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

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Originally Posted by CarlD

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

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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.

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Originally Posted by CarlD

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)

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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 :)

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

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

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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.

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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.

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.

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

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

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...)

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.)

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

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