Microwave moisture measurement system for lumber drying

The goal of this project was to develop a prototype moisture sensor system suitable for a hardwood dry kiln based on the microwave transmission measurements of the complex dielectric constant of the wood. In this project, prototypes of two designs of microwave-based moisture sensor probes (launchers) working in the frequency range from 4.5 GHz to 6 GHz were developed and tested. A prototype set of battery powered electronics that both provides the microwave excitation and records the amplitude and phase of the returned signal after passing through the wood was built and tested. The sensors and electronics built in this project allow a swept frequency microwave transmission measurement through a small area of a board. Using the prototype electronics and launchers, measurements of moisture content (MC) over a range of 6 percent to 70 percent MC for red oak and 6 percent to 100 percent for yellow-poplar with standard deviations of less than 1.5 percent MC have been obtained.