EKS-HUM-006 / Moist-air state and psychrometric properties
Absolute Humidity
绝对湿度
Absolute humidity, or water-vapour density, is the mass of water vapour divided by the total volume occupied by a moist-air sample.
- Aliases
- water-vapour density, water vapor density
QUANTITY, SYMBOL AND UNIT
Quantity / objectWater-vapour density
Symbold_v, ρ_v
SIkg/m³kilogram per cubic metre
Commong/m³gram per cubic metre
Definition
Absolute humidity, or water-vapour density, is the mass of water vapour divided by the total volume occupied by a moist-air sample.
It is normally reported in kg/m³ or g/m³. Because the sample volume changes with temperature and pressure, water-vapour density is not conserved during simple heating or compression even when the vapour mass is unchanged.
Formula and variables
d_v = m_w / V Conditions: The volume is the total volume occupied by the moist-air sample at the stated temperature and pressure.
| Symbol | Meaning | Unit |
|---|---|---|
d_v | absolute humidity or water-vapour density | kg/m³ |
m_w | mass of water vapour | kg |
V | total sample volume | m³ |
Relations and distinctions
Absolute humidity and humidity ratio
Absolute humidity uses total sample volume as the denominator. Humidity ratio uses dry-air mass and is often more convenient for air-stream mass balances.
Absolute and specific humidity
Specific humidity divides vapour mass by total moist-air mass, not by volume.
Engineering meaning
- Use water-vapour density when moisture mass per room or enclosure volume is the required quantity.
- Keep temperature and pressure with a reported value when comparing different air states.
Common misconceptions
MisconceptionAbsolute humidity is an absolute, temperature-independent measure of moisture.
Accurate explanationIts denominator is volume, so the numerical value changes when the same air mass expands or contracts.
Application boundaries
- The term is used inconsistently outside psychrometrics; state 'water-vapour density' and units to remove ambiguity.
- Water-vapour density alone does not establish saturation, condensation risk or dehumidification capacity.
Supporting sources
-
ASHRAE / SRC-ASHRAE-F25-CH01
ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics ↗
2025 SI edition / Chapter 1, Basic Parameters, Equation 10; lines 527–533
Supports: Absolute humidity is water-vapour mass divided by total sample volume; The quantity is alternatively called water-vapour density; The SI basis is mass per volume
Maintenance record
Absolute Humidity
- Maintained by
- Yakeclimate Engineering Team
- Authoritative sources verified
- August 3, 2026
- Terminology checked
- August 3, 2026
- Last updated
- August 3, 2026