TM   750-116 q.  Periscope,  M901  (fig  3-35). NOTE Desiccant must be replaced every time unit is purged. Do not check pressure at any time oth- er than during the pressurizing cycle. Loss of pressure resulting from such attempts will shorten the time between maintenance actions. Refer to appendix A for listing of applicable publications. (1)   Upper   Section. Pressurizing in excess of 10 psig (68.95 kPa) can cause damage to seals. (a) Remove  desiccator  by  unscrewing  container  part (hex  head)  from  tank  periscope  body. (b) Hold  container  part  upright  and  remove  slotted cap of desiccator by turning counterclockwise using screw- driver and discard old desiccant. To prevent damage to tank periscope, purge charging lines by blowing pressurized nitro- gen through lines. This will remove any con- tamination in lines. (c) Close  pressure  regulator  valve  (CCW).  Open nitrogen tank valve and observe nitrogen tank pressure gauge, indicating tank is pressurized. (d) Remove  dust  cap  on  filler  valve.  Install  hose assembly and loosen self-sealing screws in both upper and lower  sections. (e) Open pressure regulator valve (CW) to purge pressure of 4-5 psig (27.5 to 34.4 kPa) on low pressure gage. Check that nitrogen is being discharged from self- sealing  screw  holes. (f) Alternately  open  and  close  self-sealing  screw holes so one hole is open and another is closed every 15 minutes. (g) Purge until humidity indicator is blue (approxi- mately 30 minutes). (h) Close  pressure  regulator  valve  (CCW). (i) Remove  desiccator  from  tank  periscope  body. Remove slotted cap and install new desiccant. (j) Lubricate  preformed  packing  on  desiccant  cap lightly with grease or silicone compound and install cap and desiccator. Torque cap to 20 to 30 in lb. (2 to 3 Nm). (k) Slowly open pressure regulator valve (CW) and pressurize upper section for 10 seconds to value given be- low for ambient air temperatures. (l) Close nitrogen tank supply valve (CW) and re- move hose assembly. Install dust cap on filler valve. Ambient  Air  Temperature  List Air Temp. Pressurize to: Above  +110°F  (43°C) 4.0 psig +85°F to +110°F (25°C to 43°C) 3.0 psig +55°F to +85°F (13°C to 29°C) 2.0 psig +30°F to +55°F (-1°C to 13°C) 1.0 psig +5°F  to  +30°F  (-15°C  to  -l°C) 0.5 psig Below  +5°F  (-15°C) 0  psig Figure  3-35  M901/M981  Tank  Periscope (2)   Lower   Section. Pressurizing  assemblies  in  excess  of  10  psig (68.95 kPa) can cause damage to seals. (a) Remove desiccant cap and old desiccant from desiccant container and install desiccant cap. (b) Close  pressure  regulator  valve  (CCW).  Open main valve on nitrogen tank and observe nitrogen tank pres- sure  gage  indicating  tank  is  pressurized. (c) Remove dust cap on filler valve and install valve extension  and  hose. (d) Remove  l/4-inch  upper  and  lower  self-sealing screws. (e) Open  pressure  regulator  valve  (CW)  to  purge pressure of 4-5 psig (27.5 to 34.4 kPa) on low pressure gage.. Check that nitrogen is being discharged from self- sealing  screw  holes. 3-16