Francesco here, examining the flight data…
The camera has three internal temperature sensors that are used by the camera’s software to monitor its functioning and to alter the way the photos are processed after being shot. That seems very advanced but it’s quite standard, even in this kind-of-entry-level cameras.
The modified version of the camera software was collecting this temperature values and logging them to a file. I have graphed the temperatures that have been measured in the camera to see if they give us any interesting information.
The three sensors are mounted on the CCD (the optical sensor) on the optics (the lens mechanism) and on the battery.
The values read from the CCD and optics sensor are always the same, so I am showing only a single value.
All temperatures are in °C, the X values are minutes into the mission time. The graph ends at 161 minutes because that’s when the camera shut down because the battery was too low.
This is the resulting graph:
The first thing to say is that the -8° measured at the end of the mission is definitely not a real value, but just the lowest point of the camera’s sensor, as the end of the curve is flat. Extrapolating from the previous tendence, and ignoring every problem of linearity, accuracy and reliability of the sensor, I think that the CCD temperature at the end was about -14°
The other thing, and this is much more interesting, is the shape of both curves.
For the first minutes the temperature increases, as the camera has just been powered on and is warming.
While we gain altitude, the temperature drops significantly, almost to 0°, but at 80 minutes it starts increasing again, and at the moment of the burst (at 131 minutes) it peaks at a comfortable 22°.
From that point, it rapidly goes down.
The question is: what happens after 80 minutes, and why does the temperature rise?
The first idea is that we have actually met the temperature inversion of the tropopause: while we climb, the air temperature first decreases then (at the tropopause) stops decreasing and increases.
A simplified model of the atmosphere temperature (http://www.grc.nasa.gov/WWW/K-12/airplane/atmosmet.html) gives us the following formulas:
For altitudes <11000: T = 15.04 – .00649 * h
For 11000<altitudes25000: T = -131.21 + .00299 * h
This means that going from 25.000 to 33.000 metres the temperature increases from -56° to -32°. This increase in temperature could be responsible for the camera recovering heat, as its environment is warmer.
The other (and in my opinion, more important factor) is that during the ascent the air pressure decreases enormously. This means that the air is much less dense and is therefore less capable of diffusing the heat produced in the camera.
Air at 33.000 meters is enormously less dense than our air. In effects, to our senses it would feel more like vacuum!
But vacuum gives a very good thermal isolation (think of dewar bottles) and so, while we climb, it’s as if the camera is wrapped in a warm blanket (made of vacuum) and the heat produced by the camera is enough to warm it up.
When the balloon bursts, everything changes: the air is the same, but the box is falling at over 180 km/h and this generates a chilly wind that is again able to cold down the camera. While we descend, the denser air is able to better slow down the parachute so the wind decreases but the same density of the air is more efficient in chilling the camera.
This actually freezes the camera to below zero…
Another interesting graph is a similar one, done with the battery voltage. The camera also has a voltmeter measuring the battery level. This is used to self-monitor the battery level and first warn you (BATTERY LOW) and later shutdown the camera.
I have also logged and graphed these values, and this is the result:
The general structure is rather clear: the battery starst fully loaded (4.1 V) and the slowly loses charge with use, with the voltage suddendly dropping fast at the end of the charge.
There are three very visible “holes” in the graphs (from 0 to 3 min, from 61 to 63 min and from 80 to 83 min). These are the moments where the camera was shooting videos and therefore working harder. A stronger power was being taken from the battery, so a lower voltage was present at the battery leads. When the camera stopped shooting video, the voltage returned to its previous level, except at 63 min, when the camera only partially recovered in voltage.
I don’t know why the 60 and the 80 minutes video are so different, with the first one causing a strong step in the graph and the second recovering to almost exaclty the previous value.
The voltage keeps dropping rather slowly up to 150 minutes, where is starts falling fast, and at 161 minutes the camera shuts down.
I don’t know how much the cold hastened the fall in voltage, but I think it wasn’t too important: comparing the two graphs we see that at 150 minutes the voltage had already started falling quite fast, but the battery temperature was still at a nice, warm 14 degress. At minute 156 the battery is falling even faster, but its temperature was still at 7 degrees, a temperature that it withstanded without any problem before (at 60 minutes).
