EKS-HUM-002 / Moist-air state and psychrometric properties

Wet-bulb Temperature

湿球温度

Thermodynamic wet-bulb temperature is the temperature at which water evaporating into a moist-air sample can bring it to saturation adiabatically at the same total pressure.

Abbreviations: WB, WBT
Aliases: thermodynamic wet-bulb temperature

QUANTITY, SYMBOL AND UNIT

Quantity / objectThermodynamic wet-bulb temperature

Symbolt_wb, t*

SI°Cdegree Celsius

IP°Fdegree Fahrenheit

Definition

Thermodynamic wet-bulb temperature is the temperature at which water evaporating into a moist-air sample can bring it to saturation adiabatically at the same total pressure.

A psychrometer creates a practical wet-bulb indication with a wetted sensor in moving air. That indication can approximate the thermodynamic property, but radiation, wick condition, water temperature and air velocity prevent the two from being assumed identical.

Relations and distinctions

Thermodynamic property and instrument reading

The thermodynamic wet bulb is a defined air property; the measured wet bulb is the result of a particular heat-and-mass-transfer measurement.

Wet bulb and dew point

Wet bulb is tied to an adiabatic saturation process. Dew point is the saturation temperature at the sample's existing water-vapour content.

Engineering meaning

Use wet-bulb temperature with dry bulb and total pressure to determine other moist-air properties.
When using a psychrometer, document airflow, wick condition, radiation shielding and measurement uncertainty.

Common misconceptions

MisconceptionA psychrometer reading is exactly the thermodynamic wet-bulb temperature.

Accurate explanationIt is an approximation whose error depends on the instrument and measurement conditions.

Application boundaries

Wet-bulb values require a stated pressure basis when used for property calculation.
A wet-bulb value alone does not determine moisture load, equipment capacity or a humidity-control setpoint.

Supporting sources

ASHRAE / SRC-ASHRAE-F25-CH01

ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics ↗

2025 SI edition / Chapter 1, Section 7, especially lines 648–680
Supports: Thermodynamic wet-bulb temperature is defined by adiabatic saturation at constant total pressure; A measured psychrometer wet-bulb temperature only approximates the thermodynamic property; Radiation, water temperature and airflow affect a psychrometer reading
National Institute of Standards and Technology / SRC-NIST-HUMIDITY-CAL

Humidity Calibration Services at NIST ↗

Updated 2025-08-14 / Description and calibration-uncertainty sections
Supports: Humidity measurements require a defined measurand and calibration context

Maintenance record