Common Psychrometric Terms

Common psychometric terms as Dry Bulb and Wet Bulb Temperature, Specific Volume, Relative Humidity and Enthalpy

Psychrometry is the science of studying the thermodynamic properties of moist air and the use of these properties to analyze conditions and processes involving moist air.

The air condition can be determined by using a Psychrometric Chart or a Mollier Chart. Common properties used in the charts includes

• dry-bulb temperature
• wet-bulb temperature
• relative humidity (RH)
• humidity ratio
• specific volume
• dew point temperature
• enthalpy

With two known properties it is possible to characterize the air in the intersection of the property lines, the state-point. With the intersection point located on the chart or diagram, other air properties can be read directly.

Dry-Bulb Temperature - T_db

Dry bulb temperature is usually referred to as air temperature, is the air property that is most common used. When people refer to the temperature of the air, they are normally referring to its dry bulb temperature. Dry-bulb temperature - T_db, can be measured by using a normal thermometer. The dry-bulb temperature is an indicator of heat content and is shown along the bottom axis of the psychrometric chart. The vertical lines extending upward from this axis are constant-temperature lines.

Wet-Bulb Temperature - T_wb

Wet bulb temperature is associated with the moisture content of the air. Wet bulb temperature can be measured with a thermometer that has the bulb covered with a water-moistened bandage with air flowing over the thermometer. Wet bulb temperatures are always lower than dry bulb temperatures but they will be identical with 100% relative humidity in the air (the air is at the saturation line). On the chart, the wet-bulb lines slope a little upward to the left, and the temperature is read at the saturation line.

Relative Humidity - RH

Relative humidity is the ratio of the water vapor pressure - p_w, to the vapor pressure of saturated air at the same temperature - p_ws, expressed as a percentage. Relative humidity is a relative measure. The moisture-holding capacity of air increases with air temperature. In practice relative humidity indicates the moisture level of the air compared to the airs moisture-holding capacity.

Note ! The moisture holding capacity of air increases dramatically with temperature!

Relative humidity lines in the chart are curved lines that move upward to the right. The line representing saturated air where the relative humidity - RH is 100%, is the uppermost curved line in the chart.

Dew Point - T_dp

Dew point is the temperature at which water vapor starts to condense out of the air, the temperature at which air becomes completely saturated. Above this temperature the moisture will stay in the air. This temperature can be read by following a horizontal line from the state-point to the saturation line. Dew point is represented along the 100% relative humidity line in the psychrometric chart.

Specific Volume of Humid Air - v

Specific volume represents the space occupied by a unit weight of dry air (ft³/lb), and is equal to 1/(air density). Specific volume is shown along the bottom axis of the psychrometric chart with constant-volume lines slanting upward to the left.

Moisture Content and Humidity Ratio - x

Moisture content and humidity ratio differs from relative humidity in that it is the amount of water vapor by weight, in the air. The dry-basis moisture content of air is expressed as the weight of water vapor per unit weight of dry air. Humidity ratio is indicated along the right-hand axis in the psychrometric chart.

Enthalpy - h

Enthalpy is the measure of the total energy in the air, the energy content per unit air weight (Btu/lb_da) (dry air). Enthalpy is read from where the appropriate wet-bulb line crosses the diagonal scale above the saturation curve. Air with same amount of energy may either be dry hot air (high sensible heat) or cool moist air (high latent heat).

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