Coke Can Barometer

Coke Can Barometer (and Charles' Law Demonstrator)

A quick and easy science project using simple materials found around the home.

STEM: Teaches about Atmospheric (Barometric) Pressure and also Charles' Gas Law.

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This Instructable shows how to construct a simple barometer to measure atmospheric pressure (barometric pressure).

The barometer can also be used to demonstrate Charles' Law - That a gas expands when heated (an oversimplification, in case all those science-minded people get upset with me.)

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Empty Coke Can (Or most any drink can; or other type, see last step)

Latex Balloon (a 9-inch was used)

Rubber band, several twist ties or a zip tie (All optional; used if balloon will not stay on the can)

Drinking straw

Toothpick

Tape (like 'Scotch' tape)

Empty Cereal Box, Construction Paper or any Cardboard could also work.

3 x 5 Cards

Ink pen

Optional: Ice cubes in plastic bag

[Note that throughout where hot glue is used it may be possible to use tape.]

Tools:

Can Opener

Glue Gun and glue sticks

Scissors

We need an approximately 7 inch by 10 inch piece of cardboard for the back of the barometer.

Start preparing the cereal box by separating the box at its seam and opening it up flat. Measure out the desired size and cut out the cardboard.

Most any cardboard can be used. I just liked the thought of using a cereal box. [And I get to eat the cereal.]

A conventional manual can opener can be used to remove the top of the Coke can. It can be a bit tricky, but gently pressing down on the can while cutting and keeping the correct angle (experimentally determined) can result in the perfect removal of the top.

Cut off the neck of the balloon (diaphragm) and pull it over the can opening. I centered the balloon over the opening. Originally I shifted the balloon to the side to avoid the center 'nipple' from making the surface uneven. But pulling the balloon tight smooths out the 'nipple' and prevents bunching up of excess balloon on the side of the can opening (as seen in some photos).

Make sure there are no creases at or over the edge of the can lip. They will allow leaking of air and prevent the barometer from working or severely limiting its performance.

Check-fit the can and cardboard by holding the Coke can next to the vertical cardboard. This is the orientation that will be used when the Coke can is hot glued in place. If you try to glue the Coke can on the cardboard while laying down horizontal it is very easy to get the cardboard and the can uneven and the barometer will not stand up correctly.

Apply hot glue and attach the can to the cardboard while vertical.

If your straw has a flexible part, cut it off.

Apply hot glue into the end of the straw. Insert the toothpick so only about a 1/2 inch protrudes.

Install the straw (non toothpick end) onto the balloon (diaphragm) center by applying a very minimal amount of hot glue onto the straw and placing it in the center of the balloon. Make sure the pointer tip (toothpick) is as close to the cardboard as possible without touching.

I would suggest trying to position the straw a couple of times before actually hot gluing it in place. It can be difficult to get the straw end in the center of the balloon and at the same time get the toothpick close but not touching the cardboard.

Allow the barometer to come to room temperature. You may want to 'burp' the barometer after temperature equalization by lifting the edge of the balloon to allow the pressure in the can to be equal to the air around the can (ambient air pressure). Let the barometer sit some additional time to avoid the influence of external heat (from your hand).

Tape a 3 by 5 card to the cardboard. Mark the location of the pointer on the card. This the starting point (historical reference) in time of the present barometric (air) pressure.

Note the pointer location days later to see if the barometric (air) pressure has been raising or falling. My location has recently had very steady high pressure so my pointer has not moved very much, if at all. Very active weather patterns will provide more of an opportunity to note changes in the barometric pressure. High pressure is associated with good weather and low pressure is associated with stormy weather.

You can use a barometer application or look up the local weather (barometric pressure) on the internet and note it on the 3 x 5 card next to the pointer location. For more information on Atmospheric air pressure see Step #8.

External heat will cause erroneous readings so try to keep the barometer at the same ambient temperature to get more accurate results.

This sensitivity to heat can be used to demonstrate Charles' Law.

The barometer can easily demonstrate Charles' Law, one of the ideal gas laws.

With the barometer at a stable ambient temperature mark the pointer location on a new 3 x 5 card taped to the cardboard back. Label this reading as "Room Temperature".

Now grip the Coke can with your hand to transfer heat to the air inside it. Be careful not to crush the can. The air inside the can will expand and the balloon will bulge upward directing the pointer downward. Make a mark labeled "Warm" position.

Now (optionally) apply ice in a plastic bag to the Coke can. The air inside the can will cool and contract. The balloon will bulge inward causing the pointer to raise. Make a mark labeled "Cold" position.

