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As part of calibrating my optical gas meter I've got it logging the lowest reading seen during each logging period. This gives a very clear indication when the silvered digit passes by, even if the readings as a whole are offset higher or lower due to my blu-tac positioning or other factors. Example:

Instead of hardcoding a threshold value into the sketch to record a 'pulse', it would be more flexible to have the option to log to a subsidiary feed conditionally. In the example above, I could add a step saying 'record a pulse to feed 2 if the value is < 550' (and then changing this threshold is an easy web interface action, instead of requiring me to redo the sketch).

This is useful because each time I stick on the sensing apparatus I get different relative values: in the graph above you can see it's generally lower after I stick the sensor on (after 20:12) than before, regardless of pulses. This has made it a bit tricky to get a solid sketch.

I don't see such an option in emonCMS at present. Has anyone done (or seen this done) who could offer me any pointers?

Alternatively, I was thinking of having the sketch keep track of a long-term average (over the previous 10 minutes, say) and then counting a pulse as a percentage variation. But I figure the idea of conditional logging to feeds might be useful for all sorts of other things?