Clarification of "tied up" vs "Free"?

[DavidD]

FAS-DPD measures FC when the water turns from pink to clear. You then add R003 and if it turns pink again, you have CC (Combined Chlorine-Bad). Add R0871 until clear again and this is your CC in ppm. OTO measures only TC (total chlorine) but FC+CC always=TC.
The "Tied Up" chlorine you are speaking of will measure as FC and TC, but it is not as effective with higher CYA hence the increase in FC required with higher CYA. Hope I'm not confusing you more.

Dave