I don't think it is BleachCalc that is the root cause of any underestimation of how much chlorine is needed to shock, though you are correct that if I am right about the error, then it will underestimate how much to add. I believe the much bigger potential problem (that we don't know for certain yet) is that Ben's table tracks reasonably well with the ppm HOCl theory for his "min" and "max" columns, but not well at all for the "shock" column. Look at my FC.htm chart and you can see that (ignoring very low CYA since the required FC is so low as to be impractical to maintain) the 0.02 ppm column roughly correlates to Ben's "min" while the 0.05 ppm roughly corrleates to Ben's "max", but there is no good correlation for his "shock" column. We don't know which way the error is, if any, but if I were to hazard a guess, I would say that the proper "shock" level is at around 0.5 ppm though possibly 0.2 ppm *might* do the trick in some cases. If 0.5 ppm is correct to kill algae, then the high CYA levels in Ben's chart underestimate the amount of chlorine truly needed.

Only real-world experience will tell what we should do to go forward -- if the ppm HOCl is valid for the shock to kill algae and if so, what level is appropriate.

Richard