New China Shop Topic: How Accurate is the Pool Calculator?

[CarlD]

Today I saw this post from one of our regular posters, AnnaK, whose contributions are always welcome.



Those test kits are fun, aren't they?

At the bottom of the PoolCalc there's a feature called "Effects of adding chemicals" .

It says that adding 180 oz of 6% bleach (the 2 bottles you bought) will raise the FC in a 36,000 gal pool by 2.4 ppm.

Looks like you have things well under control and are on your way to a 5 Minutes A Day pool.

—AnnaK—

So I decided to manually calculate the effect of adding bleach in this case. I was very surprised. But rather than starting a debate in a thread helping a pool owner, I figured the China Shop was the place for it.

Just to point out at the start: Accuracy and Precision are not the same thing. In fact, they are opposite concepts. Accuracy is a description of how far off you can be, where as precision is a measure of how close you are.

Hate to tell you this but I just found that the the Bleach Calculator is inaccurate (I hesitate to say "wrong"--which is not the same thing) due to rounding off significant digits.
The correct increase is 2.3ppm, not 2.4 (actually 2.34ppm)

Pool: 36,000 gallons
Bleach strength: 6% or .06
Amount of bleach: 180 ounces or 1.406 gallons.

The underlying formula for calculating FC delivered is:
(1,000,000/36,000)*.06*1.406 = 2.34ppm, where .06 is, of course the bleach strength, 1.406 is the amount of bleach.

But the calculator rounded the bleach amount up to 1.41 gallons, which results in a ppm increase of 2.35, which it then rounded to 2.4.

So by rounding the amount of bleach up, it pushed the PPM to the rounding threshold, and then rounded up again.

Does it matter in the big picture, ie, your pool? No.
It's probably not actually 36,000 gallons either, since a drop of 2" in a 20'*40' pool is 1000 gallons, which would change the final measurement, too, and make it 2.41ppm.

But in all things B-B-B we take nothing for granted or as gospel.

Now I realize that we all want things to be easy and fast in pool care. But there's a big difference between knowing we are ball-parking a number or amount, and assuming a calculation is correct when it is not.

I ball-park all the time. I use "20,000" gallons in 99.99% of my pool calculations, when it can vary from 18,500 gallons up to 19,600--just 2-4 inches or water can make that big a change. Plus the 19,600 is just an estimate as well.

I also use my "Rule of Thumb" calculation of bleach usage and ballpark around that. But again, I KNOW I'm deliberately allowing a reduced level of accuracy. Still, the basic assumption of the rule of thumb introduces, at its root NO inaccuracy.

It assumes that a pool of 10,000 gallons IS 10,000 gallons, to within a gallon (ie, 9999.5 to 10,000.5), that the bleach concentration is to within ten-thousandth (.05245-.05255 for 5.25% bleach), and that 1 gallon is one gallon, to within a half-ounce either way, and therefore that the values of 10,000, .0525(or .06) and 1 can be used with confidence.

Clearly though, the calculator is making rounding assumptions that introduce an unnecessary (IMHO) decrease in accuracy.

Does this mean we should throw the baby out with the bath water? Definitely not!
But it does mean that if it is critical to have extremely precise results, you should consider using the formula manually.

Carl

[AnnaK]

:)

The short answer to the question then is YES. It's accurate. It doesn't claim to be precise.

Our test results go to 1 significant figure on the FC/CC test. Sample sizes likely vary from one time to the next by a ml or some fraction thereof. Pool volume is certainly not static. Bleach concentrations differ between a fresh bottle and one that's partially used. Dropper sizes change depending on humidity and static electricity. An operator's visual acuity changes with ambient light. And most importantly, goals and targets for a "balanced" pool are given in ranges, not in numbers to 4 places beyond the decimal.

'Fess up: do you REALLY care whether you raise your FC by 2.3 or by 2.4 ppm?

[CarlD]

"Fess up.."

OK. Full Disclosure: I confess. I really do NOT care if it's 2.3 or 2.4!

Saying that, I'm well aware of all the other variations. In fact, I spelled them out. I know how to test for chlorine concentration (I've posted the method a number of times). I know how to calculate pool volume (most use 7.5 gallons for gallons/cubic foot but the more precise number is 7.48). I even realize that drop size changes and the recommended way is to hold the bottle exactly perpendicular.

