Ionisation smoke detectors work by ionizing the air and causing current to flow between two electrically charged plates. When smoke enters the chamber it disrupts this current and sets off the alarm.

Photoelectric alarms are slightly more sensitive to slow smoldering fires than ionisation. They also sense darker smoke better than ionisation, and are less likely to give false alarms caused by cooking vapours.

Ionisation

Smoke alarms are essential for ensuring you have an early warning of a house fire. But with so many types of smoke alarms out there, it can be difficult to know which one is right for your home.

Ionisation:

An ionisation smoke alarm works in a different way to an optical detector. It uses a small amount of radioactive material to ionize the air in its detection chamber, causing current to flow between two electrically charged plates.

Ionisation-type smoke alarms are usually more effective at detecting fast-flaming fires than slow-burning (smoldering) fires.

As you can see in the diagram above, ionisation alarms use a small piece of a chemical element called americium to ionize the air inside their detector. As ions and electrons whiz around between the plates, the detector’s circuit thinks everything is fine and the alarm remains silent. But when smoke enters the detection chamber, it disrupts the current and sets off the alarm.

Photoelectric

The photoelectric effect is a result of the interaction of light with metal surfaces. According to Einstein, when a photon collides with a metal surface, it can transfer energy in the form of an electron to the metal.

The number and kinetic energy of these emitted electrons are directly proportional to the frequency of incident radiation. This makes the photoelectric effect a type of electromagnetic (EM) interference.

When the frequency of incident radiation is below a threshold value (indicated by ), no current is observed.

Ionisation smoke alarms have a chamber formed by two electrically charged plates and a radioactive material that ionizes the air and causes current to flow between the plates. When smoke enters the chamber, it disrupts the flow of ions and reduces the current, activating the alarm.

Combination

A guardian ionisation smoke alarm works by using tiny particles of radiation to detect smoke. These react with particles emitted from a fire and set off the alarm, much like a fire alarm you might see on TV.

These are a relatively new type of smoke detector and are often recommended by fire authorities for home use. However, they have some drawbacks.

The main problem with an ionisation alarm is that they are not as good at sensing smouldering, slow-burning house fires. Instead, they are more likely to pick up a nuisance fire alarm from cooking fumes or steam.

That is why it is highly recommended that you install both ionisation and photoelectric detectors in your home. You can buy individual ionisation and photoelectric sensors or, better still, a dual sensor alarm that contains both technologies.

Installation

Installing smoke alarms in your home is an important step in protecting your family from fire. You should place them on all floors of your house, inside every bedroom and in common areas such as a living room or hallway.

Ionisation smoke detectors use tiny particles of radiation to detect the presence of smoke. They’re more sensitive to smaller particles of smoke, such as cigarette and paper smoke, than optical detectors.

However, they’re also less efficient at detecting large smoke particles that are produced by a flaming fire before it breaks out into flames. They’re also more sensitive to cooking vapours and if placed near a cooker or toaster, they will give frequent false alarms.

For best protection, it’s recommended to have both types of smoke detectors installed in your home. There are combination alarms that have both ionisation and photoelectric sensors, which can be more effective in detecting smoke and smouldering fires.