Types of grain moisture meters and how they work
There are different kinds of moisture meters, each with its own principle of operation and underlying technologies.
Probe-type moisture meters like the GrainMate Grain Moisture Meter use a sensor to measure the temperature and humidity of the grains. It also comes along with special mathematical models to tell the equilibrium moisture content (EMC) in a grain sample.

Sesi Technologies GrainMate Grain Moisture Meter
Cup-type meters  like the John Deere Moisture Check Plus SW08120 use a high frequency capacitive circuit to measure the moisture content in a small gain sample.