Accuracy of Acid Demand Test vs Pool Calculator

[diasurfer]

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).

Hmmm, I'm not sure where I went wrong in estimating that it must be CYA = 0 but I'm not going to argue with the programmer! Using the Pool Equations spreadsheet, I can compare the results to the Wojtowicz paper I linked above and the answers are very close, so I'm confident I'm using the spreadsheet correctly. When I compare MA needed results for the same conditions except CYA = 40 vs. CYA 80, I get 20% more MA needed with the higher CYA. Similar results from looking at the Woj. paper comparing tables for CYA 50 vs 100. That's ~20% difference. Considering the Pool Calc bothers to give results to +/- 0.5 in decimal, I don't think 20% is small, but arguing semantics is a waste of time.

When I use the Pool Equations, I can see that when I add MA to reach pH Goal, then TA automatically adjusts but CYA remains fixed. Pool Calc does not adjust TA, it just provides a note saying it TA will change to with addition of MA. So I don't understand what you mean by TA making most of the correction you need to make.

Since Pool Calc is in fact using a CYA value, then why doesn't it use a variable CYA since it is already available? Is it because the equations are too complicated to code for the web if CYA is not fixed?

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.

Strange. I found acid demand no more or less difficult than the pH test, but I have an advanced scientific degree so I'm not typical. I noticed more "just ignore the demand test" posts than "I don't understand the demand test" posts from searching the archives, but what you are saying doesn't surprise me given the math-o-phobia I encounter everywhere.

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.

Fair enough. I was just thinking that if there is possibly a way to reduce some errors, why not bring it up on the Deep End. Consensus seems to be this one is not a big deal. Thanks for everyone's comments.

 

[JasonLion]

Since Pool Calc is in fact using a CYA value, then why doesn't it use a variable CYA since it is already available? Is it because the equations are too complicated to code for the web if CYA is not fixed?

The "exact" math is in fact enormously complex. Read some of the equations and macros running in chem geek's spreadsheet sometime.

For PH, Pool Calculator uses a greatly simplified polynomial model which is designed to be within +-10% over most of the range, with occasional larger errors towards the edges of the range.

Nearly all of the individual test results that are used as inputs are +-10% (or worse). The error bars on PH adjustments are thus on the order of +-30% to +-60% (depending on how well you read the PH test, which takes practice) just from the water tests and pool size used as inputs. Fortunately, much of that error, including the Pool Calculator errors, are consistent for a specific pool using a specific test kit, so people learn to compensate for the quirks of their particular pool.

 

[diasurfer]

Bleh. I had a peek. I've never been a big fan of excel for coding (or trying to understand) complicated algorithms. It's not very readable. In another post, chem geek says that it has to be solved iteratively because closed form solutions are not possible for the inverse formulas. I think other approximations are going on the spread sheet, but either way I get it. The math is hairy.