Materials Included In Kit

Additional Materials Required

Safety Precautions

Be sure that the thermometers are securely attached to the plastic handle before swinging. Inspect the assembled sling psychrometer prior to student use. Be careful not to drop or break the thermometers. Wear protective eyewear.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The assembled sling psychrometers may be saved for future use. Water may be flushed down the drain according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• This kit contains enough materials for 30 students working in pairs or 15 students working individually. Additional individual psychrometers (Catalog No. FB0453) may be purchased separately. Flinn also sells a classroom set of 15 sling psychrometers (FB1582).
• Two feet of cotton wick is included. Cut the cotton wick into ½ pieces for use. Extra wick is included as a surplus supply.
• Relative humidity and dew point values are given for temperature ranges between 32 °F and 94 °F. Plan this activity accordingly.
• The sling psychrometers may be assembled ahead of time by the instructor or in class by the students. Be sure to inspect the psychrometers before each use.
• This activity may be done over an extended period of time to study the long-term relative humidity and dew point trends.
• Have students compare their calculated values to values reported by a local weather station or the National Weather Service (see http://weather.gov).
• As an interesting side note, the heights of cumulus cloud bases can be calculated using the dew point themperature and the surface air temperature. Height of cumulus cloud bases (in feet) = 222 [temperature of air (°F) – dew point temperature (°F)].
• Dew point values may also be calculated using the air temperature (°F), relative humidity values and the dew point calculation chart found in the Supplementary Information in the Further Extensions section.

Further Extensions

Supplementary Information

Sample Data

Data Table 1. Direct Dew Point Measurement

Answers to Questions

1. Define dew point.

   Dew point is the temperature at which air must be cooled (at constant pressure and water vapor content) for saturation (dew formation) to occur.

2. Define relative humidity.

   Relative humidity is the percentage of moisture that the air is holding compared to the maximum amount it can hold at a particular temperature.

3. Which relative humidity calculation found in Part III was closer to the measured relative humidity found using the sling psychrometer in Part IV?

   Answers will vary.

References

Bonnet, R. L.; Keen, G. D. Earth Science—49 Science Fair Projects; TAB Books, USA, 1990; pp 124–126.

Introduction

In this activity, different methods will be used to determine the relative humidity and the dew point levels of air.

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. Relative humidity is defined as the percentage of moisture that the air is holding compared to the maximum it can hold at a particular temperature. For clouds to form and rain to start, the air has to reach 100% relative humidity, at the site where the rain is originating. Rain will often fall from clouds, where the humidity is 100%, into areas of much lower humidity.

A sling psychrometer can be used to measure relative humidity and the dew point level. When water evaporates, a certain amount of heat is required to convert the water into 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 measures the amount of cooling that is required for the water to evaporate at that specific temperature. If the air is very humid, the difference in temperature between the dry-bulb and wet-bulb thermometers will not be large because there is only a small amount of 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 resulting temperature difference between the two thermometers will be much greater.

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 the air temperature and dew point indicates whether the relative humidity is low or high. When the air temperature and the dew point are dramatically different, the relative humidity is low. When the air temperature and the 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%.

Materials

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. Follow all laboratory safety guidelines.

Procedure

Sling Psychrometer Assembly 

1. 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.
2. Attach the two plastic-backed thermometers together back-to-back using a small rubber band (see Figure 1).
   {13898_Preparation_Figure_1}
3. Slide both of the thermometers onto the screw through the hole used to hang the thermometers.
4. 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.
5. Place the rubber cap on the bottom end of the psychrometer handle.

 Part I. Direct Dew Point Measurement

1. Fill the metal can approximately one-half full with room temperature water.
2. Place a thermometer in the water and record the initial temperature (°F) in Data Table 1.
3. Slowly add ice to the water and swirl the can gently. As soon as moisture is felt on the outside of the can, note the temperature. This is the dew point temperature. Record the dew point temperature in Data Table 1.
4. If large droplets of water form on the outside of the can before the final temperature is recorded in step 3, the recorded final temperature will not be accurate. If this occurs, empty and dry the can and then repeat the procedure once the can has reached room temperature.

5. Obtain a preassembled sling psychrometer.
6. Measure the temperature of the air using the dry-bulb thermometer. Record this value in degrees Farenheit in Data Table 2.
7. Use a Beral-type pipet and place a few drops of water on the gauze of the wet-bulb thermometer.
8. Hold 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.
9. Spin the thermometers on the sling psychrometer for thirty seconds or until the temperature of the wet-bulb thermometer drops to a point where it remains constant.
10. After the time has elapsed, immediately record the temperature of the wet-bulb thermometer in Data Table 2. Determine the temperature difference between the dry-bulb and wet-bulb temperature measurements. This is the wet-bulb depression. Record this value in Data Table 2.
11. Use the dry-bulb temperature reading and the Dew Point Table (Reference Table 1) to determine the dry-bulb factor. Record this value in Data Table 2.
12. Multiply the wet-bulb depression by the dry-bulb factor. Record this value in Data Table 2.
13. Subtract the value obtained in step 12 from the initial dry-bulb temperature. This is the Dew Point Temperature. Record this value in Data Table 2.

14. To calculate the relative humidity, first convert the dry-bulb temperature from Data Table II from Fahrenheit to Celsius by using Equation 1. Record this value in Data Table 3.
    {13898_Procedure_Equation_1}
15. Dew point values from both Part I and Part II will be used to obtain relative humidity values. Use Equation 1 to convert both dew point temperatures to °C. Record the two temperature values in Data Table 3.
16. Calculate the saturation vapor pressure by using Equation 2 (assuming standard atmospheric pressure). Record this value in Data Table 3.
    {13898_Procedure_Equation_2}
17. Calculate the actual vapor pressures for Parts I and II using Equation 3 (assuming standard atmospheric pressure). Record these values in Data Table 3.
    {13898_Procedure_Equation_3}
18. The relative humidity values for Parts I and II can then be calculated using Equation 4. Record these values in Data Table 3.
    {13898_Procedure_Equation_4}

19. Determine the temperature on the dry-bulb thermometer. Record the temperature in Data Table 4.
20. Use a Beral-type pipet and place a few drops of water on the gauze of the wet-bulb thermometer.
21. Hold 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.
22. Spin the thermometers on the sling psychrometer for thirty seconds or until the wet-bulb thermometer drops to a point where it remains constant.
23. After the time has elapsed, immediately record the temperature of the wet-bulb thermometer in Data Table 4. Determine the difference between the dry-bulb and wet-bulb thermometers. Record this value in Data Table 4.
24. Use the Relative Humidity Table (Reference Table 2) to determine the relative humidity of the air. Record this value in Data Table 4.

Student Worksheet PDF

13898_Student1.pdf