construct and use a sling psychrometer

Using a Sling Psychrometer to Determine Relative Humidity
Mary Jane BellLyons-Decatur School District, Nebraska 

GRADE LEVEL: Suitable for junior high students.

SUMMARY: The students will construct and use a sling psychrometerto determine relative humidity. Safety goggles are required for this activityin 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 determinerelative humidity.

STUDENT AND TEACHER BACKGROUND INFORMATION: Students may needto review how to read a thermometer. The students should find two thermometersthat have the same room temperature reading before constructing their slingpsychrometer. Safety is probably the biggest concern with students. I requiresafety goggles for the construction and use of sling psychrometers. Theirsling psychrometer should have to pass instructor approval before theytry it out. Students need to be reminded to record the changes in temperatureafter the psychrometer has been spun. Having the class decide on ten ortwenty spins might be a good idea.

MATERIALS NEEDED: for each team of students…it will probablydepend 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 directionsfor making a sling psychrometer. They will construct the psychrometer,follow safety rules, and take the necessary measurements. The studentswill then determine relative humidity using a table. Relative humiditywill be expressed as a percentage.

POSSIBLE EXTENTENSIONS: They might want to take humidity at differenttimes of the day and in different locations in the school building. Theycould report the humidity on a poster in the hall near the science room,locker room, or various locations around the school. They could accessthe World Wide Web and find out the relative humidity for our area throughone of the weather sources. 

Problem: How can you determine relative humidity usinga 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.

    a. Attach two thermometers that are attachedto metal supports, back to back with two-sided foam tape.

    b. Wrap gauze around the bulb of one thermometer.Use thread to secure, this thermometer will be referred to as the
        wet-bulb thermometer.

    c. Slide a washer, both thermometers and anotherwasher onto a nail.

    d. Carefully hammer the nail into the dowel.Instructor may want students to not nail the psychrometer. Assembly maybe
        made to this point and theinstructor does the hammering of the nail into the dowel.


1)  Using a dropper, add a few drops of water to the gauzeon the we-bulb thermometer.

2)  Hold the dowel in your hand and slowly spin the thermometersaround 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 untilthe temperature of the wet-bulb thermometer no longer changes.

4)When the temperature of the wet-bulb thermometer has stoppeddropping, record the temperatures of both thermometers. Calculate the differencebetween the two readings.

5)  Using the dry-bulb temperature and the difference betweenthe two readings, determine the relative humidity. Express your answeras a percentage.


1)  What do you think will happen to the moist cloth asthe wet-bulb thermometer is exposed to air?

2)  What do you think will happed if you put lots of wateron 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 thewet-bulb temperature?

2)  What is the relationship between evaporation and relativehumidity?

3)  What would the relative humidity be if both the wet-bulbthermometer and the dry-bulb thermometer measured the same temperature?Explain.

4)  How did the relative humidity inside the classroom comparewith the relative humidity outdoors?

5)  Would you expect the temperature of the wet-bulb thermometerto be higher on a humid or on a dry day? Explain.

6)  If the humidity is high, only a small amount of perspirationwill evaporate. So how do you feel when that happens?

7)  Why do people from a southwestern state claim, "Oh it’s100 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

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