barometer

Here we construct an aneroid (without fluid) barometer from a collection of simple materials that you can find at the supermarket and local hardware store. Your barometer can be used to track changes in air pressure with weather or it can be used as an altimeter. It is also affected by temperature, so it will provide good practice in the experimental technique of "holding independent variables constant".

While we've posted this exercise on the L. M. Nixon Noon Science website, the calibration and analysis is probably at the extreme limits of the skill level of the typical 5th grader, even for our recently-simplified version. But let me know if I'm wrong!

(revision 4-29-06)

Materials

1. Large balloon, 11" or larger. Get a good quality balloon; the cheap ones slowly leak.
2. Rigid plastic or glass cup. If you select plastic, spend a little extra and get very heavy-grade cups.
3. Rubber bands (#30 size seems good, they are about 2" long. They'll need to fit tightly around whatever cup you select.).
4. Hardwood dowel (1/8" diameter x 12" long).
5. Hot glue gun and glue.
6. 8-1/2" x 11" sheet of heavy card stock, such as 90lb Bristol board.
7. Paper ruler, divided into millimeters (pick one from here, http://www.vendian.org/mncharity/dir3/paper_rulers/ )
8. 3" x 1/2" x 16" piece of lumber for a base.
9. Sharp scissors.
10. A piece of #100 grit sandpaper or an emery board for filing a point onto the dowel.

Assembly

Prepare the Aneroid Chamber

1. Blow up the balloon to stretch it out, and then release the air. Cut off the neck of the balloon so that you can reach into the balloon with your fingers and stretch the balloon out and over the top of the cup. Make sure that the inside surface of the ballon ends up on the outside so that humidity trapped in the inside of the balloon will not be trapped in the chamber. You may have to practice this several times to do it without ripping the balloon, yet at the same time obtaining a good seal over the top of the cup.

Note: Some like to stretch out the balloon by simply pulling it in their hands. My personal bias is that this does not stretch the balloon uniformly. But blowing up heavy balloons is sometimes difficult for kids.

2. Wrap several rubber bands around the top of the cup and over the balloon to help insure a tight air seal.

Add The Pointer

The long dowel will be glued to the balloon surface so that it extends from the center of the balloon outwards over the edge of the cup. Use two dabs of glue; one at the center of the balloon and one at the rim of the cup.

Assemble The Stand and Scale

Load your computer printer with some heavy paper (80 lb Bristol board, cut to 8-1/2" x 11") and print this pattern on the paper. Cut it out and fold it up to make the stand. Glue (or double-stick tape) the "tabs" to the "base".

Cut out and attach this ruler scale to the middle of the stand. Put "zero" toward the base of the stand.

Attach All Parts to the Base.

1. Position the cup and scale/stand (without glue) onto the wood base. See the picture below.

2. Use hot glue to attach just the bottom edge of the cup to one end of the base by applying two or three blobs of glue, evenly spaced, around the outside edge of the cup. Position the cup so that the end of the pointer is near the front edge of the base.

3. Use a small sanding board to sharpen the end of the pointer so that its position can be determined precisely. Then attach the scale/stand to the base so that the pointer is nearly touching the scale and is free to move up and down. .

Your First Experiment

1. I'll bet that you've assembled your barometer in your classroom or your work area at home, and that the room temperature is about 70F. When your assembly is complete, wait about 10 minutes for the cup of your barometer to come to room temperature. Then note the position of the pointer on the scale.

2. Wrap your hands around the cup snugly, but gently (don't distort the cup with the force from your hands). Wait several minutes, and record the position of the pointer.

If you are a human and not a lizard, your body temperature is about 98.6F, while the temperature of your indoor, air-conditioned surroundings is about 70F. By wrapping your hands around the cup (the aneroid chamber) you've heated the air in the cup to nearly your body temperature.

Look at the position of your barometer pointer. What do you think happened to the pressure in the cup (chamber)? This picture shows the difference in the pointer position when my barometer is at room temperature and body temperature. Which side shows what happens when my hands are on the cup? Look at the results from your own experiment to find out!

Using the Barometer to Measure Altitude - an Introduction.

