What the guys at Air gas told me was to fill the dewar all the way. If the outside of the dewar frosts up or condensation forms on the outside of the deward chances are the vacum is no good. If no rost or condensation is present. When I was researching my dewar the manufacturer had listed a normal evaporation rate. If i remember correctly the way to test mine is to fill it weigh it leave the vent open and weigh it 24hrs later. The has listed the normal evaporation rate in the manual. My dewar is what is consider a low pressure dewar. It is designed to maintain 22psi so once filled it only takes a few minutes to build enough pressure to pour. The one I had borrowed was a storage dewar it is not designed to build pressure. It took 1.5 days for enough pressure to build to be able to usethe withdrawal device on the dewar. If I wanted to use it right away I would have to pour from the dewar to the thermos.
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