But I have shown how two separate rounding functions used consecutively have generated an inaccuracy that results in .1 increase above the correct rounded figure.

Is accuracy to 4 decimal places necessary? Sounds petty to say "yes" but it's not. Bleach concentrations go into the formula out to 4 decimal places routinely (5.25% bleach is .0525 in the formula).

We estimate pool volume. We estimate bleach concentration. We even estimate amount of bleach. These are all, to the statistician, known levels of error or "noise" The formula should NOT introduce another level of "noise" if it is not necessary.

Your statement at the top "YES. It's accurate." needs to be amended: Yes, it's accurate to 1 part per million. This is certainly true and I have no problem with that.

However, the calculations CLEARLY show that it is NOT accurate to 0.1 part per million.

Carl

[AnnaK]

Which of the water balance parameters require 0.1 ppm?

Or, coming at it from another way, do the many variables we've both enumerated need an accuracy consideration to 0.0001 part per billion? Can we even measure that?

[CarlD]

Which of the water balance parameters require 0.1 ppm?

Or, coming at it from another way, do the many variables we've both enumerated need an accuracy consideration to 0.0001 part per billion? Can we even measure that?

Don't you mean 1 part per billion (1/1000 of a million).

Of course none of the majors do (pH doesn't really count). The lowest we EVER need to go is .2 (useful if CC is not clearing properly).

But you are defending a known, fixable inaccuracy in the system.

Carl

[Manslick]

What if 6% chlorine is 5.9%. OMG.

[AnnaK]

The inaccuracy of the bleach calculations within the Pool Calculator is not really germane within the context of usage.

The defense rests. The defense is just about to take her dogs swimming in a pool that may well be off by a couple parts per gazillion (that's 10 to the nth). The defense does not plan to agonize over that. Instead, she plans to dose herself with an imprecise amount of Margarita, accuracy be gosh danged.

Have a great weekend!

[Manslick]

You go girl.

[CarlD]

The reason I put this in the China Shop was so that it could be batted around and argued about. I do not like tools I know are inaccurate, and I can guestimate chlorine usage just as quickly and accurately with the Rule of Thumb--and I don't need to be near a computer to do it. I can estimate MORE accurately with just a calculator even on a cell phone.

This is not a statistical error. That would be simply noise and of no concern. It is a systemic error which is a whole 'nother kettle of fish.

The Pool Calculator generates other recommendations, including amounts of Borax and other additives. What is the accuracy of THOSE estimates?

I am reminded of Doc Brown's 2 story tall ice machine that makes one cube of ice.

Remember: The China Shop is for arguing. If you don't want to, you don't have to.

Enjoy swimming with your dogs (BTW, they are something like 30x dirtier than a person who gets in your pool. So 2 dogs is like a load of 60 bathers) (according to our vet who does water-therapy for dogs).

Carl

[waterbear]

The pool calculator is a useful tool but ANY method of calculation of chems DEPENDS on ACCURAATELY KNOWING THE AMOUNT OF WATER IN THE POOL AT THE TIME OF CALCULATION!
The Pool Calculator and BleachCalc (remember BleachCalc?) both produce very similar results (certainly close enough for pool maintenance...it's not rocket science after all--its a swimming pool!) except for the borate calculation where it is known that BleachCalc has a major error in the algorithm used to determine borate concentration.
If you want more precision and accuracy then Chem Geek's "monster" spread sheet is around (but I find it intimidating and a bit scary and I am VERY computer literate!) However, unless you are doing theoretical workd on the effects of chemical additions in a pool you once again do not need this kind of precision.
Personally I like to use the Simplified Formulas for Chemical Additions (http://www.poolhelp.com/SimplifiedDosageFormulas.pdf) from OnBalance for calcualting chems since all I need is a printout of their Formula Numbers (http://www.poolhelp.com/SimplifiedFormulaNumbers.pdf)and a calculator, which is much easier to carry to the pool than a computer or even a phone!
They even have a method to chemically determine the volume of a pool (http://www.poolhelp.com/ChemicallyDerivedPoolVolumes.pdf)that I have used several times and it certainly gets you pretty much in the ballpark, even for oddly shaped pools!

