Smoke Detection / Detectors

There are principally 3 types of smoke detection device

• Ionisation Smoke Detectors
• Photoelectric Smoke Detectors
• Infra Red Smoke Detectors

All of which are available in either conventional or addressable formats.

Ionisation Smoke Detectors

Ionisation Smoke Detectors detect the presence of smoke by means of a small ionisation chamber and a source of ionizing radiation.

When smoke enters the ionization chamber, it disrupts the electrical current field generated by the Alpa Particles and by measuring this change in electrical current an alarm condition can be generated at the required sensitivity.

The amount of radiation in an ionisation smoke detector is extremely small and is predominantly Alpha Radiation. Alpha Radiation is very weak and is easily blocked by a sheet of paper or a few centimeters of air. Ionisation smoke detectors are therefore very sake to handle but should never be dismantled without the proper equipment.

Ionisation Smoke Detectors are generally better at detecting smaller smoke particles such as that produced by rapidly burning fires but are less sensitive to the larger smoke particles generated by smoldering fires etc.

Photoelectric (Optical) Smoke Detectors

Photoelectric Smoke Detectors detect the presence of smoke by means of a small light source and a separate photoelectric light sensitive cell housed within a sensing chamber. The sensing chamber is designed so that the light source and the photoelectric cell are mounted at acute angles to each other in separate sections of the chamber.

In normal circumstances light from the light source travels along the chamber in a straight line and does not enter the section where the photoelectric cell is. When smoke enters the chamber however the smoke particles scatter the light causing some to travel towards the photoelectric cell. By measuring the amount of light reaching the photoelectric cell an alarm condition can be generated at the required sensitivity.

Photoelectric Smoke Detectors are generally more sensitive to larger smoke particles such as that generated by dense smoke or smoldering fires but are less sensitive to small particles of smoke found in rapidly burning fires.

The growing use of flame retardant materials in modern building construction has led to Photoelectric (Optical) Smoke Detectors being more widely used and are now the most common detector found in new fire alarms.

Infra Red Smoke Beams

Infra Red (IR) Smoke Beams are in two parts and consist of an IR transmitter unit and an IR sensitive receiver.

The system operates by projecting an infra red beam of light, which is invisible to the human eye, across the protected area. When the light becomes obscured by rising smoke an alarm condition is generated.

Distances of up to100 meters can be covered by Infra Red Smoke Beams and with control and indicating equipment mounted at low level IR Smoke Beams are ideal for high level detection and / or for covering large open areas such as warehouses, industrial plants and shopping centers etc. where access for installation and maintenance of traditional 'point' detectors would prove difficult.

Key: # =Most acceptable detector    0 = Acceptable detector, though not necessarily the best.

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