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Technology

Sensors
 
Multi Sensor

The Multi-Sensor combines the best features of an optical alarm – quick response to slow

smouldering fires and immunity to kitchen fumes, with the best features of a heat alarm –

quick response to a fast flaming fire and immunity from contamination. This makes it the

ideal choice for all rooms in a dwelling except a kitchen. The Multi- Sensor is less prone to

contamination and effectively eliminates the problems of nuisance alarm associated with the

ionisation and the optical type alarms.

 Multi-Sensor_Diagram_MENDE_copy

 

  1. A custom algorithm written into the integrated circuit monitors and interprets the signal from...
    • a). An ultra fast temperature sensor and ...
    • b). A high performance optical sensor .
  2. This combination allows detection of both fast flaming and slow smouldering fires.
  3. This means the alarm will respond quickly to a wider range of fire types than any of the other individual sensor types.

 

Optical

Optical sensors are more responsive to smouldering fires producing large particle smoke

typical of fires involving furniture and bedding.They are more immune to invisible smoke

produced by 'burning the toast' and similar cooking fumes. This makes them ideal for siting in

hallways close to kitchens where false alarms from ionisation alarms may be a particular problem. The

BS 5839: Pt.6: 2004 Standard recommends the use of optical alarms in circulation spaces of a

dwelling, such as hallways and landings. Optical alarms are prone to false alarm if exposed to

steam and should not be located too close to poorly ventilated bathrooms or shower rooms.

 

 how_Optical_works_copy

 

  1. A light beam is pulsed in the sensor chamber every 10 seconds to 'look' for smoke. Any smoke present has to be visible to the naked eye so that the receptor can 'see' it. If no smoke is detected, the alarm will remain in a standby state.
  2. When large particle smoke is detected, the light beam will be scattered onto the light receptor.
  3. This will then send an electrical signal to the IC (integrated circuit)
  4. This causes the alarm to sound
      
Heat

Heat alarms are less likely to cause false alarm problems as they are not responsive to any type

of smoke or fumes, only heat. Because of the potential for a slower response than smoke

alarms, they should only be used in a fire alarm system that also includes smoke alarms, and all of

the alarms must be interconnected. BS 5839: Pt.6: 2004 recommends that heat alarms should

be used in kitchens. It goes on to suggest that they may also have a role to play in the main

living room but they should not be installed in circulation spaces or areas where fast response to

fire is required.

 

 how_heat_works_copy

 

  1. A thermistor (a heat sensitive resistor) is sited in the sensor chamber of the alarm.
  2. When the temperature rises the resistance of the thermistor reduces.
  3. The IC continuously monitors the resistance of the thermistor. When this indicates the temperature is 58ºC or over, the IC sends a signal to the sounder circuit.
  4. The alarm then sounds.