This report describes the results of a study performed to measure the water
potential of loblolly pine and red oak over the full range of moisture con
tent during desorption. The matric potential as measured by the tension pla
te, pressure plate, and pressure membrane methods exhibited good continuity
with the total water potential as measured by the isopiestic method. This
not only proves the validity of the water potential measurements but also s
hows that the osmotic potential component of the total water potential is n
egligible at low moisture content. The characteristic curves allow characte
rization of water in wood at high moisture contents and thus avoid the need
to extrapolate sorption isotherm beyond the 98% relative humidity level as
was done in previous sorption studies. The results also show that, at a gi
ven water potential, the moisture contents of both species decrease with a
rise in temperature. This may be due partly to the temperature dependence o
f the surface tension of water and to the fact that entrapped air expands w
hen heated, thus displacing water out of the capillaries. The temperature d
ependence of water potential was used to calculate the enthalpy change, the
free energy change, and the product of absolute temperature and entropy ch
ange associated with moisture sorption. The data show that the logarithms o
f these thermodynamic properties vary linearly with moisture content in the
hygroscopic range but are nonlinear in the capillary range.