Lynne Wong's PhD thesis

(Guggenheim-Anderson-de Boer GAB model), semi-empirical (Henderson and Halsey
models) and empirical models (Smith and Oswin models).
Most of these equations predict EMC values from known water activity data, others from
relative humidity values. If the latter are expressed as a fraction instead of a percentage
value, they are equivalent to water activity as shown below.
The water activity a w of a substrate is defined as the ratio of the equilibrium vapour
pressure of water (
p ') in the substrate to the vapour pressure of pure water (p 0 ) at the same
temperature, i.e.
p'
p
R
100
H
aw
= = =
0
H
R
, where R H and H R are the equilibrium relative
humidity expressed respectively as a percentage and fraction.
Some commonly used moisture adsorption isotherm models are compiled in Table 5.1. All
are two-parameter equations, except for the GAB and Hailwood-Horrobin equations which
are three-parameter and Peleg which is a four-parameter equation.
Some isotherm models had been modified to take into account the effect of temperature as
shown in Table 5.2 (e.g. the modified Chung-Pfost, the modified GAB, the modified
Halsey, the modified Henderson and the modified Oswin models). All of them are fourparameter
equations including a temperature term.
175