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Smoke Alarm Sensor Types and their Differences

Smoke alarms are available with many types of sensor, each one detecting smoke or fire using a different method. This guide is designed to help highlight the differences between each sensor type, show how each sensor works, show the advantages and disadvantages of each sensor type and also highlight where each type of sensor is best suited throughout your home.

Types of Smoke Alarm Sensors

Optical Smoke Alarms

Optical smoke alarms are particularly good at detecting slow smouldering fires such as soft furnishings or electrical wiring.

Ionisation Smoke Alarms

Ionisation smoke alarms react quickly to fast flaming fires by detecting the change in ionised properties of air through smoke.

Fixed Temperature Heat Alarm

Heat alarms are ideal for smokey and dusty environments, they react to heat instead of smoke so are less likely to give false alarms.

Multi-Sensor Alarms

Multi-Sensor alarms contain two separate sensing elements – optical and heat allowing them to monitor two different by-products of fire.

How Optical Smoke Alarms Work

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 pulses a beam of light into the sensor chamber every 10 seconds to check for smoke particles.
Infra red LED
Photodiode light receptor
Insect screen
Light proof chamber cover

When a fire breaks out smoke will enter the optical chamber through the opening vents. Smoke alarms from quality manufacturers have the chamber protected with insect screens to stop bugs entering and causing false alarms.
Infra red LED
Photodiode light receptor
Light proof chamber cover

As the smoke enters the optical chamber, its particles cause the Infra red light to be scattered onto the photodiode light receptor.
Infra red LED
Photodiode light receptor
Light proof chamber cover

Once the scattered light hits the photodiode light receptor a signal is sent to the integrated circuit which causes the alarm to sound alerting the occupants to the fire.
Integrated circuit
Infra red LED
Photodiode light receptor
Light proof chamber cover

Optical smoke alarms can be situated in bedrooms and living rooms and in the ground floor hallway, the optical technology makes the alarms less prone to false alarms from cooking fumes.
Bedrooms & Living Room
Ground floor hall

How Ionisation Smoke Alarms Work

An ionisation smoke alarm works by ionising the air between 2 electrodes which are positively and negatively charged, this creates a small current inside the chamber.
Electrodes
Radioactive Source
Insect screen
Chamber cover

When a fire breaks out smoke will enter the ionisation chamber through the opening vents. Smoke alarms from quality manufacturers have the chamber protected with insect screens to stop bugs entering and causing false alarms.
Radioactive Source
Chamber cover

As the smoke enters the chamber, its particles cause the balance of current inside the ionisation chamber to be changed.
Radioactive Source
Chamber cover

Once sufficient smoke enters the chamber the change in current will cause a signal to be sent to the integrated circuit which causes the alarm to sound alerting the occupants to the fire.
Integrated circuit
Radioactive Source
Chamber cover

Ionisation smoke alarms can be situated on landings, they shouldn't be situated in areas close to kitchens as they are prone to false alarms.
1st Floor Landing

How Heat Alarms Work

A heat alarm is designed to detect heat instead of smoke, the alarm contains a thermistor which is set to respond to temperatures above 58°C.
Thermistor
Sensor chamber cover

When a fire breaks out hot air from the fire will rise and enter the sensor chamber.

When temperature inside the chamber reaches 58°C a signal is sent to the integrated circuit which causes the alarm to sound alerting the occupants to the fire
Integrated circuit

Heat alarms can be situated in the kitchen and the garage, because heat alarms don't react to smoke they are not prone to false alarms from cooking and exhaust fumes.
Garage
Kitchen

What are the Benefits of Multi-Sensor Alarms

The new Multi-Sensor from EI combines the results of the heat and optical sensors and uses the combined data to give earliest fire protection while giving full protection against false alarms.
They contain all the benefits of an optical alarm
  • For smouldering fires - visible smoke
  • No false alarms from cooking fumes
  • No environmental issues
and all the benefits of anheat alarm
  • For rapid burning - heat
  • No false alarms from cooking fumes
  • No environmental issues

Multi-Sensor alarms can be situated on the first floor landing, in living rooms and bedrooms and in the ground floor hallway. Because they monitor 2 different by products of fire their response to all types of fire is significantly improved over singal sensor alarms.
Bedroom & Living Room
Ground floor hall
First floor landing
Use heat alarm in kitchen
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