Heat Detectors

• Heat detectors are most appropriate for very dirty, dusty environments or places where a fire is likely to produce very thick smoke. These detectors incorporate two temperature-sensitive elements called thermistors. One thermistor is in contact with the surrounding air while the other is enclosed. When the air temperature rises above a certain threshold, the exposed thermistor's resistance to electric current decreases, allowing electric current to flow through the thermistor and sound the alarm.
  
  

Photoelectric Detectors

• In a photoelectric detector, a light source shines a beam of light on a straight path across a chamber in the center of the detector. When smoke enters the chamber, it scatters some of the light, causing it to strike a light sensor placed off to one side. The light sensor registers this input and sounds the alarm. Photoelectric detectors are most effective for fires that generate visible smoke; smaller particles from hot fires are more difficult to detect in this way.
  
  

Ionization Detectors

• In an ionization detector, a sealed radiation source containing a radioactive isotope like americium-241 ionizes the air in a tiny chamber with electrodes on either side. The ionized air can carry current, so an electric current flows between the two electrodes. When smoke enters this chamber, it interacts with the ions so the current ceases to flow. A microchip registers the drop in current and sounds an alarm. These kinds of systems are very good at detecting small smoke particles -- even ones so small they may be invisible to the naked eye -- but are less effective at sensing dense smoke from cooler fires.
  
  

Flame Detectors and Considerations

• Flame detectors look for light at particular wavelengths in the infrared, visible and ultraviolet regions of the spectrum. Fires often emit light at these particular wavelengths. When the sensor inside the detector registers firelight, it sends an electrical signal to trigger the alarm. These kinds of systems are most appropriate for places with high ceilings, like an auditorium or for places where wind can dissipate the smoke and heat from a fire before it reaches the detector.