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

[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