Is it not measuring the conductivity, and assuming specific chemical properties? Salt or content would change it for example. But maybe the conductivity of leaf doesn't actually change all that much. I don't know.Thanks for the response. You came up with the number 19 so I was wondering where you got that one from. I wouldn't trust that cigarmedics thing. I still wonder how a device can measure the RH of a solid matter. If it is converting the moisture content of a solid matter to an RH level then it should be calibrated at some temperature. If it is calibrated at a constant temperature then the reading is wrong when you use it in a different temperature.