Photoelectric vs ionisation smoke alarms

New legislation is requiring photoelectric smoke alarms to be installed in Queensland homes. We answer the questions, what are they, and why are they better.

For  along time, smoke alarms have all been ionisation smoke alarms. They were so common, no-one even discussed what type of smoke alarm an alarm was. You simply bought a smoke alarm, not knowing what type it was. More recently, a new type of smoke alarm has come onto the market, photoelectric smoke alarms. In this article, we’ll discuss how each one works and explain what makes one better for your home (spoiler alert, it’s photoelectric).

Ionisation Smoke Alarms Explained

We’ll start with ionisation alarms because they’re the ones we’ve all had in our homes until recently. They’re also still out there, in their millions, in Australian homes right now. 

Fun fact: ionisation smoke alarms have radioactive material in them.

That’s right, a tiny piece of radioactive material is in each ionisation detector, and it’s key to how they detect smoke. But don’t, your alarm isn’t going to go Chernobyl on you.

How it works is a current passes through the detector causing the radioactive material to ionise. Ionisation is the process of losing or gaining electrons. This can happen by radioactive decay, or through electrical charge.

These ions will naturally flow between charged plates. This flow is measured by the alarm detectors. If this flow is disrupted (yes, you guessed it, by smoke) then the alarm is triggered.  

ionisation smoke alarms infographic
1. The circuit that measures the flow of ions between the plates.
2. In normal conditions, an orderly flow of ions is detected across the circuit.
3. Smoke disrupts the flow of ions, resulting in a measurable drop in the circuit. The alarm is triggered.

Photoelectric Smoke Alarms Explained

Photoelectric sensors have been around for quite a while now. You may have triggered one when you walked through an automatic door. They’ve been used on assembly lines and in warehouses. Their adaption to detecting smoke was expected. It makes sense that this technology that was used to detect things would one day detect smoke. The thing is, when it comes to smoke alarms, you have to detect small variations from fine particles of smoke. And you have to be certain it works.  There is no tolerance for  failure in smoke alarms. The technology is now at a point where photoelectric smoke alarms are accurate and reliable.

How they work is, a beam of light shines from one end of the smoke alarm to the other. The light detector is below this beam. In a normal state, the detector sees no light. 

When smoke disrupts the beam of light, some light will strike the sensor, setting off the alarm. 


photelectric smoke alarm infographic
1. An uninterrupted beam does passes above the light sensor below.
2. Smoke particles cross the beam, disrupting the flow of light.
3. Light particles hit the sensor, activating the alarm.

Why Are Photoelectric Smoke Alarms Better Than Ionisation Smoke Alarms?

Photoelectric smoke alarms aren’t necessarily better in all circumstances. Research has shown, however, that they are better in the home.

The reason for this lies in what types of fires they are better at detecting.  

It is this latter type of fire, the smouldering fire, that is more common in the home. 

A smouldering fire is a slow, flameless form of combustion

It's A Matter Of Time

The difference between surviving a fire in your home or not surviving, is a matter of time. An extra 30 seconds can make all the difference. Early warning of a smouldering fire can be the critical factor in a situation where you need to get you and your family to safety. 

Smoke is also, by itself, quite deadly. It can make you sleepy, weak and confused. It takes only 4 minutes for smoke to make your home completely dark. Staggering around your home in the dark, in a sleepy and dazed state, is a recipe for disaster. This is why early warning is critical. 

Research has shown that smouldering fires are the most common in the home. For more information, download the Fire Safety fact sheet

Time is so critical in escaping fires, that the Queensland government also requires us to install interconnected smoke alarms in our homes. These alarms communicate with each other. When one detects a fire, they all sound off their sirens.  It’s been shown that people can sleep through a fire alarm that is located too far away from them. In some circumstances, the alarm wakes them but it took 30 vital seconds for them to be roused. Alarms are now interconnected and situated in the bedroom so that everyone is alerted the very second there is a fire.

For more information, see our smoke alarm installation and testing page. We outline the services we offer and also the government legislation as it applied to smoke alarms. 

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