I've have a Delmhorst meter for over 25 years. The 'calibration' check (hand moisture level) is changing yearly, but isn't the green zone under 20% ?Very first thing I'd do with that is, I'd have my Delmhorst moisture meter with me with the baled hay probe attached and stick the bales to ascertain the RH before even thinking about purchasing it. I keep the threshold alarm set at 25% always.