F F Voltage regulator Duration discriminator 4.3 Micrometer sensor Reset counter  clock Flicker rate discriminator SIGNAL 2.58 12V +15V +15V GND GND THRESHOLD Functional Schematic Figure 9-3.  Optical fire detection system circuit.  Kidde Continuous-Loop System Sensing Element In the Kidde continuous-loop system, two wires are imbedded  in an inconel tube filled with a thermistor core material.  [Figure 9-5]  Two electrical conductors go through the length  of the core. One conductor has a ground connection to the  tube and the other conductor connects to the fire detection  control unit. As the temperature of the core increases, electrical resistance  to ground decreases. The fire detection control unit monitors  this resistance. If the resistance decreases to the overheat  set point, an overheat indication occurs in the flight deck.  Typically, a 10-second time delay is incorporated for the  overheat indication. If the resistance decreases more to the  fire set point, a fire warning occurs. When the fire or overheat  condition is gone, the resistance of the core material increases  to the reset point and the flight deck indications go away. The rate of change of resistance identifies an electrical short  or a fire. The resistance decreases more quickly with an  electrical short than with a fire. In addition to fire and overheat  detection, the Kidde continuous-loop system can supply  nacelle temperature data to the airplane condition monitoring  function of the Aircraft In-Flight Monitoring System (AIMS). The sensing element consists, essentially, of an infinite  number of unit thermistors electrically in parallel along its  length. The resistance of the sensing element is a function of  the length heated, as well as the temperature-heating of less  Figure 9-4.  Fenwal sensing element. 9-5