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I've just done a follow-up run, connecting my TEC to 12 volts to see what happens - still running as a passive device, with no fan, and testing conducted indoors with ambient temperature of about 24 Celsius.
As expected, the ammeter shows a lot more current (about 3 amps at 12 volts, compared to 1.5 amps or so at 6 volts). Also as expected, the camera sensor reports that the temperature dropped faster than when I was powering the TEC at 6 volts, and it got about 2 degrees Celsius lower than I could get at 6 volts. This time, the big top radiator got quite hot to touch (maybe 70 degrees Celsius or so), there being a lot more heat to shift than at 6 volts.
But then, the camera sensor temperature started to climb almost as quickly as it was initially dropping, so I unhooked the battery, let everything stabilize again, and tried again - with the same result: initial rapid cooling, a short period of "thermal equilibrium" a couple of degrees cooler than I get at 6 volts, followed by temperature rise at the sensor.
Here's what I suspect is happening:
As long as the top radiator stays cool enough to re-condense the fluid in the Heat Tubes, it can pump heat away from the hot side of the TEC, and everything works nicely. However, if the top radiator gets too hot to allow the fluid to condense effectively, the Heat Tubes lose their ability to pump the heat efficiently, and the whole CPU Cooler assembly gets quite hot, and the TEC starts to heat up, including the cold side.
If I then shift back to a 6 volt supply to the TEC, it stabilizes for a few minutes while the top radiator cools down again, and then the sensor temperature drops again.
It looks like my rig can pump the heat from the TEC running at 6 volts effectively as a passive device (no fan) with ambient temperatures of up to 24 Celsius or so (maybe even a bit more?). However, the heat load from the TEC running at 12 volts exceeds the radiator's capabilities in still air (no fan) at around 24 degree Celsius; I suspect it would work fine with the supplied fan fitted, and it MIGHT even work as a passive (fan-less) device for ambient temperatures of somewhat less than 24 Celsius. (Your critical temperature could be different, depending on the passive capacity of your cooler, the heat load of your TEC, the thermal efficiency of your insulation and interfaces, and so on.)
Since I'm getting a reasonable Delta-T at the camera sensor with the TEC running at 6 volts without a fan (with a corresponding reduction in image noise), I'll probably run my cooler at 6 volts with no fan, at least for my first few imaging sessions, before I experiment with other voltages, with and without a fan.
Anyway, enough with the desk-top tests already - it's time to take this baby outside and get some REAL pictures!
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