TM 1-1500-204-23-3 3 When cell is completely drained, close drain valve and fill cell with lubricating oil, MIL-L-6081, grade 1010. Allow oil to remain in fuel cell for at least eight to ten hours. 4 Drain lubricating oil from fuel cell and save to flush other cells. 5 After two or three hours, test fuel cells with a combustible gas indicator for presence of fuel vapors.  If an unsafe condition exists, discard drained lubricating oil.  Reflush with fresh oil until a safe reading is obtained. 6 Attach a tag to the fuel filler cap with the following information printed on it: This fuel cell has been preserved with lubricating oil, MIL-L-6081, grade 1010.  No flushing is required during depreservation. (b) Alternate method.  If the proper equipment is not available or the lubricating oil supply, MIL-L-6081, is limited, use the following procedures to preserve the fuel system: 1 Drain and flush fuel cell in accordance with paragraph 4-5f(4)(a) 1 and 2. CAUTION Use  only  dehydrated  air.    Residual  moisture  may  cause  contamination  of  the  fuel  when  the cell is refilled. 2 With drains and vents open, and filler cap off, introduce into filler neck a reduced pressure air hose supplying air through a one-quarter inch orifice at approximately fifty psi.  Purge fuel tank for approximately one-half hour.  Close all drains. 3 Purge fuel cells with CO₂ or nitrogen gas. WARNING · When using a fire extinguisher bottle as a source of CO 2 for purging fuel tanks, regardless of the size of the bottle used, the fiber horn shall be removed, not only because it is too large for insertion into the tank filler neck, but to avoid generating static electrical charges which  can  build  up  by  gas  moving  rapidly  through  the  horn.    The  nozzle,  as  well  as  the bottle itself, must be grounded to the aircraft. · The CO 2 must be discharged into fuel tanks slowly, at a rate of one pound per minute.  C0₂ must be released slowly, because the rapid passage of a gas through a hose can generate static electricity. WARNING A very rapid rate of discharge allows rapid expansion of the CO₂ gas when it flows into a fuel cell.  The expanding gas can lower the temperature to the point that it will cause damage to the cell. NOTE · The  size  of  the  C0 2 bottle  used  can  be  varied  to  meet  existing  conditions.    The  fifteen pound size is handy. · The total amount recommended is based on the quantity usually needed to purge a tank or tanks of the size under discussion.  However, more may be needed to obtain a safe reading on the combustible gas indicator. · It is permissible to use nitrogen or other inert gas in place of the C0 2 gas called out in any of   the   purging   procedures.      The   same   precautionary   measures   stated   above   shall   be observed. 4 Introduce into fuel cell filler neck CO₂ or nitrogen from a tank set to discharge at a rate of not more than one pound of purging gas per minute. 5 Use not less than three pounds of C0₂ or nitrogen to purge fuel cells. 2-20