Heating a gas causes it to expand and cooling it causes a gas to contract. For more information on Charles' Law see step #9.

Well, the atmosphere really does weight heavily on my mind; well, head. Though I don't pay any attention to it.

The air pressure around us is due to the weight of the air above us. It is approximately 14.696 pounds per square inch. That is a lot of weight, but we don't notice it much because it is pressing on us from all sides. It is a similar concept to jumping into a pool or lake -- when we are under water we don't notice the weight of the water above us much because the water is also pressing on us from all sides. The same holds true for the air around us.

For various reasons the air pressure around us can vary:

High- and low-pressure systems evolve due to interactions of temperature differentials in the atmosphere, temperature differences between the atmosphere and water within oceans and lakes, the influence of upper-level disturbances, as well as the amount of solar heating or radiation...cooling an area receives. Pressure systems cause weather to be experienced locally.

Low-pressure systems are associated with clouds and precipitation that minimize temperature changes throughout the day, whereas high-pressure systems normally associate with dry weather and mostly clear skies with larger diurnal temperature changes due to greater radiation at night and greater sunshine during the day.

Barometric pressure (air pressure) is measured in many units: 14.696psi = 101,325 Pa (1,013.25 hPa; 1,013.25 mbar), which is equivalent to 760 mm Hg, 29.9212 inches Hg. The "Inches of Mercury (inHg)" unit is based upon an inverted tube of mercury (liquid) metal. The space at the top of the tube is a near vacuum with the space filled with mercury vapor. The amount of vacuum that the weight of the mercury can 'pull' (down) varies based on the pressure the atmosphere pushes up on the column of mercury. Lower ambient air pressure pushes up on the column of mercury less and hence the column of mercury that is supported is shorter, equalling less measured inches of mercury. The reverse is true. Higher atmospheric pressure pushes up on the column of mercury and increases the height of the column of mercury resulting in a greater column length of mercury. A measurement in inches of mercury - the length of the column of mercury. Normal pressure at sea level is 29.92 inches of mercury (Hg).

The Coke can Barometer can show that the air pressure varies. The needle will move up or down based on changes in the atmospheric (or barometric) pressure. A caution must be given here... A change in temperature of the Coke Can Barometer will make the reading be in error.... For a reason that is explained in the next step.

[Weather Map from US Gov., NOAA]

Gas Laws: Laws that relate the pressure, volume, and temperature of a gas.

Charles' Law, describing how gases expand when heated, was formulated by Joseph Louis Gay-Lussac in 1802, but he credited it to unpublished work by Jacques Charles. Charles' law is one of four Gas Laws:

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1. Boyle's Law, named for Robert Boyle—states that, at constant temperature, the pressure P of a gas varies inversely with its volume V, or PV = k, where k is a constant.

2. Charles' Law, states that, at constant pressure, the volume V of a gas is directly proportional to its absolute (Kelvin) temperature T, or V/T = k.

3. Gay-Lussac's Law, (more correctly referred to as Amontons's law) states that the pressure of a given mass of gas varies directly with the absolute temperature of the gas, when the volume is kept constant.

4. Avogadro's Law, under the same conditions of temperature and pressure, equal volumes of different gases contain an equal number of molecules.

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Our Coke Can Barometer demonstrates Charles' Law since it becomes obvious that the volume of the gas in the Coke can increased (expands) when it is heated by the heat of a hand touching the can. The balloon bulges allowing the gas to stay close to its original pressure; a requirement of Charles' Law. The warmer the gas, the greater its volume (When the pressure is allowed to remain the same by being allowed to expand. In other words it is not compressed by being kept in a fixed volume.) The reverse is also true. A gas contracts (decreases in volume) when cooled (and kept at the same pressure).

[Charles' Law illustration from Florida State University]

After finishing the Coke Can Barometer I realized that installing the balloon and getting it to stay on the Coke can might be a bit challenging. I found out that using an empty 'tin' can worked well.

[And I enjoyed eating the green beans that came in it!]

So you might want to consider using a 'tin' can instead of a Coke can. Try installing the balloon on each and compare.

The Coke can may have the advantage that the aluminum transmits heat faster than steel (for demonstrating Charles' Law), but the 'tin' can is easier to install (and retain) the balloon on. Heat conduction differences are probably negligible. Ease of balloon installation may win out. You get to decide :-).

Enjoy the Coke Can Barometer and watching the atmosphere's ups and downs!