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Chris I just turn off the Peltier power supply and leave the fans running with the camera still on. The internal heat generated by the camera makes sure it is not the coldest thing inside the fridge. I have a temperature monitor for fridge temperature and wait until this slowly rises to close to ambient. The fan inside circulates the air any condensation tends to stay on the Peltier internal heat exchangers. Obviously from -12C it starts off as ice. This seems to work like those frost free fridges where your iceblocks slowly sublimate away.
The fridge can then be removed or I tip the mount orientation so any moisture inside the fridge can run out through a small hole in the corner of the 'door'.
Leaving the camera on is important as it then is self heating. In fact if you turn the camera of while the fridge is at -10C say the temperature drops quickly by about two degrees. When the camera is taking a series of long exposures the fridge temperature rises by about 2C from idle ie camera on. This corresponds quite well with 80W that the Peltiers are pumping. I get a 25C drop for 80w thats about 3.2watt for every degree assuming the system is linear and it is not.
It is a lot of extra mucking about but the results are worth it. The lower the noise the fainter the objects that can be recorded. This is not for the faint hearted as you are putting a camera worth thousands inside a fridge. In winter I don't think I will go below about -10C even though the system can get to -20C or lower when ambient gets to 5C or lower.
The battery in the 5DH is a 1300mA Hr at 7.2V. With long exposures the battery only lasts one or two hours. This means the camera is using or generating four to eight watts of heat. I normally use a mains DC supply for the camera.
I always make sure the camera is on before firing up the fridge. So far no problems at all.
The take home message is as long as the camera is on, the Peltier heat exchanger is colder than the camera. This means condensation will only form preferentially on the Peltier heat exchanger.
Bert
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