Wet-Bulb Temperature Measurement

Temperature Measurement

• As the temperature of the bulb increases, the liquid in the bulb expands and rises in the stem.

Wet-Bulb Temperature

• Wet-bulb temperature is the temperature of the air as measured by an ordinary thermometer whose glass bulb is covered by a wet cloth or gauze.
• Evaporation of water reduces the temperature of the wick, and the resultant temperature is indicated by the mercury in the thermometer stem.
• Airflow passing over the wet cloth should have a velocity of at least 15 ft per second.

Humidity Ratio

• Humidity ratio is the weight of the water vapor contained in the moist air per unit weight of dry air.
• Also known as absolute humidity and specific humidity.

Enthalpy

• Enthalpy of a dry air-water vapor mixture is the heat content of the moist air per unit weight of dry air above a certain reference temperature.

Specific Volume

• Specific volume of moist air is defined as the volume per unit weight of dry air.
• Specific density of the moist air is equal to the reciprocal of its specific volume.

Psychrometric Chart

• If the dry-bulb and wet-bulb temperatures are known, the relative humidity can be read from the chart.
• If the dry-bulb temperature and relative humidity are known, the wet-bulb temperature can be determined.
• If the wet-bulb temperature and relative humidity are known, the dry-bulb temperature can be found.
• If the wet-bulb and dry-bulb temperatures are known, the dewpoint can be read from the chart.
• If the dry-bulb temperature and relative humidity are known, the dewpoint can be found.

Identification of Lines and Scales on the Chart

• Dry-bulb temperature scale extends along the sole from toe to heel.
• Wet-bulb scale extends along the instep from the toe to the top of the shoe.
• Condensation or dewpoint is the same as the wet-bulb scale.
• Relative humidity lines are located along the side of the shoe and follow approximately the same curve as the instep.
• Grains of moisture scale follows along the back of the shoe from the heel to the top.

Psychrometry

• Psychrometry is the study of the properties of the air.
• Knowledge of psychrometry and use of the psychrometric chart are essential for understanding processes involved in drying and storage of grains and other agricultural crops.
• Psychrometric chart is a graphic presentation of the conditions or properties of air, such as temperature, humidity, and dewpoint.