The bulk storage unit is normally filled with anywhere from 40 to 100 lb of liquid oxygen. However, the typical setup usually contains around 100 lb of liquid oxygen (equals 33,756 Liters of gaseous oxygen) and provides a low working pressure between 20 and 90 psig.  The storage unit can be used to deliver oxygen directly to the patient or be used to refill a portable unit carried around by the patient.

The portable, refillable carrying container is filled from the storage unit when necessary. When full, the portable container weighs between 6 and 11 lbs and provides approximately 1,025 Liters of gas. The use of a permanent storage unit and a refillable portable unit allows the patient more freedom to move about.

Oxygen becomes a liquid at temperatures below its boiling point of -183°C and takes on a pale blue color weighing 1.14 times the weight of water. When the temperature of liquid oxygen is greater then -118.6°C, the liquid will return back into a gas regardless of the pressure exerted on it. This is known as the critical temperature. 1 Liter of liquid oxygen provides 860 Liters of gas.

The liquid oxygen is kept in insulated containers (called dewars). These keep the oxygen in liquid form at a temperature of -170 degrees Celius. The container consists of a lower portion where the oxygen is in a liquid state and a smaller upper portion where the liquid has evaporated creating a gas. When the unit is being used by the patient, a flow control valve is opened to deliver oxygen to the patient. This creates a pressure gradient between the gas-filled upper portion of the container (called the head pressure) and the atmospheric pressure. Liquid oxygen passes through a warming coil, is converted to a gas, and is made available for patient delivery. When the upper portion falls below a certain pressure, liquid oxygen is drawn up from the bottom of the container to provide a constant flow to the patient. 

Since the cooled liquid oxygen is under pressure, the room temperature will cause some evaporation of the liquid into a gas creating more pressure in the container. This usually occurs when the container is not being used on a regular basis. When the pressure reaches a certain point a primary relief valve will open to vent to the outside. If the primary valve fails, a secondary relief valve will take over when the pressure reaches 10 psig above operating pressure. There is often a small venting of oxygen by the device as part of its normal operation.

Typical oxygen reservoirs contain approximately 40 Liters of  liquid oxygen (depending on the model) that may last 8-10 days at 2-Liter/min. The controls on the portable liquid oxygen container enable the patient to select oxygen flow rates that can be used with a nasal cannula, transtracheal catheter, mask, or other oxygen delivery device. Oxygen flows are usually limited to 6 Liter/min however on most models.

Advantages of LOX

1. Comes in 30 and 40 liter capacity
2. Consumes no electricity
3. Attractive design
4. Oxygen flow rate up to 15 lpm
5. Supplies oxygen continuously for up to 11 days (at 2 lpm)

Disadvantages of LOX

1. Loud noises are made when the smaller units are filled from the larger ones.
2. The connection can become frozen if the filling is not done properly. All connections should be airtight.
3. There is evaporation loss from the cannisters when they are not in use.
4. Tank needs to be refilled regularly by a service technician.