12000 gallons for my pool in the equation and came up with 16.5 oz of muriatic acid. On the other hand, using 12000 gallons in the Pool Calculator, and entering TA = 70 (test performed using speed stirrer) and 0 for borates gave 8.5 oz. This is very nearly a factor of 2 difference!

I decided to split the difference and added 12 oz of MA. (Unrelated but I also added 40 lbs of solar salt). After 24 hours of my pump running, I checked my pH and it was 7.6. From the acid demand test the day before, to get to 7.6 was one drop corresponding to 11 fl oz which was close to what I added, so the acid demand test was as accurate as one might expect from reading the color scales. From the Pool Calculator, adding 12 fl oz should have brought my pH down to 7.4, but my pH clearly did not go that low.

I then added 4 more oz for 16 total of MA and the next day I measured it at 7.5, so the original acid demand test was accurate while Pool Calculator said that 16 oz should have brought it to 7.3. I also reconfirmed my TA is at 70.

What is going on here? I thought of two possibilities for seeming inaccuracy in Pool Calculator estimates for MA. (Note that uncertainty in the volume of my pool should impact both methods in the same way).

1. Borates. I replaced 60% of my pool water and I assume tap water doesn't have borates. (Last night I added 25 lbs of boric acid but this was before I added them). It's possible my old pool guy had added something with borates but as far as I know he only added trichlor or dichlor. I actually measured them at the same time I measured 7.5 pH using the LaMotte's strips. The test strips are hard to read and it seemed to match the 15 ppm square almost as well as the 0 square, so I noted 0-15 for borates. But playing around with the Pool Calculator, for TA 70 and 12K gal pool, to add 16 oz of MA and go from pH 7.8 to 7.5 requires 23 ppm borates and that is clearly higher than I had in my pool at the time, but it can't be ruled out that some small concentration of borates could be part of the issue.

2. CYA. The Wojtowicz paper referenced above provides tables of MA needed to reduce pH to 7.2 for various start values of pH and ppm of TA. What is interesting is that he provides different tables for 50, 100, 150, and 200 ppm of CYA. He also provides a multiple linear regression showing MA needed vs. starting pH, TA, and CYA, and the factor in front of CYA is ~0.13. An example from the table is that is takes 26.4 oz MA to reduce pH from 7.8 to 7.2 at TA 90 with CYA 50 , and 32 fl oz at CYA 100. FWIW my CYA is 75.

I've read several posts claiming that the acid demand test in the K-2006 is superfluous, but since it is a more direct measurement for pH change and thus unaffected by errors in borate (and CYA) measurements, perhaps it should be acknowledged that it is the more accurate way to estimate the need for MA? Personally, I'd rather take the few extra seconds it takes to do the acid demand test and get the correct amount of MA to add the first time, rather than having to add MA twice. Wow that stuff is nasty!

Although it is acknowledged on Pool Calculator that adding CYA will lower pH, as far as I can tell it does not account for CYA level when calculating pH changes (or perhaps it assumes a fixed value). Why not?

I realize that the Pool Calculator has been verified by 1000's of users whereas this post is based on the experience of only one inexperienced pool owner over two days. Big difference. But I will also note that I've read several seemingly experienced posters talk about the need to "get to know your own pool" as far as the amount of MA to add regularly, particularly for SWCG owners dealing with pH drift. The Pool Calculator is a obviously a great tool and it's been very helpful to me. I offer this post as a suggestion for making it more accurate, if it in fact accounting for CYA as well as TA and Borates would do so.

 

[Melt In The Sun]

I think the acid demand is absolutely accurate, since it is directly measuring pH change in your pool water...which, as you wrote, means no worrying about the other numbers. The Pool Calculator depends entirely on the numbers you enter into it, which always have some slop in them. That being said, I don't use it I just dump in enough acid to get close to my target (knowing your pool is important here) and don't worry about exact numbers.

 

[onBalance]

Forgive me if I don't accurately address or understand your question, as I only skimmed through your post and am not familiar with the Pool Calculator. But this is my way of an explanation.

The acid demand test is usually very accurate because it is affected by the current content of TA, CYA, borates, sulfates, temp, chlorine residual, current pH, how low of a pH one wants to go, etc., all of which affects the amount of acid needed for a pH change. But it is the TA (actually bicarbonate/carbonate) that plays the biggest role of how much acid is needed for a particular pH change, and in either direction. The lower the TA, the less acid (or base) is needed to lower (or raise) the pH. The higher the TA, the more acid (or base) is required for a pH change.

