Adding to what has already been said, another part of the science is that the FC values in the
FC/CYA Levels are based on concentrations of "active" chlorine (HOCl - Hypochlorous Acid). In a CYA stabilised pool, most of the FC is actually bound to CYA, where it is protected from UV. It still tests as "free" chlorine (FC) in any FC-test like FAS/DPD, but it's actually not an active sanitizer.
The target FC (without SWG) in the
FC/CYA Levels is based on an HOCl concentration of about 0.05ppm, the min FC is equivalent to about 0.03 ppm HOCl. HOCl at SLAM-level is about 10 times higher than at min FC, around 0.3ppm. This level is high enough to kill algae reasonably fast, but still low enough that you can keep using the pool (as long as you can see the bottom of the pool) while maintaining SLAM-level (that's the "M" in SLAM) until all algae has been killed. As a comparison: In a public indoor pool without CYA in the water, the HOCl concentration is almost always much higher than our SLAM-level (with CYA=0, the HOCl-concentration at FC=1 is about 0.5ppm, and at FC=4 it is nearly 2ppm - that is considered "safe" for swimming by authorities responsible for public pools).
The TFP-methodology is based on understanding the water chemistry of CYA-stabilised pools, the result is the
FC/CYA Levels, an easy to follow condensation of all the science behind the scenes. The TFP chlorine levels are chosen to be high enough to ensure clear, well sanitised water - but not unnecessarily high. That is the the big misunderstanding that is unfortunately still widespread in the traditional pool industry - even though TFP's FC-levels (as tested in common FC-tests) are often higher than what the traditional industry considers safe (usually somewhere in the 1 - 3 or 4 ppm range), the levels of active chlorine (HOCl) in "traditional" pools can be much higher (by a lot...) in pools without CYA (e.g. resulting in bathing suits lasting less than a season), or much lower in pools with CYA that don't follow the
FC/CYA Levels.
Rather than throwing once some unspecified amount of "shock" in the pool, hoping that will be enough to clear the pool, TFP's SLAM-process works on a well defined chlorine level, that will over time clear any green swamp, but will keep the pool usable in the process.
As long as you stick to the
FC/CYA Levels, there is no regular "shocking" required. But should FC slip too low and a pool turns green, or there was a faecal accident, a SLAM will clear it, but that is not a one-off dose of "shock", SLAM has to be maintained until all three exit-criteria are passed - that could take any period of time from just one day to a couple of weeks. Once you're done and following the
FC/CYA Levels, no regular "shocking" required - there are are number of people round here that never had to SLAM (or "shock") their pools in years.
If you want to dive down the deep end to understand the science behind TFP, then I suggest to just do that - visit
The Deep End..., particularly the sticky threads by chem geek at the top of the list:
This thread presents my findings so far on pool water chemistry including the following: More Accurate Calcite Saturation Index (CSI) to replace Langelier Saturation Index (LSI) Calculation of ppm HOCl (disinfecting chlorine) at various levels of Total Free Chlorine (FC) and Cyanuric Acid (CYA)...
www.troublefreepool.com
EDIT: You will find a simplified and standardized version of this chart at Pool School, Chlorine / CYA Chart. Chlorine/CYA Chart by Chemgeek FC = Free Chlorine in parts-per-million (ppm) CYA = Cyanuric Acid in parts-per-million (ppm) CYA ........... Min FC3 ..... Target FC ...... Yel/Mstrd...
www.troublefreepool.com
All the "TFP science" goes back to the
1974 O'Brien scientific paper - at the end of the day (and at the end of this far too long post...), a SLAM is just the science described in that paper applied to a green pool.