On July 2nd, students from the City Colleges of Chicago launched a high altitude balloon from Pontiac High School. Some of you may have tracked our flight on APRS.fi.
We actually did two launches that day. One was a typical launch with our data trackers, and the other had a manufactured leak in the balloon. We were playing around with the design of the balloon itself, and we were interested in having the balloon go up with a slow leak. At some point it would lose its lift, and drift slowly back to earth without popping. Long story short, our leak wasn't large enough, and the intentional failure didn't happen. You can see our flight path here.
It wasn't a good day for our cameras. We flew several Raspberry Pi cameras, including an NDVI made by Public Labs. All of them failed to capture images for one reason or another. My students, being budding engineers, screamed some obscenities and then started dutifully looking for errors. We should be in much better shape on July 10th when we launch again.
One student had some success and he got some information from the BMP180 sensor using an Arduino. The plot below shows the temperature and pressure as a function of altitude.
The temperature plot looks pretty good and shows the temperature inversion that we expect when traversing the troposphere. Things get cold, and then warm again as we become awash in UV light. The peak in our temperature plot corresponds to the trough of the pressure plot. That is when the balloon popped and it started parachuting back to earth. The pressure decreased with altitude and the graph we got is exactly what we'd expect. The sensor doesn't measure altitude directly. It calculates it based on a change in pressure. The main catch is that prior to flying, we need to give the Arduino the current sea level pressure otherwise our altitude readings will be off.
My student (Giovanni Martinez) is interested in the speed of sound in the upper atmosphere. As temperature decreases, particles have less energy and bounce off each other less frequently. This leads to a decrease in the speed of sound. Similarly as air becomes less dense the speed of sound should also decrease. As particles are further apart from one another they have fewer opportunities to interact. Famously, in space...no one can hear you scream.
We are working through this data. It was taken with an Ultrasonic Sensor (HC-SR04). At room temperature on the ground, the speed of sound is about 343 m/s. We have some obvious errors in the measurement where there are erratic changes in the speed of sound. But as the temperature decreases, we do see a decrease in the speed of sound. We have a few things to look at here, and we would like to see what kind of effect temperature and pressure had independently of one another. We plan to fly this experiment again. The box it flew in was open to the air, and there were bets being taken as to whether or not it would come down in one piece. Giovanni won $5.
All in all it was a good balloon launch. Everyone had fun. Some people got usable data, and some people learned to double check their devices before sealing them up.
Stay tuned for more!