Further, when the TA is below 80 ppm, each lower 10 ppm increments of TA change, will change the amount of acid needed (for a change in pH) more drastically. With each 10 ppm increment increase on very high TA (over 180 ppm) will not change the amount of the acid (or base) needed very much for a particular desired change in pH. Charts that provide precise numbers are rather complicated (given all of the variables) and of course, are somewhat difficult to read and understand.

The reason Wojtowicz showed the effect of different levels of CYA is because he wanted to demonstrate how CYA (and borates) also buffers (like bicarbonate/carbonate alkalinity does) or changes the required amount of acid needed. But CYA or borates are not as big of a player as is bicarbonate/carbonate alkalinity in typical pool water ranges.

 

[bobodaclown]

But it is the TA (actually bicarbonate/carbonate) that plays the biggest role of how much acid is needed for a particular pH change, and in either direction. The lower the TA, the less acid (or base) is needed to lower (or raise) the pH. The higher the TA, the more acid (or base) is required for a pH change.

Maybe I'm being over simplistic but I thought it was volume of water and PH change that played the biggest role of how much acid is required to lower the PH (to go from 7.8 to 7.5). Those are the factors the Taylor book uses in their charts.

I think the difference from pool calculator and the direct test is the pool calculator is calculating based upon given information, whereas the direct test has all the factors it needs the acid demand is an actual acid calibrated to the result tables.

I equate it to baking a cake, you have the recipe, all the ingredients you have an idea what the cake will taste like (pool calculator), but you really don't know till you taste it (actual direct test). Or real world vs. hypothetical.

The buffers I though control how fast the event will take place.



Hope that makes sense.

Bob

 

[onBalance]

Bobodaclown, I stand corrected Volume of water would be the biggest factor. However, I don't think "speed of reaction" is at play here, except maybe with temperature.

 

[diasurfer]

But it is the TA (actually bicarbonate/carbonate) that plays the biggest role of how much acid is needed for a particular pH change, and in either direction. The lower the TA, the less acid (or base) is needed to lower (or raise) the pH. The higher the TA, the more acid (or base) is required for a pH change.

Maybe I'm being over simplistic but I thought it was volume of water and PH change that played the biggest role of how much acid is required to lower the PH (to go from 7.8 to 7.5). Those are the factors the Taylor book uses in their charts.

Volume of water is of course a factor but it is the same simple scale factor for both the Pool Calculator and the Acid Demand test (twice as much acid for a 20K g pool vs 10K g pool). This is easily confirmed by playing with the Pool Calculator and by looking at the Tables (or the equation summarizing the tables I gave above). And of course a larger pH change requires more acid, but the issue is for a given pH change and given pool size, what gives a better estimate of the acid needed. I think most of us agree that acid demand test is more accurate because it accounts for all the secondary factors directly.

I'll use the cake analogy, but in a way I think is better than just saying it is the difference between a recipe and how the cake actually turns out. The acid demand test is a way of tasting the batter before baking the cake and deciding what it needs. The Pool Calculator is a way of making a cake from a recipe, but in some cases, some of the minor components might not be included on the ingredient list.

OK, that is not a perfect analogy either. I would point out though that looking at old threads, prior to 2009 the Pool Calculator didn't take borates into account for the pH adjustment calculation. And I'm suggesting that it could be made more accurate now if it included CYA level (and any other minor factors).

Either way, I'll continue to use my acid demand test. Although the Test Kits Compared section of Pool School states that it is "Not really useful in most cases" (which is why I didn't even use mine for awhile when I first started testing), my experience is that it is useful in a fairly typical use case.

 

[chem geek]

Since 80 ppm CYA and 15 ppm Borates or 50 ppm CYA and 20 ppm Borates in 12,000 gallons would roughly be 16-17 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid), then that's what I figure happened. You are right that apparently The Pool Calculator doesn't account for CYA so uses some fixed CYA assumption. The Pool Equations spreadsheet does these calculations accurately. The Pool Calculator uses some internal approximate tables since calculating how much acid or base is needed to move the pH is complicated. You can see some formulas for doing the calculation in the thread pH Buffer Capacity, but I did that after The Pool Calculator was done by Jason. As noted, "the amount of acid or base needed to change the pH by a specific amount can be calculated by the difference in C_buf*K_a/(K_a+[H⁺]) at two different pH values and summing these differences for all of the buffer species." However, that hides some of the complexity since there are multiple K_a for carbonate and bicarbonate system and therefore further calculations needed to calculate the C_buf of each pair of species. I use a different technique in my spreadsheet that in some ways is even more complicated.

