How to use ashrae psychrometric charts
Dry air is shown as the x-axis, which has a relative humidity of 0%, since there is no water vapor in the air mixture. 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.
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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.
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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.