For a Newbie, I think BleachCalc is the easiest to use but the Pool Calculator that Jason from TFP put together has a bit more info for more advanced users.
BUT, if you don't accurately know how much water in the pool then NONE of these tools are going to be very useful at all.

[waterbear]

The Pool Calculator generates other recommendations, including amounts of Borax and other additives. What is the accuracy of THOSE estimates?



I have used all the calculations on it and have found it close enough for government work. so to speak. The Borax calc is very much on the money but then again your don't need that much precision. 60 oz by weight of borax and 30 liquid oz of muriatic acid will raise borates to approx 50 ppm in 1000 gallons and keep the pH pretty close to where you started. You really don't need to be more precise than that, IMHO.
If you are talking about borax for raising pH then NO calculator is going to tell you how much to add. You will need to use a base demand test and use twice the weight of borax as you would soda ash for a specific pH rise as determined by the test. Not rocket science and a lot easier than adding a bit and waiting and testing and adding a bit more and waiting and testing until you 'creep up' on your target pH.

[AnnaK]

Enjoy swimming with your dogs (BTW, they are something like 30x dirtier than a person who gets in your pool. So 2 dogs is like a load of 60 bathers)


Wouldn't that depend on the size of the dogs? And their breed? A Chinese Crested is probably a lot cleaner than a bunch of . . . oh, say, Belgians.

I do appreciate the concept of the China Shop but I'm too old to argue. I've learned that when I'm right, I'm right.

Those Margaritas were lovely :)

[CarlD]

A Chinese Crested is probably a lot cleaner than a bunch of . . . oh, say, Belgians.

DEFINITELY! Curiously the little one gets a lot dirtier. But she LIKES mud...the swamp rat!


I do appreciate the concept of the China Shop but I'm too old to argue. I've learned that when I'm right, I'm right.

Even when you are wrong! :) (ESPECIALLY when your're wrong! :) )

I love a good margarita, with lots of salt. Chilis BEST thing for years was their "El Presidente" margarita using Presidente Brandy with the Tequila. Last one I had, a couple of years back, they ruined. I was heart-broken.

[chem geek]

In the "Effects of adding chemicals" in The Pool Calculator, what you are seeing is NOT a rounding error. It is instead taking into account the fact that the Trade % of 6% bleach is actually 6.17%. The 6% listed in the ingredients for bleach is a WEIGHT percentage of bleach, but for your simple formula you need a VOLUME percentage of Available Chlorine aka Trade %. There is also a WEIGHT percentage of Available Chlorine and that is also printed on the Clorox Regular bottle as 5.7%.

Chlorinating liquid almost always uses Trade % (i.e. volume % of Available Chlorine) even though I've seen this percentage listed in the ingredients. Yes, this is all very confusing, but The Pool Calculator assumes that bleach is specified using a % weight of sodium hypochlorite but for its calculations it uses the Trade %. For chlorinating liquid, it assumes the strength is already in Trade %. Now while The Pool Calculator does this correctly in the "Effects of adding chemicals" section, in the FC Now Target section it seems to incorrectly use Weight % for all bleach entries even when they are a high enough % (such as 10 or 12.5) to imply chlorinating liquid that is more likely a Trade %.

The formulas for these conversions are as follows:

Trade % = (Weight % NaOCl) * (Specific Gravity) * (Cl2 g/mole) / (NaOCl g/mole) = (Weight % NaOCl) * (Specific Gravity) * (70.9064) / (74.44) = (Weight % NaOCl) * (Specific Gravity) * 0.9525

Weight % Available Chlorine = (Trade %) / (Specific Gravity)

The density of 6% bleach is around 1.08 while for 12.5% chlorinating liquid it's around 1.16.

Yes, this is all very confusing, but fortunately it doesn't matter that much unless you are looking at better than 3% accuracy.

[madwil]

BTW, .0525 is 3 sig digits, not four... placeholder 0 doesn't count;

[CarlD]

Richard, that's the first good defense I've read of the deviation in the Pool Calculator from the formula. THAT is an explanation I can accept. It also points out there is a potential inaccuracy in the FC section. Naturally, neither the PC nor the formula take into account any degradation or breakdown in the chlorine over time. I'm confident your coefficients are correct but where do they come from. BTW, how many grams/mole are there in CL2 and NaOCl?

