I worked some numbers and came up with a formula for chlorine demand. It's:
CD = 1 - deltadays_root(ppm_end / ppm_begin)
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:
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.