The presence of solutes in a water solution affects the colligative properties of the solution in a predictable manner, dependent on the total concentration of the solutes and independent of the number of types of solutes, their molecular weights, particle sizes, or densities. These properties include:

a) the osmotic potential, which becomes more negative with increasing concentration;

b) the freezing point, which decreases with increasing concentration;

c) the boiling point, which increases with increasing concentration; and

d) the vapor pressure, which decreases with increasing concentration of the solute.

In the physiology of a whole plant, the most important of these is the osmotic potential, which determines the ability of the plant to take up water from the environment and to generate turgor pressure, although the freezing point can be critical to the survival of cells at low temperatures. Any of these properties can be measured directly and used to calculate the total concentration of the solutes in the solution (osmolality). Once that calculation is made, the other colligative properties can be estimated. The Wescor Osmometer measures the vapor pressure (relative humidity) by measuring the dew point (temperature at which water condensate is in equilibrium with the water vapor in the overlying atmosphere).

The osmometer has a small chamber to which is sealed a thermocouple hygrometer. A thermocouple measures temperature by the voltage between two dissimilar metals that are joined together. (Actually, there are two thermocouples--the difference in their temperatures is measured by the difference in the voltages across them.)

The relationship between temperature and voltage is reversible--running a current across the interface between the metals can raise or lower the temperature of the thermocouple. A small amount (8 µl) of solution placed on a piece of paper and inserted into the chamber quickly equilibrates with the chamber atmosphere. The instrument then runs a program to determine the dew point of the atmosphere.

The program runs as follows:

1) Once the sample is introduced and the chamber is sealed, the instrument allows time for thermal and vapor equilibration. The electronic circuit measures the thermocouple voltage under "quiescent" conditions.

2) An electrical current is fed through the thermocouple junction to cool it (by the Peltier effect) below the dew point. Water condenses from the air in the chamber and forms a thin liquid film on the junction surface.

3) The cooling current is reduced, and the temperature of the thermocouple tends toward a equilibrium that reflects the evaporation or condensation of water on its surface. Since it starts cold, condensation tends to warm the junction to the equilibrium temperature (dew point). When the temperature of the junction reaches the dew point, where it stabilizes.

4) The electronics read the temperature of the junction (relative to the room temperature) by the voltage across the junction (relative to the voltage across a junction at room temperature). The temperature depression is proportional to the osmolality of the solution.

The diagram below describes the steps of the program by the temperature of the sample chamber thermocouple as a function of time.

The final reading is calibrated in milliOsmoles/kg.