Definition
The mass of water vapour per kilogram of dry air; the unit of absolute humidity, expressed as kg_water/kg_dry_air or g_water/kg_dry_air (g/kg). Unlike RH, it is independent of temperature and is the foundation of humidification/dehumidification capacity calculations.
Detailed Explanation
Humidity ratio (x or W) is the standard thermodynamic descriptor of moist-air absolute humidity: how many grams (or kg) of water vapour accompany 1 kg of dry air. The SI unit is kg_water/kg_dry_air but g/kg is more practical (so "8 g/kg" means 0.008 kg_water/kg_dry_air). The maximum value occurs at saturation and is temperature-dependent: 3.8 g/kg at 0°C, 14.7 g/kg at 20°C, 49 g/kg at 40°C.
Formula: x = 0.622 × (p_vapour / (p_atm − p_vapour)); p_vapour is the partial pressure of water vapour (Pa), p_atm the total atmospheric pressure (101,325 Pa standard). On the psychrometric chart it is shown as the vertical axis or as a logarithmic right-hand scale; constant-RH curves intersect x at each temperature. Unlike RH, x is unaffected by temperature change — that is what makes it the master reference for humidification/dehumidification capacity calculations.
Why It Matters
In humidification/dehumidification capacity calculations, x (humidity ratio) is the only valid input for absolute humidity; using an RH difference directly is mathematically wrong. Typical capacity formula:
m_water (kg/h) = ρ_air × V_air × Δx = ρ_air × V_air × (x_target − x_current)
Example: turning 22°C / 30% RH (x ≈ 5 g/kg) into 22°C / 50% RH (x ≈ 8.5 g/kg) needs Δx = 3.5 g/kg = 0.0035 kg/kg. At 10,000 m³/h, capacity = 1.2 × 10,000 × 0.0035 = 42 kg/h. Doing the same calculation with the (wrong) RH difference of 50% − 30% = 20% yields a meaningless number, because RH difference is dimensionless and has no mass interpretation.
Most engineering software and OEM tools (NKT Portal, humidifier control software, Trane Trace) operate on x values. A quick on-site estimate "every 10% RH @ 22°C ≈ 1.7 g/kg" works roughly, but a real calculation should always use a table or a tool.
Practical Example
A data centre in Izmir, supply-air analysis: summer design outdoor 35°C / 45% RH → x ≈ 16 g/kg; indoor target 24°C / 50% RH → x ≈ 9.3 g/kg. The incoming outdoor air therefore carries Δx = 16 − 9.3 = 6.7 g/kg of excess moisture; the system needs dehumidification, not humidification. When the cooling coil drops the air to 9°C (saturation @ 9°C ≈ 7.2 g/kg), condensation removes 16 − 7.2 = 8.8 g/kg. The air is then reheated to 24°C; final x = 7.2 g/kg and RH = 38%. So 8.8 g/kg of moisture is removed and the resulting overcool is corrected by reheat — this classical DX cooling-plus-reheat psychrometric process is driven entirely by the x calculation.
Engineering Note
Caveats for humidity-ratio calculation: (1) Atmospheric pressure is taken as the standard 101,325 Pa, but at altitudes above 1500 m it drops 15–20%; the x value for the same RH at the same Tdb shifts accordingly — for Erzurum, Kayseri, and Konya the correction is mandatory. (2) Air density (ρ) is 1.2 kg/m³ at sea level but drops 15–20% at altitude; in the capacity formula the two corrections compound, so ignoring them produces a 30–40% error. (3) At very high temperatures (> 50°C), use ASHRAE Fundamentals tables rather than approximate formulas.
Quick Turkish-condition reference values (sea level, x in g/kg): winter outdoor (−5°C / 75% RH) ≈ 1.8 g/kg; spring/autumn (15°C / 60% RH) ≈ 6.4 g/kg; summer outdoor (32°C / 55% RH) ≈ 16.5 g/kg; indoor comfort target (22°C / 50% RH) ≈ 8.4 g/kg; indoor process target (22°C / 30% RH) ≈ 5.0 g/kg.
NKT Application Link
NKT uses x (humidity ratio) as a standard calculation input across all humidification/dehumidification project proposals; the Portal calculator derives x automatically from airflow, temperature, RH, atmospheric pressure, and altitude. During commissioning, x is extracted from sensor readings (Tdb + RH) and compared with the design target. On NKT - Climate Track trend logs, x can be shown as an additional track, allowing operators to follow absolute humidity directly rather than RH — especially in dry rooms (lithium battery) and sensitive printing applications, where the x trend is far more interpretable than the RH trend.
