What are the different types of smoke alarm

What are the different types of smoke alarm.

Smoke alarms are a great fusion of technology and practicality- they are a fully self-contained device which houses the means to detect fires and once triggered emits and a warning alarm.  They detect the presence of a fire often well before we could either smell or hear it.

Sized in the region of 12- 18cm in diameter and around the 7cm in depth they are relatively unobtrusive and easy to DIY fit to your ceilings.

There are generally four (4) common type of smokes alarm or detectors available in the UK; each is discussed below.


Ionisation Smoke Alarms and Detectors

These are the most common and cheapest option. An ionisation alarm works by ionising the air between two opposingly charged electrodes (+ve and –ve) within the unit. When a fire breaks out the smoke (or smoke particles) enter the alarm. The smoke particles cause the previously balanced current inside

the unit to be altered. Once the amount of smoke or smoke particles have reached a threshold level the current becomes altered such that it triggers the integrated circuitry to sound the alarm function.

Ionisation alarms are sensitive to small particles produced by flaming fires whilst being slightly less sensitive to slow burning ones.


Optical Smoke Alarms

Optical alarms, also known by the more technical term of photo-electric smoke alarms, work on the detection of light principle. Within the unit a LED (light

emitting diode), generally in the infra-red range, emits a beam of light into the chamber every 10 seconds or so. When a fire breaks out smoke enters the unit through the vents.  This smoke causes the emitted infra-red light to be scattered, some of which is then hits the photodiode light receptor (also contained within the unit).  Once the light hits the receptor it triggers the integrated circuitry to sound the alarm function.

Optical alarms are more expensive than ionisation ones but are more effective at detecting slow burning fires. They can generally be sited on the ground floor, hallways and bedrooms.


Heat alarms

Heat alarms for the detection of fires are becoming more popular. They very simply detect high heat rather than smoke particles.

Within the alarm is s thermistor connected to the alarms circuitry. Once the detected temperature breaches a pre-set threshold the alarm is triggered. The alarms use the guiding principle of “heat rises” as the basis for early detection.

This type of alarm will not be triggered by smoke, smoke particles, exhaust fumes, cooking fumes or steam.  For this reason they can be set in both garages and kitchens.


Combined or multi sensor smoke alarms

As can be noted above from the brief descriptions, each type of detection system has its own strengths.  For this reason combined smoke alarms, using two of the above mentioned three detection methods, are becoming more common place.


Most smoke alarms, regardless of detection type, do look remarkably similar. 

The detectors are generally powered by batteries or mains electricity.  Recent building regulation changes means that new build and substantially renovated properties will need to have hard wired alarms powered by the mains electricity supply (often with battery back-up).  You should be replacing alarm batteries every 12 months for peace of mind.  That said there are smoke detection units which have a battery which will last for either 10 or even 15 years.