Figure 4-1 is the gas flow schematic for the Model 111. Room air enters the compressor, where it is raised to a pressure of approximately 80 to 90 psi (4560 mm). At 25o C the saturation water vapor pressure is approximately 24 mm. Therefore, most of the water condenses out, and falls to the bottom of the tank. Out of the 4560 mm of pressure in the tank, only 24 mm is due to water vapor. When this air is later expanded to atmospheric pressure (760 mm) the water vapor pressure is reduced to approximately 4 mm. This corresponds to a dew point of slightly less than 0o C.