 

[duraleigh]

my experience is that it is useful in a fairly typical use case

Well, not really. Millions of pool owners have never used it and are able to maintain their pools just fine.

I think, if you simply feel you need the extra precision, then the acid demand test is helpful. However, "in a fairly typical use case", most folks are happy by simply adjusting pH using the pool calculator. BBB is not about scientific precision but rather about a practical, convenient and accurate (enough) way to manage your pool correctly and successfully and I don't believe the acid demand test is a necessary part of that.

 

[UnderWaterVanya]

I would argue that since the Acid demand test (like the pH test in general) relies on color matching there is an inherent inaccuracy - also since the number of drops is fixed to particular steps (can't get half a drop) it's got fixed inaccuracy built in - 1 drop = X; 2 drops = Y but I really needed X + C however there is no way to know that from the test.

Ideally you would combine the details from both sources and learn your pool well enough to know which was more accurate in your case and adjust the results to suit the situation. I see nothing wrong with using Acid demand if the pool calculator results are not working well for you - but at the same time see no reason it should be assumed that it is completely accurate given the color match requirements and the limit on precision of the actual demand portion of the test.

 

[diasurfer]

my experience is that it is useful in a fairly typical use case

Well, not really. Millions of pool owners have never used it and are able to maintain their pools just fine.

I think, if you simply feel you need the extra precision, then the acid demand test is helpful. However, "in a fairly typical use case", most folks are happy by simply adjusting pH using the pool calculator. BBB is not about scientific precision but rather about a practical, convenient and accurate (enough) way to manage your pool correctly and successfully

We're not talking about decimal places here - IMO being off by a factor of two (8 oz vs. 16 oz) as it was in my case is about more than just scientific precision. For me it was about one trip out to my shed to dose my pool with that nasty muriatic acid (if I had just trusted the acid demand test results) vs two (or more if I had continued to trust the Pool Calculator on subsequent doses and continued underdosing), plus the additional pH testing required to check if the additional MA doses are getting you to your pH goal. From what relatively small amount of I have read of these boards in the last six weeks, I can tell people are happy with BBB in general, as I am sure I will be, but I'm just saying that they might be even happier if they didn't have to go through as much trial and error to figure out how much MA to dose their pool with.

and I don't believe the acid demand test is a necessary part of that.

I agree. The acid demand test is probably the quickest and easiest way to do the pH component of BBB because it is the most accurate (the first point I'm trying to make), but it is not necessary. The Pool Calculator is sufficient, but why not make it more accurate (the second point I'm trying to make)? I wasn't aware of chem geek's excel spreadsheet until he posted it (thank you). When I enter the same values from my case above in the Pool Equations spreadsheet and the Pool Calculator, I get them to reproduce the same add 8.5 oz MA result (my original case) only when I enter CYA = 0 in the Pool Equations spreadsheet. So that it is the assumption in the Pool Calculator that chem geek mentions: CYA = 0, or more likely, CYA is not an input.

When I enter CYA = 80 and Borate = 15 in the Pool Equations spreadsheet, the MA dose is 16-17 as chem geek stated, consistent with my acid demand test results. Leaving CYA = 0 and setting Borate = 15, the dose is 13 oz, while leaving Borate = 0 and setting CYA = 80, the dose is 12 oz. So in my case, leaving Borate out of the calculation vs. CYA out of the calculation have a roughly equal effect ... each alone will underestimate the MA dose by a factor of 1.5, and together a factor of 2.

Since CYA is one of the 5 basic pool chemicals everyone tests for as part of BBB, I can't think of any reason it would be better to NOT include it in the Pool Calculator's calculations as far as making it a better tool. As I said before, at one point it didn't include Borates either, and now it does, so why not add CYA too? But practically speaking, chem geek's posts show the mathematics are not straightforward and perhaps the solutions aren't easy to implement in a Java web/app tool. Also I recall coming across a post that Jason no longer owns the Pool Calculator so that sounds like an impediment too.

 

[UnderWaterVanya]

But practically speaking, chem geek's posts show the mathematics are not straightforward and perhaps the solutions aren't easy to implement in a Java web/app tool. Also I recall coming across a post that Jason no longer owns the Pool Calculator so that sounds like an impediment too.

