Adding BBB

[chem geek]

Why do they ignore the CYA level and why Ben's chart doesn't use the 10 X CC rule to calculate the FC needed to shock? I'm sure that CYA and CC levels are totally independent and it seems that both chemical species should be considered when it comes to calculate the FC needed to shock.

There are many possible answers as to "why" but that really doesn't matter. The chlorine/CYA relationship in terms of the equilibrium chemistry, and specifically the fact that the FC/CYA ratio roughly determines the amount of disinfecting chlorine (hypochlorous acid), was definitively determined in a scientific paper written and presented at a symposium in 1973 and published in 1974. That's nearly 35 years ago. By comparison, the dangers of smoking were first known in the 1950's and only more fully acknowledged by that industry in the 1990's, a period of roughly 40 years. Perhaps there's pride in trying to break a world record here.

But even ignoring that chlorine/CYA relationship, one can also ask why the following basic rules are not taught by APSP Tech or NSPF CPO:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases CH by 7 ppm.

Those are chemical facts independent of concentration so should be easy "rules-of-thumb" taught in courses, wouldn't you think? A little simple math assuming just 1 ppm FC per day using Trichlor tabs/pucks means that CYA would climb by over 100 ppm in 6 months, ignoring dilution (from splash-out, backwashing, etc.).

As for shocking, Ben's chart is referring to a shock level used to clear algae at a reasonable rate. Though it can also be used to more rapidly clear Combined Chlorine (CC), most outdoor pools exposed to sunlight and properly maintained with sufficient FC relative to CYA simply don't get much measurable CC. Technically speaking, even the 10x rule for CC is inaccurate. The chemistry behind the 10x is for the amount of chlorine (in ppm FC which is a "chlorine gas" equivalent) needed to oxidize an amount of ammonia (in ppm Nitrogen) and comes from an actual stoichiometric (ppm-based) amount of 7.6 that becomes more optimal in the range of 8-10. However, for CC (which is measured as chlorine, not ammonia-nitrogen), you technically only need half of the CC amount in FC to "break" it (assuming the CC is monochloramine). The extra chlorine for shocking to get rid of CC just makes it go faster, but in a properly managed pool the disinfecting chlorine is continually breaking down ammonia/urea from bather sweat and you do not need to shock.

There are many myths in the industry and I'm slowly putting together something of a training program that I hope will correct or augment current training (see this post), but this will take time and the industry has so far not been receptive to change. I don't give up, however, because there's something about truth that drives persistence.

Richard