lukepools,
I don't know how to say this in a politically correct way, but the relationship between chlorine and cyanuric acid (CYA) has been known definitively for at least 34 years and higher CYA levels do make a difference in spite of what Bioguard (Chemtura) is telling you. The paper published in 1974 in this link determined the equilibrium relationship between the "active" chlorine, hypochlorous acid, and the chlorine combined with CYA (chlorinated cyanurates). Though the paper is technical, you should read the initial "Introduction" in the first 2 pages as well as the "Summary" in the last 2 pages. Additional scientific studies in the late 70's and into the 80's showed that CYA significantly reduced chlorine's effectiveness, mostly showing that hypochlorous acid was the primary disinfectant and that hypochlorite ion and the chlorinated cyanurates had little disinfecting capability. Oxidation-Reduction Potential (ORP) values also show that it is hypochlorous acid that has the primary oxidizing potential.
Ben Powell, who created
The PoolForum and
PoolSolutions, noticed the relationship of FC and CYA in terms of a pool's tendency to produce algae. He found that if he kept the FC level at a certain level relative to CYA, that algae growth could be prevented with chlorine alone, even in pools that were high in algae nutrients (e.g. nitrates, phosphates). He came up with a
Best Guess CYA chart. We've since modified that chart taking into account the chemistry determined in 1974 as well as additional analysis of what users have reported in newer real pools, including SWG pools (and yellow/mustard algae), to produce the
Chlorine / CYA Chart.
You can certainly have a pool with a high CYA level, but if you do not proportionately raise the FC level to be, in a manually dosed pool, at least 7.5% of the CYA level, then the pool may develop algae. I say "may" because algae growth also depends on other factors including nutrients, sunlight, and amount of circulation determining local chlorine levels.
As for adding chlorinating liquid or bleach to a vinyl pool, you need to be more careful, but it can absolutely be done safely. With most concentrated chemicals, the safest approach is to add them slowly over a return flow with the pump running, preferably at the deep end. After adding the chemical, lightly brush the side and bottom of the pool in the area where you poured to ensure thorough mixing, especially if there is no floor drain. Once the chlorine is mixed in the bulk pool water, it is identical regardless of its source. It's only if you dump in a chemical quickly that it can settle to the bottom or stay concentrated in one area and cause problems.
Richard