My understanding - which is not informed by much other than reading here - is that the Pool Calculator is semi-abandoned. The current owner is not responsive to requests about it. I think it would be a great to update it but I think that's a low probability event.

What happens if you adjust your borates numbers in pool calc to some increased number like CYA + Borates - does it work then? It might be a simple work around for people who have issues with accuracy.

 

[UnderWaterVanya]

Now I'm confused. I tried running PoolCalculator.com with a 12000 gallon plaster pool with SWG. I set the TA to 70 (and 80 at one point) and CYA to 80 and Borates to 15. Salt is 3500. pH is set to 7.8 with a goal of 7.5:

EDIT: TYPO EARLIER SAID 4 not 14! My results show 14oz of 31.45% MA

What did I do wrong? I was trying to recreate the original posts issues but I can't seem to get it to give me the very low result. With an 80 TA this grows to 15 oz which is very close to the 16 oz. What else could I have set differently from the OP? (I tried 13K gallons also which gives me 16 oz with an 80 TA...)

 

[diasurfer]

I would argue that since the Acid demand test (like the pH test in general) relies on color matching there is an inherent inaccuracy - also since the number of drops is fixed to particular steps (can't get half a drop) it's got fixed inaccuracy built in - 1 drop = X; 2 drops = Y but I really needed X + C however there is no way to know that from the test.

Ideally you would combine the details from both sources and learn your pool well enough to know which was more accurate in your case and adjust the results to suit the situation. I see nothing wrong with using Acid demand if the pool calculator results are not working well for you - but at the same time see no reason it should be assumed that it is completely accurate given the color match requirements and the limit on precision of the actual demand portion of the test.

Well the inaccuracies of color matching are going to affect acid demand test vs Pool Calc the same as far as pH starting point, but it's true that with acid demand you have to do color matching twice. (If your starting pH is at the end of your scale, this is a good thing - I started with a couple of 8.0 max measurements on my kit but could tell they weren't the same due to the demand test). But I'll also point out that to use Pool Calc accurately, you also have to know your borates, which is another color match if you use the strips. I've only done each test a handful of times and IMO the pH test has atomic clock precision compared to those borate strips! Also, as far as the increments on the pH changes, I got 1 drop to 7.6 and 2 drops to 7.4 and used 1.5 drops to estimate 7.5 and that ended up being dead-on compared to the Pool Equation spreadsheet.

Nothing is "completely accurate" but IMO it's pretty clear that acid demand test is more accurate. But I think you have it reversed, TFP should tell people to use it if they have it, rather than ignore it and use Pool Calc from the beginning. It's an easy test, whereas Pool Calc has known deficiencies for pH estimation.

 

[diasurfer]

But practically speaking, chem geek's posts show the mathematics are not straightforward and perhaps the solutions aren't easy to implement in a Java web/app tool. Also I recall coming across a post that Jason no longer owns the Pool Calculator so that sounds like an impediment too.

My understanding - which is not informed by much other than reading here - is that the Pool Calculator is semi-abandoned. The current owner is not responsive to requests about it. I think it would be a great to update it but I think that's a low probability event.

What happens if you adjust your borates numbers in pool calc to some increased number like CYA + Borates - does it work then? It might be a simple work around for people who have issues with accuracy.

That's too bad. Second idea sounds like it could be a dangerous hack in the wrong hands. Maybe TFP should start recommending use of chem geek's spreadsheet instead of Pool Calculator.

 

[diasurfer]

Now I'm confused. I tried running PoolCalculator.com with a 12000 gallon plaster pool with SWG. I set the TA to 70 (and 80 at one point) and CYA to 80 and Borates to 15. Salt is 3500. pH is set to 7.8 with a goal of 7.5:

EDIT: TYPO EARLIER SAID 4 not 14! My results show 14oz of 31.45% MA

What did I do wrong? I was trying to recreate the original posts issues but I can't seem to get it to give me the very low result. With an 80 TA this grows to 15 oz which is very close to the 16 oz. What else could I have set differently from the OP? (I tried 13K gallons also which gives me 16 oz with an 80 TA...)

