of instruments which measure relative humidity through its effect on the
electrical properties, such as conductivity or resistivity, of materials.
Thus the resistance of lithium chloride, or the capacitance of anodized
aluminium, changes with changes of relative humidity.
A method known as the Landrock–Proctor method depends on
gravimetrically measuring changes in water content of samples of the
material after equilibration with atmospheres of known relative humidity
which can be obtained using saturated solutions of a number of inor-
ganic salts. If the sample has a lowerawthan the atmosphere then it will
gain weight, if it has a higherawthen it will lose weight. By carrying out
measurements of weight change over a range of relative humidities it is
possible to extrapolate to the relative humidity which would cause no
weight loss and thus corresponds to theawof the sample. Figure 3.12
shows the result of such an experiment with samples of Madeira cake and
Table 3.11 shows the water activities of a variety of saturated salt
Figure 3.12 Weight changes of samples of madeira cake at different ERH values
44 Factors Affecting the Growth and Survival of Micro-organisms in Foods