Nondestructive Testing of Wood Using a Single Sided Nuclear Magnetic Resonance Device

Hydrogen nuclear magnetic resonance (NMR) is a powerful tool to measure the number of hydrogen nuclei in wood. By measuring various characteristics of the NMR signal (amplitude and relaxation times), the amount of hydrogen in different physical states of their molecular environment, (e.g. liquid and solid state) can be determined. Convential NMR equipment requires samples to be placed into the bore of a magnet and into a small detection coil. Therefore this "enclosing" technique is not suitable for inspections of larger objects, as wood pariticle boards or for in-field inspections of wood logs. For the measurements presented here a single-side access NMR instrumentation (Southwest Research Insitute, San Antonio, Texas) was used, which offers the feasibility to do NMR measurements external to the probe. The NMR signal is generated in a flat disk-shaped volume (sensitive volume) which is 1 mm thick and located 26 beneath the probe surface. By scanning the sensor towards the sample, depth profiles of the NMR signal can be obtained up to a maximum depth of 26 mm with a depth resolution of 1 mm. In one of our present studies this new instrumentation was used to determine the moisture profile in a disk of a beach log. A sharp increase of the moisture content at the bark/marrow interface was clearly detected by the NMR measurements. Moreover, some bends of the signal profiles in the marrow could originate from moisture variations in the annual rings. The goal of a second study is to use this NMR instrumentation for in-situ measurements of the moisture and the density distribution of wooden pressboards.