Improving experimental techniques that probe wood-moisture interactions
Almost all properties of wood and other cellulosic materials depend on moisture content. These materials absorb moisture from the surrounding environment or release it depending on the relative humidity and temperature of the air. Equilibrium moisture content (EMC) is defined as that moisture content at which wood is neither gaining nor losing moisture, when the relative humidity and temperature are constant. The EMC-temperature-relative humidity relationship is fundamental for understanding wood-moisture interactions and is used in a host of technological applications ranging from wood drying to the design of wood structures. Traditionally EMC has been measured manually by repeatedly weighing samples in a constant temperature and humidity environment until no change is detectible. Recently, computer-automated instruments have been developed, known as dynamic vapor sorption (DVS) instruments, which allow for continuous measurement and precise temperature and humidity control. DVS instruments are usually programmed with a series of humidity steps, and each step typically is held until the rate of change in sample mass is below an arbitrary threshold. Prior to this study by Forest Service researchers, the literature had claimed that a threshold of 0.002 percent per minute yielded EMC values accurate within 0.1 percent. The researchers found that this claim was unsupported by any data and that wood and other cellulosic materials continue to change moisture content over much longer times than captured by any previous DVS measurements. Although the DVS data at short times were consistent with previous measurements, the data at long times reveal that prior methods had much larger errors in EMC than claimed. The researchers’ ongoing work is aimed at developing robust DVS methods that accurately characterize EMC.