Determination of Density and Moisture Distribution in Chip Boards with One-Sided Access Nuclear Magnetic Resonance (OSA-NMR)

The profiles of density and moisture across the thickness are the most important measuring parameters for characterizing and controlling the product quality in chip and fiber board fabrication. Usually, the density profile of small samples is determined by laboratory methods. Nowadays, the gross density profile in wood boards can be measured on-line with measuring devices based on the scattered x-ray method. But due to the similar scattering and absorption properties of solid and liquid wood components, the accurate determination of the more important dry density is difficult with x-ray methods. Based on the characterization of the hydrogen nuclear magnetic resonance (¹H NMR) relaxation, solid and liquid wood components can be distinguished, which enables to measure the moisture content as well as the dry density in wood in a single process. By using an NMR instrumentation, which allows one-sided access to the specimen, the profiles of density and moisture can be determined in arbitrary large objects as chip boards. Consequently, OSA-NMR provides a new approach for process integrated quality control of timber products. Using the available equipment, wood density can be determined with an accuracy between 13 and 23 kg/m3 and the measurement error in determining wood moisture is between 0.4 an 1.0 mass-%. Today, the resolvable depth increment of the measuring profiles is about 1 mm. But with a modified equipment distinctly higher depth resolutions should be realizable.