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Sorry for the extra work - my thoughts clearly didn't help.

I'm not sure the graph is showing what I intended though. Periods where the red line is almost flat should show as little change (=0) on the blue line. It looks almost like it shows the background noise instead of filtering it out. The blue line should only be positive and rise when there is a change in the red line.

The only other thing I can suggest given the almost random possibilities of measurement is to find the peak and back off some degree, perhaps 10%, and repeat the peak check continuously so it tracks any changes caused by drift or environmental influences.  I would assume a peak would occur fairly frequently, specially during hours when water is being used.

Brian.