Top fill liquid oxygen unit: A liquid oxygen unit comes with a stroller/carry set/portable oxygen device. The stroller/carry set/portable device can be filled depending the type of vessel provided on the side or on the top of the base vessel. The side fill models are easier and safer as the connection valves don't get the possibility to freeze. Depending the country one would require the medical liquid oxygen one could be provided with either one of these systems. The top fill is generally the oldest but most familiar system to oxygen users. A problem that could occur if the stroller/carry set/portable oxygen device is filled is that the valves freeze and either one can not take the stroller of the base unit or the valves of one or the two units does not close completely so the equipment will empty quickly as it leaks oxygen. To close the valves or separate the machines one can use hand luke water to defrost the ice.