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n. ~ A combination of relative humidities and temperatures that produce equivalent effects on the permanence of materials.


An isoperm forms a continuous line when plotted on a graph with axes for relative humidities and temperature.

(Sebera 1994) The isoperm method arises from one idea: the rate of deterioration of hygroscopic materials such as paper is influenced by the temperature and percent relative humidity of its surrounding environment. The isoperm method combines and quantifies the preservation effects of the two environmental factors, temperature and percent relative humidity, and presents the results in a readily comprehensible and usable graphical form. ¶ What is at first sometimes confusing but is essential to an understanding of isoperms is that relative rather than absolute rates of deterioration (and paper permanence) are employed. That is, if r1 and r2 are the (absolute) rates of deterioration of a specific paper under two sets of temperature and relative humidity conditions we shall not deal with r1 and r2 individually but only with their ratio r2/r1 which measures the relative change in the deterioration rate resulting from the change in environmental conditions.