Definition
The mass of water vapor per unit volume of air (g/m³). It shows the actual moisture content of air regardless of temperature. Absolute humidity is used in industrial moisture load calculations and seasonal comparisons.
Detailed Explanation
Absolute humidity expresses the total mass of water vapor an air mass carries per unit volume, given in g/m³ or kg/m³. Unlike relative humidity, it is temperature-independent: the same 12 g/m³ value represents the actual amount of water in the volume whether the air is at 5°C or 25°C. This property makes it the most reliable parameter for seasonal comparisons, outdoor design condition analysis, and energy simulations.
At 20°C, saturated air carries approximately 17.3 g/m³ of water vapor; at 30°C, about 30.4 g/m³. The moisture-carrying capacity of air grows exponentially with temperature, which is the fundamental reason why moisture load increases so dramatically in summer. In cooling and dehumidification system design, the outdoor absolute humidity is a critical input for accurately determining capacity.
Calculation
ρv = mv / Vtotal
ρv: absolute humidity (kg/m³ or g/m³) mv: mass of water vapor in the air (kg) Vtotal: total air volume (m³)
Using the ideal gas approximation: ρv = Pv / (Rv × T)
Rv: specific gas constant for water vapor = 461.5 J/(kg·K) T: temperature (K) Pv: partial pressure of water vapor (Pa)
Practical Example
Consider a 1000 m³ food storage warehouse. Outdoor design conditions are 30°C, 75% RH (about 22.8 g/m³ absolute humidity), while the indoor target is 18°C, 50% RH (about 7.7 g/m³).
Assuming 1 air change per hour (1 ACH) through door openings and infiltration, the additional moisture load entering the warehouse per hour is:
(22.8 − 7.7) g/m³ × 1000 m³/h = 15,100 g/h ≈ 15.1 kg/h
This figure is used directly in dehumidifier capacity selection. A calculation based on relative humidity could have over- or under-estimated capacity by 20–30% due to the temperature difference.
Engineering Note
Absolute humidity (ρv, g/m³) and specific humidity (W, g/kg dry air) are two commonly confused concepts:
• Absolute humidity is measured per unit volume; since air density also changes with temperature, it does not entirely remove the temperature effect. • Specific humidity is measured per unit mass of dry air, does not change when air is heated or cooled, and is the standard y-axis of psychrometric charts.
In dehumidifier capacity calculations and energy balances, specific humidity (W) is preferred because the dry air mass passing through the device remains constant. Seasonal data, outdoor design tables, and meteorological reports, on the other hand, are usually given in absolute humidity or dew point; these values should be converted to specific humidity before calculation.
