Dr Jacoby, This is stupid question but have you tried to fit the carboy in the fridge with that cap on? If it is an under counter fridge like mine you will have a problem. My Carboys wouldn't fit in mine so I have to use Plastic buckets. I have done 3 fermentations in my under counter fridge and had no problems with allowing the carbon dioxide to escape . It seems to always find away out.
I have my probe in a rubber bung in the lid of the plastic buckets. I have had no issues. The gasses do fill the fridge though. When I have fermented lagers, the stench of Sulpher dioxide was overwhelming when you opened the fridge. <!-- s:!: --><img src="{SMILIES_PATH}/icon_exclaim.gif" alt="

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