Air sampling port (typical) Air inlet tube (3) Exhaust ducts Lower cargo  smoke detector Smoke detector fans FWD Smoke detector Zone 1 sampling ports Zone 2 sampling ports Zone 3 sampling ports Water separator (3) Heater (3) Manifold Suction lines 17-16 Figure 17-20.  Smoke detector system.  Figure 17-19.  Smoke detector installation. Lavatory Smoke Detectors Cargo Compartment Extinguishing System •  Master warning lights come on. •  Fire warning aural operates. •  A cargo fire warning message shows. •  Cargo fire warning light comes on. The fire extinguishing bottles contain Halon 1301 or  equivalent fire extinguishing agent pressurized with nitrogen.  Tubing connects the bottles to discharge nozzles in the cargo  compartment ceilings. The extinguishing bottles are outfitted with squibs. The squib  is an electrically operated explosive device. It is adjacent to  a bottle diaphragm that can break. The diaphragm normally  seals the pressurized bottle. When the cargo discharge switch  is activated, the squib fires and the explosion breaks the  diaphragm. Nitrogen pressure inside the bottle pushes the  Halon through the discharge port into the cargo compartment.  When the bottle discharges, a pressure switch is activated  that sends an indication to the flight deck that a bottle has  been discharged. Flow control valves are incorporated if the  bottles can be discharged in multiple compartments. The flow  control valves direct the extinguishing agent to the selected  cargo compartment.  [Figure 17-21] The following indications occur in the cockpit if there is  smoke in a cargo compartment:  the other set of diodes. The detector sends a fault message. The smoke detector has multiple sampling ports. The fans  draw air from the sampling ports through a water separator  and a heater unit to the smoke detector.  [Figure 17-20]  The cargo compartment extinguishing system is activated  by the flight crew if the smoke detectors detect smoke in  the cargo compartment. Some aircraft are outfitted with two  types of fire extinguisher containers. The first system is the  dump system that releases the extinguishing agent directly  when the cargo fire discharge switch is activated. This action  extinguishes the fire.  The second system is the metered system. After a time delay,  the metered bottles discharge slowly and at a controlled rate  through the filter regulator. Halon from the metered bottles  replaces the extinguishing agent leakage. This keeps the  correct concentration of extinguishing agent in the cargo  compartment to keep the fire extinguished for 180 minutes. Airplanes that have a passenger capacity of 20 or more are  equipped with a smoke detector system that monitors the  lavatories for smoke. Smoke indications provide a warning  light in the cockpit or provide a warning light or audible  warning at the lavatory and at flight attendant stations  that would be readily detected by a flight attendant. Each  lavatory must have a built-in fire extinguisher that discharges  The master warning lights and fire warning aural are  prevented from operating during part of the takeoff operation. 