Discrepancy between poolcalculator and poolmath for FC shock level

[raksamit]

I'm a big fan of the TFPC method, but like to use the poolcalculator.com website for calculations (it prints better).

I have always seen very good agreement between the calculators on both sites, but have found a discrepancy (6 ppm) for the FC shock level. Please see sample values below.

Is there any way to determine which one is "right"?

Size: 15900 gallons

FC: 4.5 now, 9.0 target
pH: 7.3 now, 7.0 target
TA: 100 now, 90 target
CH: 525 now, 525 target
CYA: 60 now, 60 target
Salt: 950 now, 950 target
Borate: 40 now, 40 target

Temp: 89

CSI: 0.04 now, –0.29 target

Suggested FC Levels: SWG=3, Normal= 5-9, Shock= 18 (poolcalculator) 24 (poolmath), Mustard = 34

 

[Butterfly]

We recommend PoolMath now because we can update it unlike the abandoned poolcalculator. JasonLion (admin here) wrote the software.

Welcome to TFP

 

[chem geek]

The Pool Calculator used shock levels that were a bit lower and more similar to the original Bens' Best Guess Chart, but PoolMath is using the values in the Pool School Chlorine / CYA Chart which for shock levels is roughly an FC that is 40% of the CYA level while for yellow/mustard algae shock level it's roughly an FC that is 60% of the CYA level.

As for "right", shock level isn't as clear cut as the minimum FC level for algae inhibition (regardless of algae nutrient level) because higher levels kill algae faster. If you go too low when shocking, then there might not be enough chlorine to penetrate established algae clumps fast enough to kill it all off. Using the lower level in The Pool Calculator will probably still work, but may be somewhat slower in bleaching the algae from green to cloudy white.

 

[raksamit]

Thanks, chem geek, for the history and details.

I haven't had to shock / SLAM this year, so keeping my fingers crossed that this is for reference only.