Psychrometrics definition: The mass of water vapour contained in 1 m³ of moist (wet) air is known as which property?

Introduction / Context:
HVAC design and atmospheric science rely on precise humidity terminology. Mixing up absolute humidity with specific humidity or relative humidity leads to calculation errors in coil sizing and comfort analysis.

Given Data / Assumptions:

• A fixed volume reference of 1 m³ of moist air is considered.
• We want the mass of water vapour contained within that volume.
• Standard psychrometric definitions apply at a stated pressure and temperature.

Concept / Approach:
Absolute humidity is defined as the mass of water vapour per unit volume of moist air, typically kg/m³. This is distinct from specific humidity (also called humidity ratio), which is the mass of water vapour per unit mass of dry air (kg/kg of dry air), and from relative humidity, which is a percentage comparing actual vapour pressure to saturation vapour pressure at the same temperature.

Step-by-Step Solution:

Verification / Alternative check:
Using the ideal gas relation for water vapour, m_v = (p_v * V) / (R_v * T), where p_v is partial pressure of water vapour, gives an absolute humidity value (kg/m³) that changes with temperature and barometric pressure.

Why Other Options Are Wrong:

Specific humidity (humidity ratio): mass of vapour per kg of dry air, not per m³ of moist air.Relative humidity: a dimensionless ratio (percentage), not a mass measure.Degree of saturation and dew-point temperature are different psychrometric concepts unrelated to mass per unit volume.

Common Pitfalls:
Using absolute humidity in place of humidity ratio when doing air mixing or coil calculations; most HVAC formulas are written in terms of humidity ratio (kg/kg of dry air), not kg/m³.

Final Answer:

absolute humidity