I'm thinking out loud here but maybe we are thinking to hard on this. We are trying to detect ice, not detect the conditions for ice, or even detect the lost of air flow.
I'm thinking place a sensor somewhere past the metering device at a given (x) distance away from the line (we want t detect ice, not frost).
This sensor could be a switch monitoring small voltage drops between two post (y) distance away from each other. That are "tripped" when ice forms between the two post.
Or it could be a heated temp switch that has to be completely iced over before it starts the defrost. If place in the correct location(where the ice is going to build up first) then it should start the defrost in time. Might even have to add a delay.
Just some ideas.
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