Hopefully, you live near some hills that are high enough to show a large change in atmospheric pressure between the bottom and the top (if you're in Kansas, you're out of luck, in more ways than one). A hill of about 1000 feet elevation is certainly enough to show a large motion of the pointer on your barometer. The elevator in a ten story building will show enough change on your barometer to be visible. Next time you go to San Francisco, take the elevator up the Transamerica Tower and watch the change on your barometer! Take a picture of yourself with your barometer at the ground floor and at the top floor. Make sure your photo shows the pointer on your barometer.

In our neighborhood close to L. M. Nixon Elementary near Palo Alto, CA, we are lucky to be close to "mountains" of about 2000 feet in altitude. Our students can use a topographic, or "topo" map, and ask an adult to drive them up Page Mill Road from near sea level at the bottom near Nixon Elementary, up to the top at Skyline Boulevard. By stopping at identifiable landmarks at intervals of 500 feet or less, you can record the position of the pointer at each spot. But as you do this, think about the effect of temperature on the position of the pointer of your barometer. What can you do to keep the temperature of your barometer constant? Your car may have "climate control" that can help. But you might also try to use your hands as a temperature control. Remember, unless you are feeling sick, your body temperature is quite constant!

If you are an L. M. Nixon Elementary student, we've posted all of the relevant topo maps for our drive from Hwy. 280 all the way up to Skyline Blvd here. Here is a "topo" map for Nixon school; can you use it to find the elevation at school? (about 140 feet above mean sea level).

Atmospheric pressure decreases by about 0.25 inches of mercury for each 250 feet in altitude (or 10 millibars per 100 meters). So a climb of two thousand feet in altitude will show a change in pressure of about 2 inches of mercury, which is a greater pressure change than you might usually expect to see due to changes in the weather. But it will allow us to "calibrate" our barometer so we will know how much motion to expect as the weather changes. This is about how far my barometer moves when I drive from Skyline Boulevard down to sea level.

Using the Barometer to Monitor the Weather - More Introduction.

We'll need to know the actual pressure at "Nixon School Level" (nearly sea level). The World Wide Web helps us to do this. In our location, use this website:

http://www.wrh.noaa.gov/mesonet/getobext.php?wfo=mtr&sid=sfo&num=48&raw=0

to find the current atmospheric pressure at San Francisco (SFO) airport. We'll use this to find the atmospheric pressure at Nixon school.

Do This

If you are a Nixon student, you will need some help from a parent! See if you can convince them to read the instructions first and help you finish your project.

1. Before you start your day's adventure of collecting barometer data, you will need to find the atmospheric pressure at sea level. Here is a part of the page for the SFO website that I "grabbed" on the morning of April 21, 2006. The column on the far left is the time of day and the column on the far right is the atmospheric pressure ("Station Pressure") in "inches of mercury". What is this number? This is the height of a column of mercury that can be supported by the pressure of the atmosphere.

You will need to put this information into a chart. Get a copy here, and print it out for your experiment.

Write down the date and time that you start your data collection, and put the atmospheric pressure at SFO into your chart like this:

Now start your journey at Nixon school. You might like to use our already-prepared map that shows a list of identifiable landmarks and the corresponding altitude. Decide what temperature control technique you intend to use with your barometer (either the air-conditioned-car-and-home technique or the hold-it-in-your-hands technique). Record the pointer position of your barometer at Nixon school, and again as you reach each 250 foot increment of altitude. Here is what the information looks like for my barometer; yours will be different because many things about your barometer will be different.

Now we're ready to fill in the rest of the numbers in the "Pressure" column. Atmospheric pressure changes by about 0.25 inches of mercury for each 250 foot change in altitude. So start with the number for the pressure at SFO and subtract 0.25. Your answer goes in the "250 ft." space.

Keep going all the way down the "Pressure" column, subtracting 0.25" for each change of 250 feet.

Your calibration is all finished! If you want, you can even calculate what your pointer position should be at the San Francisco Airport, and what the air pressure would be at Nixon school (about 0.14 inches of mercury less than at SFO). See if you can think about how that might be done with the information you have on your chart.

Now you can use your barometer to monitor the weather! I added the actual air pressure values to the scale of my barometer so that I could monitor the barometric pressure without looking at my chart. When the weather is turning from clear to stormy, the air pressure often falls. See if you can predict the change in the weather just from watching your barometer.

Try to build a science fair project with your barometer!