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Sling Psychrometer
Introduction
In this activity, a sling psychrometer will be constructed and the relative amount of humidity in the air will be measured. The dew point will then be found using the relative humidity value.
Concepts
Relative humidity - Water vapor
- Dew point
Background
Water vapor is the gaseous, invisible form of water in the atmosphere. It is better known as humidity. When the air in the atmosphere contains a large amount of water, the air feels very humid. The opposite is true when the air is relatively void of water vapor—the air feels very dry. When air holds the maximum amount of moisture, dew or frost will be present and small droplets will begin to form as clouds. As the clouds become saturated with water droplets, they will become too dense to hold all of the droplets and the droplets will start to fall towards the Earth’s surface in the form of rain or snow. This is known as 100% humidity.
A sling psychrometer can be used to measure the relative humidity of the air. Relative humidity is the percentage of moisture air is holding compared to the maximum it can hold. When water in the air evaporates, a certain amount of heat is required to convert the air into water vapor. Therefore, a cooling effect takes place when evaporation occurs. A sling psychrometer consists of two thermometers; a dry-bulb and a wet-bulb. The dry-bulb thermometer measures the temperature of the surrounding air while the wet-bulb thermometer records the amount of cooling that is required for the water to evaporate at that specific temperature. If the air is very humid, the differences between the dry-bulb and wet-bulb thermometers will not be large because there is little evaporation. However, if the air is arid or dry, a large amount of evaporation takes place (which causes a cooling effect on the wet-bulb thermometer) and the difference between the two thermometers will be greater.
To use the Relative Humidity Table (see the Sling Psychrometer Student Worksheet), first find the temperature difference between the dry- and wet-bulb thermometers. Locate this value on the Relative Humidity Table. Now use this value and the final temperature of the dry-bulb thermometer to obtain the relative humidity reading.
Dew point is defined as the temperature at which air must be cooled (at constant pressure and water vapor content) for saturation (dew formation) to occur. When the dew point is below freezing, (32 °F), it is commonly referred to as the frost point.
The dew point is an important measurement used to predict the formation of dew, frost and fog. Since atmospheric pressure varies only slightly at the Earth’s surface, the dew point is a good indicator of the air’s water vapor content. High dew points indicate high water vapor and low dew points indicate low water vapor content.
The difference between air temperature and dew point temperature indicates whether the relative humidity is low or high. When the air temperature and dew point are dramatically different, the relative humidity is low. When the air temperature and dew point are close to the same value, the relative humidity is high. When the air temperature and dew point are equal, the relative humidity is 100% (see the Dew Point Calculation Chart).
To find the dew point, use the temperature of the air (from the Sling Psychrometer Student Worksheet) and your relative humidity percent reading. Find each of these values on the Dew Point Calculation Chart (see the Sling Psychrometer Student Worksheet) and locate the corresponding dew point value.
Materials
Cotton wick, 1
Handle, plastic
Pipet, Beral-type
Rubber band, small
Rubber cap
Screw
Screwdriver
Sling Psychrometer Worksheet
Thermometers, plastic-backed, 2
Safety Precautions
Be sure that the thermometers are securely attached to the plastic handle before swinging. The instructor should inspect the assembled sling psychrometer prior to use. Be careful not to drop or break the thermometers. Wear protective eyewear.
Procedure
Assembly
- Construct a wet-bulb thermometer by slipping a small piece of cotton wick over the bulb of one of the thermometers. The other thermometer is the dry-bulb thermometer.
- Attach the two plastic-backed thermometers together back-to-back using a small rubber band (see Figure 1).
{10356_Procedure_Figure_1}
- Slide both of the thermometers onto the screw through the hole used to hang the thermometers.
- Twist the screw carefully into the end of the plastic handle (with the predrilled hole it it) until 3 or 4 mm of the screw’s shaft remains above the handle.
- Place the rubber cap on the bottom end of the psychrometer handle.
Procedure
- Determine the temperature on the dry-bulb thermometer. Record the temperature on the Sling Psychrometer Worksheet.
- Use a Beral-type pipet and place a few drops of water on the gauze of the wet-bulb thermometer.
- Place the plastic handle in your hand and slowly rotate the thermometers around the screw. The spinning motion will accelerate the evaporation rate of the water.
- Spin the thermometers on the sling psychrometer for thirty seconds or until the wet-bulb thermometer drops to a point where it remains constant.
- After the time has elapsed, immediately record the temperature of both thermometers on the Sling Psychrometer Worksheet. Determine the difference between the dry-bulb and wet-bulb thermometers. Record this value on the Sling Psychrometer Worksheet.
- Use the Relative Humidity Table to determine the relative humidity of the air. Record this value in the data table.
- Use the relative humidity value and the Dew Point Calculation Chart to determine the dew point. Record this value in the data table.
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