The oxygen flow metre is attached to either a bottle of oxygen or a medical wall supply of oxygen. This oxygen is pure, as it is 100% oxygen. So, anything that comes out of that oxygen flow metre has a FiO2 rate of 100%.

An oxygen flow metre is used with medical oxygen cylinders. This is used for regulating the supply of oxygen to the patient from the cylinder. The gauge ranges from 0-250 K per cm² and the flow-meter range is 0-10 litres per minute.

It chiefly consists of gas pressure gauge, pressure reducer, safety valve, flow tube, flow control valve, and humidification bottle, and other parts. Oxygen flow meter is primarily used to regulate the flow of oxygen, oxygen humidification.

The working principle of a flow-meter is that the oxygen in the bottle (i.e. Oxygen Source) is reduced by the pressure reducer. The pressure action adjusts from the original high-pressure oxygen to low-pressure oxygen, and after the flow is regulated by the oxygen flow meters, it can be output by the output nozzle at a continual flow value.

The normal flow rate of oxygen is usually 6 to 10 litres per minute and provides a concentration of oxygen between 40-60%. This is why they are frequently referred to as medium concentration masks, as 40%-60% is believed to be a medium concentration of oxygen.