Читать книгу Osmosis: The Molecular Theory - Larry Howlett - Страница 9

We select vapor pressure as an important property of moist air since it is useful for many engineering applications. A psychrometric chart for atmospheric air that includes vapor pressure is shown in figure 3. At normal pressure, there is a unique relation between temperature, relative humidity, and moisture vapor pressure of air. The dependent thermodynamic properties of moist air are relatively insensitive to small changes in atmospheric pressure.

If we apply this thermodynamic data for air (figure 3) to the equilibration chart (figure 2), we can develop a unique thermodynamic property graph for the seed separate from the atmosphere (figure 4). Each value for temperature and relative humidity determines a specific vapor pressure value for the atmosphere. We propose that the seed must exhibit this same vapor pressure be in equilibrium with the atmosphere. We propose that the vapor pressure data shown in figure 4 represents a characteristic property of the seed that is independent of the storage atmosphere.

We hypothesize that the seed placed in a small-evacuated container at constant temperature will lose some water to the container to produce a moisture vapor pressure in the container. Since the moisture loss from the seed to the container is small, the seed moisture content will remain approximately constant. Thus, we propose that the moisture vapor pressure in the container will correspond to the seed properties shown in figure 4.

When a seed has equilibrated to its environment, the moisture content in the seed is in equilibrium with the seed's environment.

We postulate that the moisture vapor pressure in the seed must equal the moisture vapor pressure in the environment as a condition of equilibrium. We propose that this equilibrated value of seed moisture content, vapor pressure, and temperature are state thermodynamic variables of the seed. Thus, these conditions represent a unique characteristic of a seed that is independent of the storage atmosphere and the process by which the seed attained the temperature and moisture content. See Figure 4 Thermodynamic properties of seed.