BROOKS AUSTRALIA RFP6 Installer ISSUE 4
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1.2.1 Ionisation model
Ionisation Smoke Alarms are certainly the most popular in domestic situations, due
in no small measure to the fact that they are the least costly to produce. They
respond to a sufficiently wide range of fire to be of general use and they are
particularly responsive to fast flaming fires where little visible smoke may occur.
They are seldom troubled by dust or insect contamination. However, ionisation
alarms are vulnerable to irritating nuisance alarms caused by cooking fumes, or
portable gas heaters. Excessive nuisance alarms may lead to the alarm being
disconnected. Furthermore ionisation alarms will be slower to respond to smoke
produced by slow smouldering fire. This type, with consideration to the above, can
be installed in passageways and areas leading to bedrooms.
Operating Principle
Radioactive source ionises the air molecules within the sensing chamber.
Advantages
• Least expensive to produce
• Dual chamber compensates for changing ambient conditions
• Less prone to false alarms due to dust and steam
• Suitable for general use
Disadvantages
• Contain radioactive material
• Cannot be disposed of in landfill
• Must be returned to supplier or Health Department
• Cannot be sent via the post
• Prone to false alarms due to fumes from cooking or gas/oil heaters
• Not sensitive to dark smoke from a slow smouldering fire
1.2.2 Photo-Electric model
Optical Smoke Alarms on the other hand sense visible smoke particles, again they
respond to a sufficiently wide range of fires to be of general use, but they are
particularly responsive to smouldering fires and the dense smoke given off by foam
filled furnishings or overheated PVC wiring. They are much less prone to nuisance
alarms from cooking, and furthermore, contain no radioactive material. However, all
optical smoke alarms are prone to nuisance alarms caused by dust or insect
contamination. Mesh screens are effective in reducing insect ingress, but it is
impossible to make the alarms dust proof, since they would then be effectively
smoke proof! For this reason it is essential that optical smoke alarms are always
kept clean. This type, with consideration to the above, should be installed within
bedrooms and living areas adjacent to kitchens.
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