Spring 2006 Name__________________________
Objective: To show how humidity affects the properties of air, and to show how changes in the dew point and rate of vaporization can be used to determine the relative humidity.
Read the entire experiment and answer all questions on the first two pages.
Relative Humidity is the measure of the amount of water vapor that is in the air compared to the amount that the air is capable of holding. In this experiment, we will use two different effects of humidity to obtain two separate measurements of humidity, then the results of the two methods will be compared. We will also compare the humidity inside the lab room with that outdoors.
When the relative humidity is 100% the air is saturated, it will not hold any additional moisture. (Water will not evaporate under these conditions.) At 25oC 100% humidity would mean that water represents 2% of all the components of the air.
Relative Humidity = 0%, means there is no water in the air, and therefore evaporation is maximum.
The dew point is the highest temperature at which dew will condense from the air to the object. The higher the humidity, the closer the dew point is to the ambient temperature. At 100% humidity, the dew point will be the same as the ambient temperature. At a low humidity, the dew point would be much below ambient temperature.
The wet-bulb dry-bulb method of determining humidity depends on the fact that evaporation cools, and the faster the evaporation the faster the cooling. On a day of low humidity evaporation is faster so there is greater difference between the temperatures of wet and dry bulbs of identical thermometers.
Complete the following questions before coming to class.
[Put an X by the correct answer on all the questions that follow. Note that there are some on the next page that must also be answered before class.]
On days of high humidity, would one expect evaporation to be
( ) faster, ( ) slower, or ( ) the same as days of low humidity. Explain why.
We know that because of the heat of evaporation of water, that the faster water evaporates, the greater its cooling effect. This effect can also be used to determine the relative humidity if the air. (This is called the wet-bulb---dry bulb method of determining humidity.)
The amount of water air can hold varies with temperature. Does cooler air hold ( ) less, ( ) more, or ( ) the same quantity of water than warmer air? Why?
The relative humidity is relative to what? ( ) Relative to the amount of moisture air can hold at 0°C, ( ) relative to the amount of moisture air can hold at that same temperature.
What effect would you expect heating the air indoors would have on the relationship between the relative humidity indoors and outside? Would the heated air have a ( )higher or a ( )lower humidity? Explain your answer.
Would you expect the difference in temperature between a dry thermometer and one wrapped in a wet towel on a day of low humidity to be ( )greater or ( )less than on a day with high humidity? Why?
Describe what dew point is and how knowledge of the dew point can be used to determine humidity.
Describe how measurements of a wet bulb and dry bulb can enable the determination of humidity.
1) [Note: You need only one of these "wet bulb" thermometers at each lab table.] Tie about a 4 inch square piece of paper towels with a string tightly around the bulb of your thermometer. Be sure that the bulb is completely covered with the towel. It is best to fold the towel over the bulb before tying it. Dip this in room temperature water and place the thermometer with the wet paper in a place where it will not be broken but that air is free to circulate on all sides (do not lay it on the table or allow it to project from the side of the table). Read the temperature approximately every two minutes while you continue with the rest of the experiment. You do not need to know the exact times between your readings. The important measurement is the lowest temperature.
2) Place a dry thermometer close to the one with the wet bulb. Be sure this also has its bulb where the air can freely circulate around it, and use this to give you the room temperature values. Be sure this is placed where no one will bump it and air is free to circulate around the bulb. Then take the temperatures from time to time as you proceed with the rest of the experiment. Always record the wet and dry temperatures at the same time
Room Temperature [Dry bulb temperature] (approximately every two minutes)
______C, ______ C, ______ C, ______ C, ______ C, ______ C, ______C,
Wet bulb temperature (about every two minutes)
______C, ______ C, ______ C, ______ C, ______ C, ______ C, ______C.
3) To do this part of the experiment you need 3 beakers and another thermometer. One beaker should have water whose temperature is above that of the air in the room and the other water whose temperature is below that in the room. Pour water from these two beakers into a third that has a thermometer as a stirring rod. Mix the water from the warm and cool beakers until the temperature in this beaker is one degree above the temperature on the dry bulb thermometer in part 2 of this experiment.
When you achieve this temperature, remove the thermometer from the water and without drying it off place it on the ledge where air can circulate around the bulb. Record its temperature every 15 seconds until the blanks below are full.
______C, ______ C, ______ C, ______ C, ______ C, ______ C, ______C.
What happened to the temperature of the thermometer during this time period?
Why did this happen?
Use the thermometer to transfer a few drops of water to the back of your hand. Does the temperature of the water seem hotter or cooler than the air in the room? ______________________________
You know the temperature of the water is warmer than the air, so why doesn’t it feel warmer?
How would you expect this result to be different in a room where the humidity is 100%?
Dew Point determination
4) Fill a glass beaker not more than ˝ full of water from the cold tap. Take the temperature of the water______ C. Add small amounts of ice to the water, continually noting the temperature and constantly stirring. (Do not add more ice until all the previous lot is melted.) Carefully observe the surface of the beaker, rubbing it with your finger, and note the temperature at which water first condenses on the surface. ______ C. (Don't wait for drops to form, record the first temperature at which you can see there is "fog" on the can). This temperature is called the "Dew point". Be careful not to breathe on the can as the humidity in your breath is much higher than that in the room and will result in an incorrect value for the humidity.
5) Warm the water up with some tap water; dry off the outside of the can. Repeat the entire procedure, to obtain a second reading for the dew point. ______ C.
Why does the dew form on the cup?
What does the temperature at which the dew forms have to do with the amount of humidity in the air? Will the dew form at a higher temperature when the humidity is higher or lower? Why?
6) The highest of the two readings is ______°C and we will use this as the value of the dew point inside the lab.
7) Use the reading part 6 to figure the dew point depression (the number of degrees below room temperature that dew formed). This is = [room temperature ______°C] - [(dew point) ______°C] = (item 2) (item 6)
Dew point depression = [t-d ______ c].
8) Now go outdoors and repeat the dew point measurement. Take one dry thermometer to measure the temperature outside. Record the temperature outside________°C and the dew-point_______________, and the difference between the two [t-d= ].
Ask the instructor for the dew point and wet/dry bulb humidity charts.
9) From the dew point chart read the humidity. To do this you must use the dew point depression calculated on item 7.
The Dew-Point chart is at the top of your page, look on the left-hand column until you find the number that corresponds to your dew-point depression, then read the relative humidity from the column that is under the number closest to the air temperature. Relative humidity inside = ______ %
10) Repeat the entire process to obtain the relative humidity outside = ___%.
11) Now use the wet and dry-bulb chart (at the bottom of the page containing the charts) to calculate the humidity by that method. [Room temperature ______°C] -[lowest temperature recorded from the thermometer with the paper towel ______°C] =[t'-t ______°C]. Read the t-t' depression from the middle of the chart and follow under that number until you reach the room temperature (from the left-hand column). Relative humidity by this method =______%.
Calculate the percent difference between the two humidity readings taken in the lab room.
(item 9) (item 11)
% dif = [humidity from dew-point]-[humidity from wet/dry bulb] =
[largest of the two]
% dif =
Why does the wet bulb thermometer have the lower temperature?
If the humidity were higher, would you have measured a higher or lower wet bulb temperature?
Considering the weather today, why do you think the humidity indoors and outdoors gave the results that you observed? Explain completely. Failure to give your opinion will result in a lower grade.
Is there anything that is unclear about this experiment?