How Smoke Detectors Work

Smoke detectors are the most common fire detection device. They give early warning of fires that produce smoke before significant heat — which includes many fires in homes, hotels and offices.

Optical (photoelectric) detectors

Most modern commercial systems use optical smoke detectors. Inside the detector is a dark chamber with a light source and a light sensor positioned so that, normally, light does not reach the sensor.

When smoke enters the chamber, the particles scatter the light, and some reaches the sensor. When the scattered light passes a threshold, the detector signals an alarm.

Optical detectors respond well to smouldering fires that produce larger smoke particles — such as fires involving furniture, bedding and wiring insulation.

Ionisation detectors

Ionisation detectors use a small radioactive source to ionise the air in a chamber, creating a tiny current. Smoke particles disrupt the current, triggering the alarm. They respond well to fast-flaming fires with small particles. Their use has declined in many markets, partly because of disposal requirements for the radioactive source.

Multi-sensor detectors

Multi-sensor detectors combine optical smoke sensing with heat sensing (and sometimes other sensors). By analysing several inputs, they can respond to a wider range of fires and reduce false alarms.

Other smoke detection types

• Beam detectors project a light beam across large open spaces such as warehouses and atria.
• Aspirating detectors draw air through sampling pipes to a sensitive detector, providing very early warning in critical areas.
• Duct detectors sample air inside ventilation ducts.

Where not to use smoke detectors

Smoke detectors can produce false alarms where there is steam, cooking fumes, dust or exhaust — such as kitchens, bathrooms, laundries and some plant rooms. Heat detectors are usually used in these areas instead.

Placement and maintenance

• Detectors are normally mounted on ceilings, where smoke collects.
• Spacing and positioning follow the design standard and room layout.
• Detectors should be kept clean, tested periodically and replaced at the end of their service life.
• Never cover or remove detectors, even temporarily, without arranging alternative precautions.

Omega designs and installs detection systems suited to each environment. Request a consultation.