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Chlorine Demand

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just a footnote on this procedure from my experience in a bromine spa with ozone: the decay rate is NOT linear. if it is, then you have bad guys growing. for healthy water, a good bromide bank and a strong, corona-discharge ozone generator, the sanitizer decay rate asymptotically approaches some very small non-zero number over some long period of time. I have personally measured decay rates of 10%, which implies a very long time before you can use the tub if you start at 10ppm FC equivalent!

My ozone generator runs continuously (I can't shut it off), so I'm very careful not to shock with anything, including MPS (which also produces bromine). Happily, if the water is that healthy then it doesn't need to be shocked :-)

My tub runs at about .5ppm continuously sustainable residual, with no bather load and healthy water - a very nice bonus for vacations and such. the only caveat is that with an outdoor tub, there are alot of things that can upset the perfection, such as a stray critter, some organic matter from nearby trees, or even a wind that kicks up some dust that gets through the crannies of your cover.

so in an odd twist, the consequences of very healthy water in a bromine spa with ozone is that you have to watch that upper limit very carefully!

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Hi..

Chlorine Demand is a pool water chemistry topic that is getting more attention due to changing climates, consumer's water chemistry and understanding of the problem. It is also becoming a greater issue in many private & commercial pools.

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I worked some numbers and came up with a formula for chlorine demand. It's:

  • CD = 1 - deltadays_root(ppm_end / ppm_begin)

Where:

  • deltadays = the number of elapsed days
  • deltadays_root = take the nth, deltadays, root, so if deltadays was 2 it would be a square root, if 3 then cubed root, etc.
  • ppm_begin = starting ppm
  • ppm_end = ending ppm

I've cross checked the formula with a few points in the table (in this thread) and the numbers match up.

I was using excel to make the calculation and there isn't an nth root function available. You can use 1/deltadays with an exponential to get the same result though. Here's the formula in excel:

  • CD = 1 - ((ppm_end / ppm_begin)^(1/(deltadays)))

Anyway, the chlorine demand of my hot tub is tiny. Once I checked and it was 6.5%, the next 2 times I checked it was 2.3%. Is this normal? Here are my numbers:

  • PH: 7.5
  • FC: 8
  • CH: 150
  • Alkalinity: 40 (low, but my PH seems to staying right around the 7.5 range)
  • CYA: I've added about 19 ppm, via dichlor, since my last refill on 2/29/2020
  • Borates: None yet, planning to add this weekend.

We haven't used the tub in weeks. Does that explain the low chlorine demand? I haven't even cleaned the filters since the refill.

In my first two tests the water temp was about 99 degrees, and CD was 6.5% and 2.3%, in my last test, of 7 days elapsed, the temp was 80 degrees, and the CD was again 2.3%. My hot tub is about 450 gallons. It has an ozone generator, but it's 11 years old and the bulb has never never been changed.

Thanks,
Samuel

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