Understanding Dry and Wet Bulb Temperature
Psychrometry is the study of moist air behaviour, bridging thermodynamics and atmospheric science. Two temperature measurements form the foundation: dry bulb temperature (the ambient air temperature measured with an ordinary thermometer) and wet bulb temperature (measured by a thermometer with a water-saturated wick exposed to moving air).
The wet bulb temperature is always lower than dry bulb because evaporative cooling from the wick depresses the reading. The difference between these two temperatures reveals the air's moisture content—a small difference indicates humid air, while a large gap suggests dry conditions. This simple measurement pair unlocks the entire psychrometric state.
Applications span building climate control, textile manufacturing, grain storage, and pharmaceutical cleanrooms. Any process involving moisture removal or air conditioning relies on these principles.
Humidity Ratio and Relative Humidity
Humidity ratio (ω) expresses the mass of water vapour per kilogram of dry air—typically ranging from 0 to 0.03 kg/kg in practical systems. It differs from specific humidity, which measures water vapour relative to the total moist air mass, a subtle but important distinction for precise calculations.
Relative humidity (Φ) is the ratio of actual partial vapour pressure to saturated vapour pressure at the current temperature, expressed as a percentage. At 100% relative humidity, the air is saturated; below that, moisture can still be absorbed.
- Humidity ratio is an absolute measure—it doesn't change with temperature alone
- Relative humidity is temperature-dependent—cooling air increases its relative humidity without adding moisture
- The two properties often lead to different design conclusions when air is heated or cooled
Key Psychrometric Equations
These relationships define the thermodynamic state of moist air. The partial vapour pressure equation corrects for atmospheric effects when wet bulb temperature is measured in the field. Relative humidity, humidity ratio, and dew point follow directly from saturation properties.
P_v = P_w − 0.00066 × P_a × (T_a − T_w) × (1 + 0.0115 × T_w)
Φ = 100 × P_v / P_s
ω = 0.621945 × P_v / (P_a − P_v)
T_dp = 243.5 × ln(P_v / 6.112) / (17.67 − ln(P_v / 6.112))
h = 1.006 × T_a + ω × (2499.86 + 1.84 × T_a)
v = 287.042 × (273.15 + T_a) × (1 + 1.607858 × ω) / (P_a × 100)
P_v— Partial vapour pressure (mbar)P_w— Saturated vapour pressure at wet bulb temperature (mbar)P_a— Atmospheric pressure (kPa)T_a— Dry bulb (ambient) temperature (°C)T_w— Wet bulb temperature (°C)Φ— Relative humidity (%)P_s— Saturated vapour pressure at dry bulb temperature (mbar)ω— Humidity ratio (kg water/kg dry air)T_dp— Dew point temperature (°C)h— Enthalpy of moist air (kJ/kg dry air)v— Specific volume (m³/kg dry air)
Specific Volume and Enthalpy in Moist Air
Specific volume represents the volume occupied per kilogram of dry air and directly affects sizing of ducts and equipment. As humidity increases, specific volume rises—moist air is less dense than dry air at the same temperature and pressure.
Enthalpy (h) quantifies the total thermal energy of moist air and is crucial for calculating heat transfer in cooling and heating coils. The enthalpy equation shows two contributions: sensible heat from temperature change and latent heat from moisture content. The multiplier 2499.86 kJ/kg represents the latent heat of vaporisation of water, while 1.84 kJ/(kg·°C) accounts for the specific heat of water vapour's temperature dependence.
In HVAC systems, enthalpy differences between inlet and outlet air streams determine compressor work and heat rejection rates. Designers plot these states on a psychrometric chart, where enthalpy becomes a diagonal line, enabling quick graphical solutions.
Practical Considerations When Using Psychrometric Data
Real-world psychrometric analysis requires attention to several common pitfalls.
- Altitude Effects on Atmospheric Pressure — Atmospheric pressure decreases with altitude, significantly altering saturation properties and partial pressures. A Denver location at 1600 m experiences 83% of sea-level pressure, which raises relative humidity and lowers dew point temperature calculations. Always confirm your altitude input to avoid misspecifying humidity control setpoints.
- Wet Bulb Temperature Measurement Errors — Wet bulb readings depend on air velocity across the wick and water purity. Stagnant air or contaminated water produces erroneously high wet bulb values, overstating actual air moisture content. Use a sling psychrometer with smooth, steady motion for accurate field measurements.
- Saturation Curve Nonlinearity — Saturation vapour pressure is highly nonlinear with temperature—it nearly doubles between 0°C and 20°C. Small temperature errors cause large errors in saturation pressure and subsequent calculations. Digital calculators handle this better than hand-drawn psychrometric charts, which have limited resolution.
- Degree of Saturation vs. Relative Humidity — Degree of saturation (μ) measures actual humidity ratio against saturated humidity ratio at the same temperature and is not the same as relative humidity. The two properties diverge significantly when air is heated or cooled, potentially causing confusion in specification documents. Always clarify which metric is intended in design contracts.