A dry bulb thermometer measures the ambient air temperature, while a wet bulb thermometer measures the temperature of the air after evaporative cooling. A maximum thermometer records the highest temperature reached during a specific time period. These thermometers are used together to determine relative humidity and can be part of a psychrometer.
Dry Bulb Thermometer:
Measures the actual air temperature.
The bulb is exposed to the air and not covered by anything.
Wet Bulb Thermometer:
Measures the temperature of the air after it has been cooled by evaporation.
The bulb is covered with a wet cloth or wick, which is continuously moistened.
Maximum Thermometer:
Records the highest temperature reached during a period of time.
It often has a constricted section in the mercury column, which prevents the mercury from flowing back after the temperature rises, allowing it to "remember" the highest temperature.
Relationship between Dry Bulb and Wet Bulb:
The difference between the dry bulb and wet bulb temperatures is related to the humidity.
In dry air, the wet bulb temperature will be significantly lower than the dry bulb temperature.
In humid air, the difference between the two temperatures will be smaller.
At 100% relative humidity, the dry bulb and wet bulb temperatures will be the same.
Applications:
Weather Monitoring:
Dry bulb and wet bulb thermometers are used to measure atmospheric humidity.
Building Science:
They are used to determine the state of humid air, which is important for HVAC systems.
Industrial Processes:
Wet bulb temperatures are important in some industrial processes, particularly those involving moisture.
Human Health:
Wet bulb globe temperature (WBGT) is used to assess heat stress on humans.
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