I am really hoping that somebody experienced with water chemistry could advice me what happens when ORP values are crashing after increasing inhibitor to the pool.
I am having 6 000 litr pool / spa with automated ORP control (Spaboy) and Onzen SWG. The water to the pool is coming from the drill well so according to operators manual I am adding every other week Arctic Best Defense stain and scale inhibitor (methylene-phosphonic acid).
The problem is that every time after adding the methylene-phosphonic acid the ORP values are crashing at the pool and this situation takes days to come back normal. ORP value is even going below zero and somehow the software stops sanitizing at that point. I have tried to test free chlorine amount but because of the really low natural (0,2-0,5) level of free chlorine (system is designed that way) it is almost impossible to tell if there is sufficient amount of chlorine or not. Eventually the water turns cloudy but phenomenom is something that is also described on inhibitors label.
What I would like to know is what really happens to pool water when you add methylene-phosphonic acid on it? Does acid "eat" all the free chlorine away or is it just mixing the ORP-meter readings?