In order to pump down two Bigfoot dewars simultaneously, we purchased a second vacuum pump and the other necessary components. Since we tend to order dewars by the pair, doing this will save us about three months getting the dewars ready for patients after a new purchase.
Recently, while testing the boil-off rate of liquid nitrogen from Bigfoot-1, we took the opportunity to gather more data on the temperature differential between the bottom and top of the dewar, should nitrogen supplies be discontinued. (The dewars boil off between ½ and ¾ inches per day, on average.) There was a 5 degree difference between the boiling point of liquid nitrogen and the top of the pod. At the same time, there was a temperature difference of 40 degrees between the top of the pod and the bottom of the dewar’s lid.
Engineer Hugh Hixon estimates that when the last drop of liquid nitrogen evaporated, the temperature at the top of the pod would be no warmer than -180 degrees C. These excellent results are largely because of the conductive nature of the pods themselves, which are made of aluminum.
If we were to lose access to liquid nitrogen, it would take more than 90 days to boil off all the nitrogen. The more efficient dewars would go significantly longer.