It has been proven that supplemental oxygen increases oxygen saturation and reduces dyspnea in hypoxemic COPD Patients. There are different oxygen delivery systems available, some of them operating with liquid oxygen and others by concentrating the ambient air (concentrators). An advantage of the concentrator system is, that there is no refill required, but only a power plug and plug socket, enabling the patient for higher mobility and to be more autonomous. There are only few publications about whether portable oxygen concentrators have a comparable efficiency on oxygen saturation to liquid oxygen devices with continuous flow (LOD) or not. Until now there are no official recommendations about how to convert the oxygen flow rate for LOD (liter per minute) into the corresponding level of the POC.