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The humidity ratio, W, is the ratio of the mass of water vapor to the mass of dry air: X w = n da / n = p w V / RT = p w = p w / p X da = n da / n = p da V / RT = p da = p da / p To solve for humidity ratio, the calculations assume that moist air is a mixture of independent perfect gasses, water vapor and dry air:Ĭombining these two equations using the perfect gas equation: pV = nRT:Ĭombining and rearranging the equations above: Note that there are 7000 grains of moisture per pound of moisture. It is commonly measured in units of pounds of moisture per pound of dry air (lb moisture / lb dry-air ), grains of moisture per pound of dry air (grains / lb dry -air ), and grams per kg (g/kg). When reading a psychrometric chart, it is the horizontal line that intersects the right axis of the chart. Also known as the moisture content or specific humidity. Humidity Ratio, W: The mass of water vapor present per unit mass of dry air. When reading a psychrometric chart, the specific enthalpy is the diagonal line intersecting the leftmost straight line. It takes into account both the sensible and latent heat components. Specific Enthalpy, h: The total energy content per unit mass of moist air. When reading a psychrometric chart, the relative humidity lines are the curved lines that span the chart. Specific Volume: The volume occupied by a unit mass of moist air. P s ​ = saturation water vapor partial pressure at the pressure, p, and the dry-bulb temperature, Temperature, db P w​ = water vapor partial pressure at the total pressure, p, and dew-point temperature, Temperature, d p The equation to calculate relative humidity is: Relative Humidity, RH%: The ratio of water vapor in the air to the maximum amount of water vapor the air can hold at a given temperature. The dewpoint temperature is the temperature at which the humidity ratio is equal to the humidity ratio at saturation: When reading a psychrometric chart, it is the horizontal line that intersects the curved leftmost line. Dew Point Temperature is also referred to as the condensation point because it is the temperature at which the water turns to liquid from vapor in the airstream. When reading a psychrometric chart, it is the diagonal line that intersects the curved leftmost line.ĭew-point Temperature, T dp:The measure of the dry-bulb temperature at the point that water vapor starts to condense to liquid or be removed from the air. It is used to determine the humidity and adiabatic saturation properties of air. Wet-bulb Temperature, T wb: The temperature measured by a thermometer with its bulb covered in a wet wick exposed to moving air. When reading a psychrometric chart, it is the horizontal axis at the bottom of the chart. It represents the sensible heat component of the air. Please pardon the dust while we complete this work :)ĭry-bulb Temperature, T db: The temperature of the air as measured by a standard thermometer. ** CONSTRUCTION ZONE ** We are actively updating the information below. The following psychrometric chart equations and terms have been adapted from the ASHRAE Fundamentals Handbook (I-P Edition), Chapter 1, Psychrometrics.