For example, an air mixture at 100% relative humidity is shown on the saturation curve because the amount of water vapor in the air mixture is equal to the maximum amount.
Relative Humidity: Relative humidity is the ratio of the amount of water vapor in the air mixture - compared to the maximum amount of water vapor the air can hold at that dry bulb temperature. A wet bulb temperature much lower than the dry bulb temperature is an indication of dry air. A wet bulb temperature that is equal to the dry bulb temperature, indicates saturated air. Wet Bulb Temperature: This temperature takes into account the moisture in the air. It is called dry because the temperature is not affected by the moisture in the air. Dry Bulb Temperature:This is the temperature most people are familiar with, as they are always shown on thermometers and thermostats. The psychrometric chart as previously discussed, graphically shows the theromdynamic properties of Moist Air (air-water mixture, humid air, etc.). Typical Psychrometric Questions: Mixtures of Moist Air, Humidification/Dehumidification, Heating/Cooling Questions. Movement on the Psychrometric Chart: What causes each type of movement on the chart (Right to Left, Up and Down). Air Properties on the Psychrometric Chart: What does each property tell of the air-water mixture?. Now that you understand the basics of a psychrometric chart, we will now go into detail on the following (3) Pscyhrometric Chart Topics: The psychrometric chart for the PE exam also only shows the typical temperatures encountered by a typical HVAC and Refrigeration Engineer. For the PE exam and for this guide, it is assumed that the psychrometric chart is based on atmospheric pressure (14.696 psia or 1 atm or 29.921 in. As you can see, pressure is not shown on any axes, because it is constant. The psychrometric char does not account for variations in pressure. Moist air on a psychrometric chart ranges from Dry Air to Saturated Air. Moist air is defined as an air-water mixture. The Psychrometric Chart describes the various properties (Dry Bulb Temperature, Wet Bulb Temperature, Enthalpy, Humidity Ratio, Relative Humidity and Specific Volume) of moist air.
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