In this activity, students measure relative humidity in the air using just a temperature sensor, by comparing the wet bulb and dry bulb temperatures.

How humid is it?

Some days it feels “dry” and some days it feels “muggy,” even though the temperature is the same. This is due to different amounts of water vapor in the air. Describe why you think humid air would feel different to us from dry air.

There is always some water vapor in the air, but we can’t see it directly. How do you think one could measure it? Think of observable physical objects that might change if the air were more or less humid.

Relative humidity is a measure of how much water is in the air. At any one temperature there is a maximum amount of water the air can carry. The relative humidity is a percent of this maximum. For example, when air holds all the moisture it can, the relative humidity is 100. If more moisture is added, or if the temperature goes down, some moisture condenses out as liquid water droplets in the form of fog or mist. When air holds only half the maximum amount of moisture, the relative humidity is 50.

There are several different ways to measure humidity. In this activity you will use a very simple and accurate technique, called the wet bulb – dry bulb method. It depends on the fact that water cools when it evaporates. When the relative humidity is low, evaporation is rapid and there is lots of cooling. When the relative humidity is high, evaporation is slow and there is very little cooling. If the relative humidity is 100%, there is no cooling at all!

The temperature of a dry temperature sensor and a wet temperature sensor are compared. The greater the difference, the lower the relative humidity.

Explain in your own words why you think the evaporation of water causes cooling.

• Fast-response temperature sensor
  • See Data Collection Part 1 for commercial sensors
  • See Data Collection Part 2 for the ITSI Probe Kit thermocouple sensor
• Small piece of gauze (such as from a band-aid)
• Cup of water

This activity addresses NSES standards for earth and space science and inquiry at grades 5-8 for structure of the earth system
(http://books.nap.edu/readingroom/books/nses/6d.html#es).

There are no special safety concerns in this activity.

• After you have done this activity once, you can use it as part of the other activities that require monitoring relative humidity. Skip directly to the Collect Data section and make your dry bulb – wet bulb measurements.
• Note: If you need to make repeated relative humidity measurements, it may be easier to work with another team, using one temperature sensor for the dry bulb measurement and one for the wet bulb measurement.

Part 1: using a commercial temperature sensor

1. Measure the air temperature with a dry temperature sensor. You can wave the sensor back and forth to be sure it reaches the temperature of the air.
2. Dry bulb temperature =______.
3. Wrap gauze (bandage gauze is good) around the temperature sensor and hold it in place with a twist tie or a piece of tape. Be sure that the gauze around the sensor is fully exposed to the air. Attach it to a pencil or ruler so that you can wave it around and it won’t fall apart.
   
4. Dip the gauze in room-temperature water.
5. Start recording temperature. Wave the sensor vigorously back and forth to create as much evaporation as possible. Note the lowest temperature the sensor reaches. It may take a few minutes for the sensor to reach its lowest temperature. Dip the gauze into water and try again, to see if you can get the temperature even lower. You may repeat this several times and wave the sensor for several minutes before you reach the lowest possible temperature. Note: you can save several datasets by using the new button on the left.
6. Lowest (wet bulb) temperature =______.
7. Calculate the difference between the two measurements.
8. (Dry bulb) – (Wet bulb) =_______.
9. Use the following chart to determine the relative humidity. The change in temperature (Dry bulb – wet bulb) is shown in red. http://www.novalynx.com/reference-rh-table.html

Relative humidity =____.

Part 2: using the ITSI Probe Kit thermocouple temperature sensor

1. The thermocouple sensor consists of two junctions of dissimilar metals. The voltage output is proportional to the difference in temperature of those two junctions, so it cannot measure the absolute temperature. Here is the circuit. Consult your Probe Kit Manual for a wiring diagram.
   
2. Measure the air temperature with another temperature sensor. You can wave the sensor back and forth to be sure it reaches the temperature of the air.
3. Dry bulb temperature =______.
4. Wrap one thin layer of gauze (bandage gauze is good) around one of the two thermocouple junctions. Hold it in place with a twist tie or a piece of tape. Be sure that the gauze around the sensor is fully exposed to the air.
   
5. Attach the thermocouple junctions and wires to a pencil or ruler so that you can wave them around and they won’t fall apart.
   
6. Start recording temperature. Note the reading when the two junctions are at the same temperature. If the signal jumps around, your junctions may not be very good.
7. Hold one junction with your finger for a few seconds and then let it go. You should see a graph roughly like this, either upward or downward.
   
8. Allow the two junctions to return to room temperature. Note the reading.
9. Clear the graph. Start recording temperature.
10. Dip the gauze in room-temperature water.
11. Start recording temperature. Wave the sensor vigorously back and forth to create as much evaporation as possible. Note the lowest temperature the sensor reaches. It may take a few minutes for the sensor to reach its lowest temperature. Dip the gauze into water and try again, to see if you can get the temperature even lower. You may repeat this several times and wave the sensor for several minutes before you reach the lowest possible temperature. Note: you can save several datasets by using the new button on the left.
12. Greatest change in temperature = _________.
13. Use the following chart to determine the relative humidity. The change in temperature is shown in red. http://www.novalynx.com/reference-rh-table.html

Relative humidity =____.

1. Measure several times. Do you get the same temperature values each time?
2. Compare your results to that of other student groups. Are they the same?
3. Why do you think it’s important to wave the wet bulb sensor back and forth?
4. Why do you think it’s not correct to blow on the wet bulb sensor to cause evaporation?
5. Why do you think the cooling effect of evaporation is greater for dry air than for humid air?
6. Here is a picture of a commercial “sling psychrometer” that uses the same principle as your measurement. Explain what the parts are and how you think it works.
   

Based on what you have observed, would sweating help cool you more in a desert, or in a rainforest? Why?