Aha! I was getting 14 too. My result of 8.5 oz MA in the OP was based on borates of 0. Put that in Pool Calc with TA = 70 and that is what you should get. At the time I assumed borates = 0 because I hadn't added any, had replaced 60% of my water with city water, and didn't think my old pool guy had added any. Then once I tested them, I noticed that I couldn't decide between 0 or 15 ppm being a better match, so there might be some borates in the pool. But that is part of my point of the nice thing about the acid demand test - you don't have to know you're borate accurately (or at all).

As chem geek pointed out, my issues with Pool Calc where a combination of not knowing my borates level, and Pool Calculator not accounting for CYA. I encourage you to play around with his spread sheet as well. There are small differences even when you enter CYA = 0 in his spreadsheet.

 

[UnderWaterVanya]

I guess I see your point - but I would have picked a borates level of 8 (splitting the difference between 0 and 15) and then you would have gotten a 12oz result not an 8.5oz result - which is so much closer to the 16 you seem to have needed. My overall point is that there isn't a 2x difference if you only drop out the CYA - it's more like a 15-33% difference.

I also see the point that color matching is involved in borates, and pH so inaccuracy is inherent. I just do not see the delta being massive. I could live with using the acid test or without it - frankly when I test the pool and it hits 7.6 vs. 7.5 - I'm not going back for more acid - I'm going to be happy with the result - adjusting pH isn't very precise already.

EDIT: Earlier I had the number at 13 - which was based on a TA of 80 not 70 - too many numbers!

 

[diasurfer]

I didn't pick a borates level of 8 from the beginning because I had never done a borates test. At the time, I didn't think I had any. I bought the strips because I decided to add borates but I was trying to adjust pH first which is when I came across the issue. Before adding the borates, I did a test and was surprised that I had somewhere between 0 and 15 ppm borates already. But that's my particular story.

I think the wider issue (the reason I started a Deep End thread) is that, as you state pretty accurately, leaving CYA out of the equation underestimates MA dose by 15-33%.

It goes the other way too. I now know that 7.8 is considered ideal compared to 7.5. Let's say I decided to raise it back to 7.8 (I'm not going to). According to the Pool Calculator, and the Pool Equation spreadsheet with CYA = 0, I should add ~19 oz of borax. With Pool Equation and CYA = 75, I should add 25 oz, or 33% more.

These could be viewed as a good thing since it's a built in method of avoiding overshooting the goal, but if one is already factored in that they are creeping up on a goal (and didn't know about this issue in Pool Calc), then they are going to creep up on it frustratingly slow.

 

[ping]

The calculator gets you in the ballpark and tends to error on the safe side. Once you know your pool, you most likely won't need the calculator, and you will know what and how much you need to add to keep your pool balanced. I don't remember the last time I used it for my pool.

 

[duraleigh]

The calculator gets you in the ballpark and tends to error on the safe side. Once you know your pool, you most likely won't need the calculator, and you will know what and how much you need to add to keep your pool balanced. I don't remember the last time I used it for my pool.

Exactly. ANY test an individual chooses to use in his management routine is just fine if that individual finds value in it.

Our goal at TFP is to teach a level that fits the vast majority of folks and to make that level is as simple as possible but still adequate.

I have had my pool long enough that I can do a pH test and adjust it correctly simply by eyeballing how much acid I pour out of a gallon. I have a fairly large pool so I get a forgiving margin of error but I bet there's a bunch of y'all that have learned the "rhythm" of your pool that can do the same thing.

 

[JasonLion]

Pool Calculator assumes CYA is 40 and does not correct for the CYA value you enter. The error caused by having a different CYA levels is actually fairly small, since changing the CYA level also changes the TA level, and that makes most of the correction you need to make (though certainly not all).

The acid demand test has a couple of advantages. It works even if the PH starts outside the range of your test kit. It only has two measurement errors, instead of three (PH twice, instead of PH, TA, and Borate). It also works when you don't have any idea what your TA and/or Borate levels are. However, it have one fairly large, but difficult to evaluate disadvantage, which is that a fair percentage of novices seem completely unable to master it at all. The PH test in general seems to be fairly intimidating for some people, and add another few steps to it and it seems to get beyond what they are capable of.

There is one other large issue here. "Exact" calculations are a myth, and simply don't happen in practice. Regardless of how well your math matches theory, there are errors at every step in the process, initial measurement errors, math mistakes, chemical measurement errors, pool size estimation errors, and so on. To keep things stable you need to close the loop, test, adjust, and test again to see where things turned out. Because of that, calculation errors (within reason) are not crucial.