service life recommended by the manufacturer is usually in  terms of years. Cartridges are available with a service life  of 5 years or more. To determine the unexpired service life  of a discharge cartridge, it is usually necessary to remove  the electrical leads and discharge line from the plug body,  which can then be removed from the extinguisher container.  Be careful when replacing cartridge and discharge valves.  Most new extinguisher containers are supplied with  their cartridge and discharge valve disassembled. Before  installation on the aircraft, properly assemble the cartridge  in the discharge valve and connect the valve to the container,  usually by means of a swivel nut that tightens against a  packing ring gasket.  [Figure 9-19] If a cartridge is removed from a discharge valve for any  reason, it should not be used in another discharge valve  assembly, since the distance the contact point protrudes may  vary with each unit. Thus, continuity might not exist if a used  plug that had been indented with a long contact point were  installed in a discharge valve with a shorter contact point. Regular maintenance of fire extinguisher systems typically  includes such items as the inspection and servicing of fire  extinguisher bottles (containers), removal and reinstallation  of cartridge and discharge valves, testing of discharge tubing  for leakage, and electrical wiring continuity tests. The  following paragraphs contain details of some of the most  typical maintenance procedures. The preceding material in this chapter is general in nature,  addressing the principles involved and general procedures to  be followed. When actually performing maintenance, always  refer to the applicable maintenance manuals and other related  publications pertaining to a particular aircraft. Fire extinguisher containers are checked periodically to  determine that the pressure is between the prescribed  minimum and maximum limits. Changes of pressure with  ambient temperatures must also fall within prescribed  limits. The graph shown in  Figure 9-18  is typical of the  pressure temperature curve graphs that provide maximum  and minimum gauge readings. If the pressure does not fall  within the graph limits, the extinguisher container is replaced. The service life of fire extinguisher discharge cartridges is  calculated from the manufacturer’s date stamp, which is  usually placed on the face of the cartridge. The cartridge  The following section discusses the fire detection and  extinguishing system of the B777 aircraft. The information  is included only for familiarization purposes. Boeing 777 Aircraft Fire Detection and  Extinguishing System 9-13 Fire Extinguisher System Maintenance  Practices 5.  Failure to obtain an alarm signal when the test switch  is actuated may be caused by a defective test switch or  control unit, the lack of electrical power, inoperative  indicator light, or an opening in the sensing element  or connecting wiring. When the test switch fails to  provide an alarm, the continuity of a two-wire sensing  loop can be determined by opening the loop and  measuring the resistance. In a single-wire, continuous- loop system, the center conductor should be grounded. 4.  Moisture in the detection system seldom causes a  false fire alarm. If, however, moisture does cause an  alarm, the warning persists until the contamination is  removed or boils away, and the resistance of the loop  returns to its normal value. 3.  Kinks and sharp bends in the sensing element can  cause an internal wire to short intermittently to the  outer tubing. The fault can be located by checking the  sensing element with an ohm meter while tapping the  element in the suspected areas to produce the short. 2.  Fire alarms and warning lights can occur when no  engine fire or overheat condition exists. Such false  alarms can be most easily located by disconnecting  the engine sensing loop connections from the control  unit. If the false alarm ceases when the engine sensing  loop is disconnected, the fault is in the disconnected  sensing loop, which should be examined for areas  that have been bent into contact with hot parts of the  engine. If no bent element can be found, the shorted  section can be located by isolating the connecting  elements consecutively around the entire loop. Figure 9-18. Fire extinguisher container pressure-temperature chart. Temperature (°F) Pressure (psig) −40 −30 −20 −10  0  10  20  30  40  50  60  70  80 900 800 700 600 500 400 300 200 100 0 Minimum ga uge reading Maximum gauge reading