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

[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 from OnBalance for calcualting chems since all I need is a printout of their Formula Numbers 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 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.

[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 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. Unfortunately, Jason sold The Pool Calculator to pSIFlow Technology Inc. and they don't seem to be doing any maintenance on it.

[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;