Apparatus and methods for improving the safety and efficiency and decreasing the cost of producing liquid oxygen with a small-scale use liquefaction device, according to various embodiments of the present invention. In one embodiment, a switch is electrically coupled to a storage dewar pressurizing means, the switch positioned to be activated by a portable dewar upon engagement of portable dewar with storage dewar. Cryocooler and/or cooling fan enter low power mode when dewar liquid level reaches a predetermined level, and to return to a fall power mode when dewar liquid level drops to another predetermined level. A cold finger of the cryocooler extends within the dewar and may prevent overfilling of the dewar. The cold finger has a temperature gradient. As the gas liquefies and fills the dewar, the liquid level rises only to a level on the cold finger at which the temperature exceeds the boiling point of oxygen.