A production-oriented review of high-frequency measurement methods for forest products

Forest products measurements based on high-frequency techniques such as microwaves, radio waves or electrical impedance spectroscopy is a growing and maturing field. As high-frequency moisture content meters are being commercialized, the question arises if similar technology can be used to measure additional forest products properties. This review aims to survey literature on high-frequency measurements of properties such as density, spiral grain and heartwood content. Applications to primary forest products such as logs and wood chips are here prioritized over secondary products such as sawn timber and pellets. To promote technical and commercial relevance, the literature search focuses on peer-reviewed and grey literature published within the last twenty years and on commercially available measurement systems. Furthermore, this review focuses on applications under production conditions, taking environmental, logistical, and economic factors into account. High-frequency methods are generally fast, non-destructive, harmless, insensitive to disturbances, and allow for interior inspection and bulk measurement. Disadvantages include high operator skill-level, difficulty in separating frozen and unfrozen material, as well as insufficient studies carried out in a production environment. Moisture content and density measurements are mature applications with high demonstrated accuracy. Spiral grain, knot characterization and bark thickness show potential. The measurement of decay, foreign objects, tree species and chemical analysis of resin or heartwood content need further evaluation, while the measurement of strength properties is unlikely to have sufficient accuracy to compete with other techniques. Promising applications include measurement through large volumes of material, for example in wheel loader buckets or on trucks.