Definition
The air temperature measured by a standard thermometer. Shown on the x-axis of the psychrometric chart. Used together with wet bulb temperature to calculate the relative humidity of air.
Detailed Explanation
Dry bulb temperature (Tdb or DBT) is the actual thermodynamic temperature of air, measured without influence from moisture. It forms the x-axis of the ASHRAE psychrometric chart and is the value people perceive as "temperature" in everyday life. A standard mercury, alcohol, or electronic thermometer measures this value when the sensor is directly exposed to the air stream.
In HVAC design, dry bulb temperature alone is not sufficient; the air conditioning load cannot be calculated without information about moisture content. Nevertheless, it is the most commonly used practical control setpoint, because most human comfort sensation (radiant heat transfer and dry air temperature) is perceived through this value. Typical comfort ranges are 20–22°C in winter and 24–26°C in summer.
Measurement Standards
Industrial dry bulb temperature measurement standards:
Pt100 (DIN EN 60751 Class A): Accuracy: ±(0.15 + 0.002·|t|) °C ±0.15°C at 0°C, ±0.35°C at 100°C
NTC thermistor: Accuracy: typical ±0.1 to ±0.5°C Low cost, fast response, lower calibration stability
Thermocouple (Type T): Accuracy: ±0.5°C for Class 1 Wide range (−200 to +350°C)
For the condition "sample temperature = measurement temperature", the sensor must be placed at least 5 cm normal to the air stream and isolated from radiant heat sources.
Practical Example
In a data center cooling room, the setpoint is configured as 22°C dry bulb temperature. The sensor is mounted on top of a rack and, due to incorrect positioning, reads the hot exhaust air: the actual cold aisle temperature is 18°C while the sensor reads 26°C.
The control system performs additional cooling based on the 26°C reading → the aisle drops from 18°C to 14°C → falling below the dew point starts condensation → risk of equipment corrosion.
The correct solution is to place the sensor in front of the cold aisle, at rack inlet level, in accordance with ASHRAE TC 9.9 recommendations. Sensor placement is as critical as temperature measurement itself.
Engineering Note
To use dry bulb temperature correctly in HVAC design:
• Temperature alone should not be the sole control parameter — especially in environments with condensation risk, it must be monitored together with humidity or dew point. • Thermal stratification (temperature varying with height) is important in tall facilities; a single sensor may not represent the entire volume. • Solar load effect: sensors mounted on exterior walls can read 3–5°C higher than actual air temperature due to solar radiation. • Velocity effect: in low-velocity zones below 0.5 m/s, local hot or cold pockets may form around the sensor.
ASHRAE Fundamentals defines dry bulb temperature for outdoor design conditions (DOC) at the 0.4% annual exceedance frequency — that is, an extreme value exceeded for only 35 hours per year. The NKT energy simulation tool follows this standard.
