Microwave Moisture Measurement System for Hardwood Lumber Drying

The goal of this project was to develop a prototype microwave-based moisture sensor system suitable for the kiln drying of hardwood lumber. The moisture sensors developed are battery powered and are capable of communicating with a host kiln control system via spread spectrum wireless communications. We have developed two designs of the sensors working at 4.5 to 6 GHz with linear response to moisture content (MC) over a range of 6–100%. These sensors allow us to make a swept frequency microwave transmission measurement through a small area of a board. Using the prototype electronics and sensors, we have obtained measurements of MC over the above MC range for red oak and yellow poplar with standard deviations of less than 1.5% MC. We have developed data for board thickness corrections and for temperature corrections for the MC measurement system.