Determining Relative Humidity

By Mary Jane Bell

GRADE LEVEL:

Suitable for junior high students.

SUMMARY:

The students will construct and use a sling psychrometer to determine relative humidity. Safety goggles are required for this activity in my classroom while the psychrometers are being constructed and used. Students will take measurements of the temperature on each of the two thermometers, determine the difference between the two readings, and use a table to determine relative humidity.

STUDENT AND TEACHER BACKGROUND INFORMATION:

Students may need to review how to read a thermometer. The students should find two thermometers that have the same room temperature reading before constructing their sling psychrometer. Safety is probably the biggest concern with students. I require safety goggles for the construction and use of sling psychrometers. Their sling psychrometer should have to pass instructor approval before they try it out. Students need to be reminded to record the changes in temperature after the psychrometer has been spun. Having the class decide on ten or twenty spins might be a good idea.

MATERIALS NEEDED:

For each team of students…it will probably depend on how many thermometers you have available.

  • Two thermometers mounted on metal backs.
  • Double-sided foam tape, poster mount tape will work.
  • Two small washers.
  • One nail.
  • One dowel approximately 15 centimeters, smooth and sliver free.
  • Hammer…may be shared by more than one group.

LEARNING OBJECTIVES:

The student team will follow directions for making a sling psychrometer. They will construct the psychrometer, follow safety rules, and take the necessary measurements. The students will then determine relative humidity using a table. Relative humidity will be expressed as a percentage.

POSSIBLE EXTENSIONS:

They might want to take humidity at different times of the day and in different locations in the school building. They could report the humidity on a poster in the hall near the science room, locker room, or various locations around the school. They could access the World Wide Web and find out the relative humidity for our area through one of the weather sources. 

Problem: How can you determine relative humidity using a handmade sling psychrometer?

Materials: Per team of four.

  • One Sling Psychrometer…materials list follows if not already constructed.
  • One Medicine Dropper
  • Small Container of Distilled Water
  • Two thermometers mounted on metal backs.
  • Double-sided foam tape, poster mount tape will work.
  • Two washers
  • One nail
  • One dowel approximately 15 centimeters, smooth and sliver free!
  • Hammer…may be shared with more than one group.

Construct a sling psychrometer following the steps given below.

  1. Attach two thermometers that are attached to metal supports, back to back with two-sided foam tape.
  2. Wrap gauze around the bulb of one thermometer. Use thread to secure, this thermometer will be referred to as the wet-bulb thermometer.
  3. Slide a washer, both thermometers and another washer onto a nail.
  4. Carefully hammer the nail into the dowel. Instructor may want students to not nail the psychrometer. Assembly may be made to this point and the instructor does the hammering of the nail into the dowel.

Procedure:

  1. Using a dropper, add a few drops of water to the gauze on the we-bulb thermometer.
  2. Hold the dowel in your hand and slowly spin the thermometers around the nail. This spinning motion will speed up the evaporation process. CAUTION
  3. Determine how many times you will spin the sling psychrometer, check and record the temperature of both thermometers. Keep spinning until the temperature of the wet-bulb thermometer no longer changes.
  4. When the temperature of the wet-bulb thermometer has stopped dropping, record the temperatures of both thermometers. Calculate the difference between the two readings.
  5. Using the dry-bulb temperature and the difference between the two readings, determine the relative humidity. Express your answer as a percentage.

Observations:

  1. What do you think will happen to the moist cloth as the wet-bulb thermometer is exposed to air?
  2. What do you think will happed if you put lots of water on the wet-bulb thermometer?
  3. Which of the two thermometers measures the air temperature?
  4. What is the relative humidity of the classroom?
  5. What is the relative humidity of the hallway?
  6. What is the relative humidity of the outside?

Analysis and Conclusions

  1. What is the relationship between evaporation and the wet-bulb temperature?
  2. What is the relationship between evaporation and relative humidity?
  3. What would the relative humidity be if both the wet-bulb thermometer and the dry-bulb thermometer measured the same temperature? Explain.
  4. How did the relative humidity inside the classroom compare with the relative humidity outdoors?
  5. Would you expect the temperature of the wet-bulb thermometer to be higher on a humid or on a dry day? Explain.
  6. If the humidity is high, only a small amount of perspiration will evaporate. So how do you feel when that happens?
  7. Why do people from a southwestern state claim, "Oh it’s 100 degrees today, but it’s a dry heat?"
  8. What temperature scale were they using?

Source:  Parts of this laboratory lesson were taken from the following:

Exploring Earth's Weather

  • Prentice Hall Science 1994 copyright