Madwil, I suppose it's a difference of definition but the VARIABLE "Concentration" must be defined as 4 digits to the right of the decimal point because available VALUES can use any of the 4 places...LC would be expressed as .125, while regular bleach is .0525. Each uses only 3 digits, but 4 are still needed.

Carl

[madwil]

That's why it's a place holder...
0's at the end of a number, without decimal places, are not significant.
0's after the decimal, before numbers, are not significant if 0 precedes the decimal.
Otherwise, you could call 2 ppm .000002 and say it's 6 sig dig, when it is only 1!
Consider scientific notation- a.bc x 10 to n power- a.bc is sig, not the location of the decimal point, with regards to accuracy/precision of measurement
When you mult/div numbers, the lowest number of sig dig in any of the factors is the number of sig dig in the product
When you add/sub, the number with the highest multiple of 10 becomes you last sig dig (1,250 + 2, is still 1250 for significance...)

[waterbear]

People, we are losing sight of the fact that we are talking about a swimming pool and the amount of water in the pool needs to be accurately known (and not rounded off to the nearest 1000 gallons or so) or this all become moot.
It's a swimming pool, not a science experiment and in the real world we are just splitting hairs here that are not important. Now I am the first one that insists in precision and accuracy when doing a scientific experiment (lest anyone forget I have a very similar scientific background like chem geeks) BUT you need to take in account your largest margin of error (which in our case would be the volume of water in the pool) and not try and get a precision smaller than this limiting factor because it is invalid! Yes, I know that errors are cumulative but if we are talking about errors that differ on such a significant order of magnitude they become insignificant!

We also have to take into account the precision of our testing methods and any errors introduced by our testing methodology.

What it boils down to is that the 'guestimate' that CarlD provided of one gallon of liquid chlorine adds approx. it's strength of FC to 10000 gallons of water is about the amount of precision that we can expect in real world situations of pool maintenacne and it is most certainly close enough for our purposes of maintaining a pool.

(If anyone is old enough to remember using triple beam balances or balance pans remember that the weight is taken after three equal oscillations of the pointer and not when the pointer had come to rest. This is my point, you have to realize what the limit of your precision is.)

[Watermom]

Y'all just need to go swimming and have some fun.

[CarlD]

You don't think arguing is fun???? That's why we have the China Shop.

There's difference between measuring with something you know is off and correcting for it: "My speedometer always reads 5mph fast"
and something you believe is correct and isn't "But Officer, I had it NAILED at 55mph on my speedometer, not 65".

Can't go swimming. It's nasty, cold and raining. Therefore, arguing is a valid alternative source of fun! :)

Carl

[waterbear]

Can't go swimming. It's nasty, cold and raining. Therefore, arguing is a valid alternative source of fun! :)

Carl

Explains everything!:rolleyes:
Beautiful sunny weather here, been swimming for several months now and the pool stays a comfortable 85 degrees without my heat pump running! Getting ready to jump back in before I start grilling! :p

[chem geek]

BTW, how many grams/mole are there in CL2 and NaOCl?
CL2 is 70.9064 g/mole while NaOCl is 74.44 g/mole. That's where those numbers came from in the formula in my post.

[PoolDoc]

Hey All, Richard;

Tell me that you have taken into account the time and temperature related decomposition of bleach? Odds are that 6% bleach started as 8 or 9 in hopes it would still be 6 when YOU got it. If they filtered the nickel and iron out, and kept it cool, it probably IS 6% or more. If not, well, not.

Ben

[chem geek]

The idea is just to start out with what is the most likely value -- that claimed by the manufacturer. As I wrote, it's not that important, but if you're going to have an automatic calculation you might as well start with the correct intended number. Errors accumulate so there's no point in starting with something you know is likely to be wrong.

6% Bleach doesn't degrade very fast because the rate of degradation goes roughly as the square of the concentration. 12.5% Chlorinating liquid degrades more quickly. See the chart at the bottom of this page (http://www.odysseymanufacturing.com/about_product.htm). As for impurities, you'll note that this manufacturer does remove catalyst metals from their source water and I checked with Hasa on the West Coast who is the source of the chlorinating liquid I use as well and I also checked with Clorox about their bleach. Most of the big players take care to produce a quality product. I can't speak for the off-brand Ultra bleaches.

