Here you can see the flight hardware packed inside the polystyrene foam container. The Geiger Counter, DAQ system, and camera were all placed inside and turned on. I placed a couple pieces of Uranium glass inside so the Geiger Counter had something to record during the test.
This is the setup inside of the cooler. The best way to get really low temperatures around the box was to place the block of dry ice above it. I used some more pieces of polystyrene foam to mount the dry ice above the box. You can also see the temperature sensor that I used to derive the temperature inside the cooler.
This is a picture of the box before placing the dry ice on top.
Now the piece of dry ice is placed and ready to go.
I put the cooler lid back on with the temperature sensor wires connected. The equilibrium temperature inside of the cooler and outside of the flight hardware box was found to be -36 degrees Celsius, right around what I expect the temperature at 100,000 feet to be. After two hours, a thermal equilibrium was established with a temperature of -5 degrees Celsius inside the flight module. The heat generated from the electronics and excellent insulation properties of polystyrene foam aided in establishing this equilibrium. At the conclusion of the test, all of the electronics were still functioning perfectly.
Overall I was pleased with the performance of the electronics under the conditions. The next task will be to perform the same experiment with hand-warmers placed inside the flight module. In doing so I hope to establish a factor of safety by running closer to the middle of the operating temperatures of all the electronics. Further, temperatures will actually be lower than -40 degrees Celsius on the journey up to 100,000 feet, so having some active heat source other than the electronics themselves will be critical.