a pressure regulator 23 and a flow controller 24 and then into the oxygen surge tank 25 this tank serves as a reservoir for the oxygen prior to entering a high pressure oxygen compressor 29 in this embodiment an oxygen sensor 27 such as a douglas scientific model 120j with accuracy of u2212 1 percent or another oxygen sensor known to those skilled in the art is located between the oxygen storage tank and the oxygen compressor the model 120j is preferred because it uses acoustic technology which is very reliable it rarely needs calibration and it is not affected by changes in altitude as shown in fig2 the oxygen sensor is connected to the programmable logic control plc unit 51 and the levels measured by the oxygen sensor may be displayed on the touch screen interface e g 57 also in this embodiment a purge valve 28 is located between the oxygen storage tank and oxygen compressor flow controllers 38 39 are associated with both the oxygen sensor and purge valve the oxygen gas is delivered to the high pressure oxygen compressor from which it is compressed into oxygen cylinders e g 40 a 40 b 60 a 60 b 60 c for example the cascade of oxygen cylinders shown in fig3 typically the oxygen gas is compressed into cylinders up to 2 200 psig when this pressure is reached the oxygen filling apparatus of the present invention automatically de energizes in a preferred embodiment the oxygen.