13. Remove the grounding strap. Purging an Oxygen System Filling LOX Systems 16-19 Figure 16-32.  A temperature-compensating pressure refill chart is  used by the technician to ensure proper oxygen cylinder pressure  in the aircraft system. Temperature has a significant effect on the pressure of  gaseous oxygen. Manufacturers typically supply a fill chart  or a placard at the aircraft oxygen refill station to guide  technicians in compensating for temperature/pressure  variations. Technicians should consult the chart and fill  cylinders to the maximum pressure listed for the prevailing  ambient temperature.  [Figure 16-32]  When it is hot, oxygen cylinders are filled to a higher pressure  than 1,800 psi or 1,850 psi, the standard maximum pressure  ratings of most high-pressure aircraft oxygen cylinders. This  is allowable because at altitude the temperature and pressure  of the oxygen can decrease significantly. Filling cylinders  to temperature-compensated pressure values ensures a full  supply of oxygen is available when needed. When filling  cylinders on a cold day, compensation for temperature and  pressure changes dictates that cylinders be filled to less than  the maximum rated capacity to allow for pressure increases  as temperature rises. Strict adherence to the temperature/ pressure compensation chart values is mandatory to ensure  safe storage of aircraft oxygen. Note that some aircraft have temperature compensation  features built into the refill valve. After setting the ambient  temperature on the valve dial, the valve closes when the correct  amount of oxygen pressure has been established in the aircraft  cylinder. A chart can be used to ensure proper servicing. The inside of an oxygen system becomes completely  saturated with oxygen during use. This is desirable to deliver  clean, odor-free oxygen to the users and to prevent corrosion  caused by contamination. An oxygen system needs to be  purged if it has been opened or depleted for more than 2 hours,  or if it is suspected that the system has been contaminated.  Purging is accomplished to evacuate contaminants and to  restore oxygen saturation to the inside of the system The main cause of contamination in an oxygen system  is moisture. In very cold weather, the small amount of  moisture contained in the breathing oxygen can condense.  With repeated charging, a significant amount of moisture  may collect. Additionally, systems that are opened contain  the moisture from the air that has entered. Damp charging  equipment, or poor refill procedures, can also introduce water  into the system. Always follow manufacturer’s instructions  when performing maintenance, refilling, or purging an  oxygen system. Cumulative condensation in an oxygen system cannot  be entirely avoided. Purging is needed periodically. The  procedure for purging may vary somewhat with each aircraft  model. Generally speaking, oxygen is run through a sound  oxygen system for a number of minutes at a given pressure  to perform the purging. This can be as little as 10 minutes at  normal delivery pressure. Other systems may require up to  30 minutes of flow at an elevated pressure. Regardless, the  removal of contaminants and the resaturation of the inside  of the system with oxygen is the basis for purging. It is  acceptable to use nitrogen, or dry air, to blow through lines  and components when performing maintenance. However, a  final purging with pure oxygen is required before the system  is serviceable for use. It is important to ensure storage cylinders are refilled if they  are used during the purging process. Be certain that there are  no open lines and all safety caps are installed before returning  the aircraft to service. The use of LOX in civilian aviation is rare. The most common  and safest way to fill a LOX system is to simply exchange  the storage unit for one that is full. However, filling LOX on  the aircraft is possible.  Temperature (°C) Temperature (°F) Gauge pressure (PSI in hundreds) -40 -30 -20 -10 0 10 20 30 40 50 60 -40 -22 -4 14 32 50 68 86 104 122 140 20 19 18 17 16 15 14 13 12 11 10 9 8 7 70°F 900 at 70°F 1,000 at 70°F 1,100 1,200 at 70°F 1,300 1,400 at 70°F 1,500 1,600 at 70°F 1,700 1,800 at 70°F