The Gas Laws Are Out of This World!

By Tom Kuntzleman and Josh Kenney

A few years ago, we launched a weather balloon during our summer science camp. The balloon reached an altitude of 30 km (100,000 ft)! Among other things, this project ended up being a great way to teach campers about the gas laws and how atmospheric pressure decreases with altitude. Initially, I had no idea how to carry out such an experiment. However, I knew a former student of mine (Josh Kenney, a high school physics and chemistry teacher) who had successfully launched a weather balloon. So I invited him to come to our campus to conduct this experiment for our campers. In return for his efforts, I gave Josh the GoPro camera that we purchased to use in our weather balloon experiment. Josh operates a science related YouTube channel called The Science Classroom and uploaded a video of our weather balloon launch to his channel. Check out the video below:           

You will notice that we attached an unopened bag of chips, a partially inflated balloon, and some other items (marshmallows!) to the payload of the balloon in view of the camera. These items were attached for students to observe the effects of decreasing atmospheric pressure.

There are certainly easier ways of demonstrating the gas laws, the decrease of atmospheric pressure with altitude, and the effects of decreasing pressure on gases. However, this experiment generated enormous excitement among our campers. We’ve done a lot of really cool experiments at Cougar Science Camp, but without question this is one of the all-time greats.  

While the process of carrying out this experiment is quite involved, it is still very doable. Below you will find some very helpful tips if you decide to try this experiment with your students. Be sure to let us know if you launch a weather balloon! Alternatively, let us know some new and different things to try to attach to the payload of a weather balloon for when we try this experiment again! (We are tentatively planning another launch during June, 2016). 

Materials / Cost

1. 600 g weather balloon $40.00
2. SPOT GPS tracker $79.99 (to follow the balloon during its trip and to locate and recover it upon its return to Earth).
3. Parachute $20.00
4. Helium (large tank rental from Party City) $119.00
5. Styrofoam cooler, PVC pipe, duct tape, nylon rope $36.00
6. GoPro Camera $400.00
7. Small balloons $1.00
8. Hand warmers  $3.00

Assembling the Payload

A styrofoam cooler was used to house the camera. The camera was placed inside the cooler and a small hole, fitted for the camera lens, was cut in the cooler. In view of the camera, we attached a piece of ½ inch PVC pipe to the cooler and taped a partially inflated balloon to the end. We packed hand warmers inside the cooler to keep the camera warm and then securely fastened the lid with duct tape.

Launch Preparation

Prior to launch, we needed to:

1. Estimate the trajectory of the balloon
2. Determine the required amount of helium

Trajectory of the balloon

The weather balloon drifted in atmospheric air currents. This drift was from west to east during the summer in southeast Michigan. This website was useful to estimate the impact point of the balloon. At this site, the estimated balloon trajectory was calculated based on current weather data. We also used this website to decide on a launch day when the balloon would drift the shortest distance from our starting position. Therefore, the trajectory prediction was used to decide if the current weather conditions would make for a successful launch.

Amount of helium

We calculated the lift required using the formula:

Required Lift (g) = Weight of payload x 1.5. See: http://www.highaltitudescience.com/pages/how-to-inflate-a-weather-balloon

We attached a force meter to the weather balloon as we inflated the balloon with helium. As the balloon filled with helium, it pulled upwards on the force meter. When the force meter matched our calculation required lift, we tied the balloon shut.

Results

Our spot GPS device did not provide altitude data. We estimated the maximum altitude of our weather balloon based on the manufacturer’s burst altitude for a 600 g balloon which was claimed to be 29 – 32 km. See: http://www.highaltitudescience.com/products/600-g-near-space-balloon

The combined gas law was used to calculate the pressure at the maximum altitude of our weather balloon (30 km):

combined gas law

Where P is pressure, V is volume, T is temperature, the subscript 1 indicates launch site conditions and the subscript 2 indicates conditions at balloon burst altitude.

The initial volume of the partially inflated balloon was estimated by filling a separate balloon to the same size with a measured volume of water. The final volume of this balloon at burst altitude was estimated by measuring the radius of the balloon and length of the chip bag in captured video images. Using the known length of the chip bag, the radius of the balloon was determined by ratio. The volume of the balloon was then calculated using the equation for the volume of sphere.A standard thermometer was used to measure the initial ground temperature. Temperature at 30 km was approximated using known temperatures at that altitude (approx. -47 ^oC).

Initial volume of the partially inflated balloon : 20 cm³

Final volume of the partially inflated balloon, based on spherical volume and an estimated radius of 7 cm³ from pictures in the video: 1400 cm³

Initial pressure : 755 mmHg

Initial temperature : 26^⁰C (299K)

Final temperature : -47^⁰C (226 K)

Calculated pressure at final altitude: 8 mmHg. In excellent agreement with pressures of 5 – 10 mmHg cited for atmospheric pressure at an altitude of 30 km (see, for example, http://usatoday30.usatoday.com/weather/wstdatmo.htm).