Evan (waterbear),

Thanks for the info on chloramines and their use in killing algae. I did read something about this in some non-scientific articles, but couldn't find good data on algae kill levels (for different algae types) for either chlorine (HOCl) or chloramines (NH2Cl) except for an EPA report that essentially said that 0.1 ppm chlorine (in sea water, so chloramines certainly were formed) killed most marine plankton. On the "Save Swimming with Elevated Chlorine" forum you mentioned that there were studies showing the relationship of ORP to different types of algae that grew in that environment -- I would appreciate links to those studies or a summary of results and whether there was ammonia present (so that chloramines would have formed), what the pH was, etc. I can roughly translate ORP to ppm HOCl at least to get an idea, both for maintenance and shocking.

You also commented on the other forum (if I should respond there instead, let me know) about the use of ORP sensors and how these measure oxidation potential which is not the same as disinfection capability. I believe you are right about this, though there is some correlation since the primary oxidizer in pool waters is chlorine (HOCl) which dominates the ORP reading. So in some sense, ORP acts as a proxy for HOCl. You are right, however, that it also has other pH dependencies and the actual reactions that are being measured appear to be close to 1 electron instead of the 2 one would expect with HOCl oxidation. I've had numerous discussions about this with several ORP sensor manufacturers which led me down the path towards comparing calculated ppm HOCl against the Oregon Commercial Spas study where I found that calculated (not even real-time) HOCl concentration was at least as good if not slightly better than real-time ORP (with one exception at low chlorine and zero CYA that was very, very strange).

I plan to start two new threads in the China Shop. One will be non-technical and will be focussed on getting more real-world information from users who are battling algae (maybe some will even be willing to try some experiments) -- perhaps Ben will move this out of the China Shop, but I'll leave that up to him. The other new thread will be technical and will address the issues I described above regarding ppm HOCl and ORP, giving you graphs, spreadsheets, etc.

Richard