MIRROR: G K Bhat
The ionisation type smoke detectors, which we discussed in our previous topic , though very sensitive and accurate, there are few draw backs in it. Basically as we discussed, there is a small amount of radioactive material called americium-241. If inhaled it is harmful. Though lot of precautions has been taken still the possible danger is there. Further , disposal of such material is also very difficult and cannot be disposed off easily like any other items. Due to these reasons the country has banned this type of detector and it is not being used now in India. As an alternative, at majority of the places the optical type fire detectors are being used now.
How Optical Smoke Alarms Works:An optical smoke alarm (also called photo-electric smoke alarm) works using the light scatter principle. The alarm contains a pulsed Infra-red LED which emits a beam of light into the sensor chamber every 10 seconds (it is variable in different detectors according to the manufacturers specifications) to check for smoke particles. This is also called air sampling.
Action when Smoke Enters the Optical Chamber: When a fire breaks out or smouldering takes place smoke will enter the optical chamber through the opening vents. While buying this type of detectors ensure that the vents of the smoke detectors through which the smoke has to enter the chamber of the detector , must be protected with proper material in such a way that it should not prevent the entry of the smoke but should prevent the entry of insects etc. Also ensure that the protectors of the vents are cleaned periodically so that they are not blocked.
When smoke enters into the chamber of the detector, the Infra-Red rays of the Light is getting disturbed. The disturbed infra-red rays when falls on its photodiode receptor, the receptor senses the disturbance.
Technique of the Alarmactivation: Once the scattered lightrays hits the photodiode light receptor the signal which is constantly being sent to the integrated circuit gets disturbed and the variation in the signal is sensed by the Integrated circuit of the sensor. The variation causes triggering of thealarm/hooter connected to the sensor.
While installing these type of detectors, ensure that the detectors are placed properly at right place. The detectors are screwed tightly to the ceiling and straight.
Which is best, ionization or photoelectric? The question doesn’t arise now when ionisation type detectors are banned in the country that, which detector is good or bad. The choice is with us to go for optical type fire detectors only. However in academic point of view it is always better to discuss the merits and to compare the items available in the world market. According to a detailed study by the “US National Institute of Standards and Technology “Ionization type detectors provided somewhat better response to flaming fires than photoelectric alarms, and photoelectric alarms provide (often) considerably faster response to smouldering fires than ionization type alarms. The reason is very simple, the optical type detectors need enough smoke to get the light rays disturbed which has to activate the receptor and Integrated circuit to which the receptor is connected. In view of the above , one has to be very careful while selecting the detector since the highly sensitive transmitter and receptor devices connected to the Integrated circuit and the integrated circuits itself will cost more . Therefore looking at the cost of the device by saving few hundred rupees,one should not go for the lower quality products.
Will be Continued.
G K Bhat
MIRROR: G K Bhat