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Clausius-Clapeyron Program

This page is from the Clausius-Clapeyron Program on-line help.

Program Description

The aw of a material at a given moisture is dependent on temperature. As temperature increases, isotherms shift down and to the right as shown below. Thus, at constant moisture content, the water activity increases with temperature. This may influence shelf stability.

Isotherm plot

This effect has been modelled by the Clausius-Clapeyron equation.


  • Qs = heat of adsorption, a function of moisture content (cal/mole)
  • R = the gas constant, 1.987 cal/mole
  • T = temperature in degK

If you measure the Aw of a material at at least two temperatures over a range of moistures, an isotherm can be produced for any temperature. Aw's at three or four different temperatures will improve the accuracy.

Note: The isotherm data at each temperature must first be entered into the isotherm program, and stored in the database.

Using the Program

  • To enter data, select the " Data" from the " Program menu".
  • Enter the product name and the number of data sets (2-4) in a dialog box, ie the number of different temperatures at which you have isotherms stored on the database.
    • The first dialog box will ask you to select the first product/temperature from the database.
    • The second (third and fourth if necessary) will ask for the other product/temperatures.
  • Temperatures can be selected in any order, but it is sensible to select them in ascending order.
  • The program will then calculate Qs values for the full range of moistures, and plot a GAB isotherm for the first data set.
  • A dialog box will ask for a temperature at which aw's will be estimated. An isotherm for this temperature will also be plotted.
  • You can select " aw estimate" for an estimate of the aw's at a new temperature.
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Last modified: 10-Aug-2008