Isoperms
Notes
[1] We should recognize that Equation 2 applies to papers in equilibrium with their environment. Individual closed books and tightly packed manuscript boxes are slow to respond to environmental % RH changes. Though their outside edges may equilibrate in minutes or hours, it often requires months for the moisture content in the center of a book to reach its equilibrium value for a specific % RH. For convenience we shall assume equilibrium with atmospheric conditions for both temperature (a very good approximation) and relative humidity (good only over relatively long time intervals).