[PoolDoc]

Hi Richard;

I was sort of teasing, though I do think excessive precision is a bane of good pool care.

That said, because I bought, sold, and delivered 15% bleach for 15 years, I investigated bleach stability as much as I could, given the paltry Internet resources available at the time.

One possible oversight in your remarks above: according to the man whose company* made the bleach I started off with, the significant source of metal contamination of bleach was not the source water, but the chlorine liquid. Nickel and some other trace contamination originated in the manufacturing process; iron in the transport and piping process. Producing low-metal bleach required post manufacture filtration of the bleach itself. Fully effective filtration apparently results from the use of bleach tolerant micro-filtration.

So, based on what I was told, a bleach manufacturer who's hyping source water filtration is really just dodging the question.

The first company I purchased from did not even make high purity bleach at the time -- their business was build on selling to the cotton chenille makers in Dalton, Georgia, and it was a fast delivery, high turnover process. By the time I was dealing with them, their business was with cotton sock makers in Alabama. But even in summer, the bleach didn't decompose faster than their 2x weekly deliveries. The second company did make high purity bleach, but only on an erratic schedule, and my purchase volume wasn't large enough to change that schedule. However, water filtration would have been a non-issue for them both, since they were using limestone spring or well water.

Ben

* His father-in-law revolutionized bleach production -- so I was told -- by dumping sugar in bleach, resulting in the formation of gluconic acid which chelated the iron in the bleach.

[HofstraJet]

If you are talking about borax for raising pH then NO calculator is going to tell you how much to add. You will need to use a base demand test and use twice the weight of borax as you would soda ash for a specific pH rise as determined by the test. Not rocket science and a lot easier than adding a bit and waiting and testing and adding a bit more and waiting and testing until you 'creep up' on your target pH.

So The Pool Calculator's suggestions for how much Borax to add for a specified rise in pH are incorrect?

PS - love the academic debate. Keep it up!

[waterbear]

Acid and base demand cannot be calculated without a chemical test since they depend on other factors such as TA and temperature and how fast the chemical is added. (for example muriatic acid will have an almost instantaneous effect on pH while cyanuric acid, because of its slow dissolving nature, will not have a appreciable MEASURABLE effect since the slow drop in pH that occurs as it dissolves will be offset by the bicarbonate buffer in the water and the pH rise from outgassing of CO2 and the drop in TA that it will cause is going to be smaller than the precision of our TA test.)

A calculator like the pool calculator or a treatment table can only approximate how much of a pH rise or drop a certain amount of a chemical can cause that may or may not have ANY basis in reality! IF your pool is in what is considered a normal range of balance and temperature then the results can and will be off, particularly for large changes in pH. In fact, here is a direct quote from the pH section of the pool calculator:
Note: pH calculations depend on TA and Borate. Results are approximate and can be off significantly for large pH changes. Changing your pH will also change your TA.

IF you want a more precise way to determine how much of a chemical you need then do an acid or base demand test! (since base demand tests are set up for soda ash--sodium carbonate--it is lucky that twice the amount of borax by weight--sodium tetraborate decahydrate--produces about the same pH rise so you just need to double the weight of soda ash when substituting borax for pH increasing.

[chem geek]

As waterbear noted, knowing an accurate amount of acid or base to add to move to a specific pH is best determined through an acid/base demand test such as found in the Taylor K-2006 test kit. However, The Pool Calculator does a decent job estimating the amount and accounts for the various buffer systems in the water including carbonates (TA), borates and CYA. However, at the time the calculator was made the accurate calculations in my spreadsheet (http://richardfalk.home.comcast.net/~richardfalk/pool/PoolEquations.xls) were too complicated to put into the calculator so Jason created tables/formulas that approximated the effects. Since that time, I've found simpler formulas for the accurate calculation and this is discussed in the thread pH Buffer Capacity (http://www.troublefreepool.com/ph-buffer-capacity-t31321.html). Unfortunately, Jason sold The Pool Calculator to pSIFlow Technology Inc. and they don't seem to be doing any maintenance on it.