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The pressure of the cryogen within a Dewar determines the operating temperature since the cryogen is typically in a saturated state. Thus, the operating temperature of a Dewar is directly related to the ambient pressure external to the Dewar and the flow losses associated with venting cryogen. Given the low vapor pressures of some cryogens, such as solid hydrogen, the vent flow from Dewars used in space can enter the transitional and molecular flow regimes.