One of the conditions inherent to the freeze drying process is that of a proper vacuum. An evacuated chamber is essential to the vapor-pressure relationship necessary for the sublimation of ice directly into vapor. For sublimation to occur, the vacuum level must be at least below the triple point of water which is well documented to be 0 degrees celsius and 4.58 Torr. Once below this point is it possible to change the state from ice directly from vapor without passing through a liquid phase.
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Tech Brief: Considerations When Specifying Ultimate Freeze Drying Vacuum