is installed in the converter system to relieve excessive pressure. Supply Sequence The supply sequence of the liquid oxygen system consists of two modes of operation—the economy mode, in which gaseous oxygen is fed from the converter, and the demand mode, in which oxygen flows from the converter as a liquid and vaporizes to a gas in the feed line. In the economy mode of operation (fig. 4-15, supply sequence, view C), limited demand upon the system allows the converter to supply gaseous oxygen directly as a result of drawing off the gaseous oxygen stored within the top of the converter. At approximately 82 psi, the pressure opening valve unseats and allows gaseous oxygen to flow from the converter to the supply system. Oxygen then flows from the upper (gas) portion of the converter, rather than the liquid side. When the amount of oxygen demanded by the crew exceeds the supply capabilities of the economy mode, the pressure opening valve closes. As the crew continues to draw upon the oxygen supply, the supply system pressure becomes lower than that of the converter. When a pressure differential of 5 psi occurs, the differential check valve opens (fig. 4-15, supply sequence, views C and D) and allows liquid oxygen to flow into the supply line, thus creating the demand mode. Converter pressure will build up while the system is operating in the demand mode. As the pressure again approaches 82 psi, the pressure opening valve will again unseat, switching the supply sequence back into the economy mode. The converter automatically switches itself back and forth between the economy and demand modes while supplying oxygen to the crew. SYSTEM MAINTENANCE Extreme care must be exercised when installing units in an oxygen system. The life of the pilot and crew depends on the thoroughness with which the AME does this job. All maintenance of LOX systems must be done in accordance with the instructions contained in the applicable MIM. The AME assigned to do LOX system maintenance should also be familiar with the various instructions pertaining to handling LOX and maintenance of the related equipment. The actual removal and installation procedures used in maintaining LOX systems will vary from 4-24 one aircraft to another; however, the following precautions will apply to almost any aircraft system. 1. Use only tubing assemblies that have been tested, cleaned, capped, and properly identified as oxygen lines. CAUTION If lines are fabricated locally, ensure that only clean, oil-free tubing and fittings are used. Also ensure that no hydraulic fluid is used in the fabrication procedure. 2. Use only the type of fittings specified for the particular oxygen system. Never use fittings with pitted or otherwise disfigured cones or imperfect threads. 3. It is MANDATORY that EXTREME CAUTION be exercised with regard to cleanliness of hands, clothing, and tools. It must be emphasized that all items that come into contact with the oxygen system must be free of dirt, oil, or grease. 4. Use thread antiseize tape that is approved under specification MIL-T-27730A. 5. When installing tubing assemblies between fixed units, the tube assembly should align without the use of undue force. 6. The torque values specified for the particular oxygen system should be strictly adhered to when tightening the fittings. 7. If a section of line is left open or disconnected during an installation, the open fittings must be covered with suitable caps or plugs. When making connections, be certain that no lint, dust, chips, or other foreign material is allowed to enter the oxygen system. 8. Upon completion of the installation of a tube assembly or component, a pressure check of the system should be conducted. The system should be pressurized and the connections checked with a leak-test solution conforming to specifica- tion MIL-L-25567B. After the connections have been checked, the leak-test solution should be washed off with clean water. 9. The aircraft liquid oxygen system should be purged after the replacement of any compo- nent or tubing assembly. 10. The type of clothing and footwear that is worn when maintaining and servicing a liquid oxygen system is an extremely important factor. Do not wear anything that will produce sparks or static electricity, such as nylon clothing or shoes