ROTRONIC HygroLab - Manual (Page 41)

IN-E-HyLab-V4_11
Rotronic AG
Bassersdorf, Switzerland
Document code Unit
Instruction Manual
Document Type
HygroLab bench top indicator version 4:
instruction manual
Document title
Page 41 of 51
© 2006; Rotronic AG IN-E-HyLab-V4_11
Water activity is usually defined under static conditions of equilibrium. Under such conditions, the
partial pressure of water vapor (p) at the surface of the product is equal to the partial pressure of
water vapor in the immediate environment of the product. Any exchange of moisture between the
product and its environment is driven by a difference between these two partial pressures.
Finally, water vapor can also be present in a gas or gas mixture. The relative humidity of a gas is
defined as %RH = 100 x p/ps, where (p) is the partial pressure of the water vapor present in the
gas mixture and (ps) is the saturation pressure, or the partial pressure of water vapor above pure
water at the temperature of the gas.
Aw and temperature
Both water activity (materials) and relative humidity (gases) are referred to the saturation pressure
(ps) or partial pressure of water vapor above pure water:
Aw = p / ps
%RH = 100 x p/ps
The saturation pressure (ps) is strongly dependent on temperature. At normal room temperature,
(ps) increases by about 6.2% for a 1°C increase in temperature. In an open environment that is not
saturated with water vapor, the partial pressure of water vapor (p) does not change with
temperature. In a closed environment, (p) changes proportionally to the °K temperature (°K
temperature = °C temperature + 273.16). At normal room temperature, the change in (p) caused by
a small change in the °C temperature is practically negligible. Because (p) does not change with
temperature while (ps) does, the relative humidity of a gas (%RH = 100 x p/ps) is strongly
temperature dependent. At 95 %RH and room temperature, an increase of 1°C results in a relative
humidity decrease of about 6 %RH. At 50%RH, the same temperature increase causes relative
humidity to decrease by about 3 %RH.
The water activity of most hygroscopic products is not as strongly dependent on temperature. At
room conditions, research data typically shows that water activity varies only by roughly 0.0005 to
0.005 Aw (0.05 to 0.5 %RH) when temperature changes by 1°C.
This is explained by the fact that the partial pressure (p) at the surface of a hygroscopic product
does vary with temperature. Above most hygroscopic products, the magnitude of the change in the
partial pressure of water vapor (p) with temperature is similar (but not exactly equal) to the
magnitude of the change of the saturation pressure (ps) above pure water.
In summary, a change in temperature causes the partial pressure of water vapor above a
hygroscopic product to change. At the same time, the partial pressure in the air above the product
is practically unchanged. It follows that any change in the temperature of a hygroscopic product
automatically causes the product to exchange moisture with the air (or gas) that surrounds it.
Moisture is exchanged until the partial water vapor pressure at the surface of the product and in the
air is equal. When measuring water activity, it is essential to keep temperature as constant as
possible.
Applications
The active part of moisture content and, therefore, water activity, provide better information than
the total moisture content regarding the micro-biological, chemical and enzymatic stability of
perishable products such as foods and seeds. For similar reasons, water activity is equally relevant