Ionization and Photoelectric Smoke Detection

It turns out, smoke detector does not necessarily equal smoke detector. As with any technology, the detection of smoke has evolved. The two most-used technologies are ionization and photoelectric. While both technologies sense the presence of smoke, the technologies are quite different. Smoke alarms for home, as well as smoke detectors for fire alarm systems employ these technologies. In this post we will explore the major differences between the two.

Smoke detection history

Smoke Alarm on Yellow Background

Many separate inventions ended up forming what we know as a smoke detector today. Fire alarms and smoke detectors used to be reserved for large commercial and industrial facilities, due to their cost and also size. The first affordable smoke detector with a battery was brought to market by the 1960’s. This detector was an ionization type detector. The technologies further advanced and allowed for the cost of the detectors to drop significantly to the levels we know today. The photoelectric smoke detector was invented in 1972, further advancing the smoke detector.


As the first invention for fire detection after the heat detector, Ionization minimized the time it required to sense a fire. Ionization uses a tiny amount of radioactive material, which ionizes the air. That is where the technology gets its name. The material used is generally Americium-241. The unit consists of two chambers. One chambers is free to the air and the other is a closed chamber. Both are exposed to the ionization and have two electrodes. the resulting current from the ionization between the electrodes is the same, until smoke enters the open chamber. It interrupts the flow of the current in the chamber and will cause an alarm condition.


Photoelectric detectors also have a chamber and look for obscuration of the air. That is where the similarities end. This technology employs a a source of light, usually emitted by a small LED and a receiver. This light beam is constant across the chamber. Should smoke enter the chamber, the light beam is interrupted and cannot be seen by the receiver anymore. It then causes an alarm condition within the detector. Photoelectric sensors are generally said to be more effective for smoldering fires, while ionization detectors shine for flaming fires.

What detector should you get?

Photoelectric detectors, being the most popular type detectors do provide adequate response to all types of fires. A recommendation is to purchase a detector that is multi-technology, such as a combination heat/smoke detector or a detector that combines the photoelectric and ionization technologies. While there is nothing wrong with a photoelectric only detector, it increases occupant safety to take it a step further. On that note, the ionization detector, while it contains radioactive material, uses Alpha radiation to ionize the air. This type of radiation has a very low penetration, which means it is stopped by the plastic around the detector and does not present a hazard.

CO Detectors

While on the topic, I would like to touch on carbon monoxide detectors, as they are closely related to smoke alarms. Many smoke alarms come as a combination unit with CO detection. CO detectors are a small chemistry lab. The presence of carbon monoxide causes a chemical reaction with an electrolyte and causes it to conduct electricity. This is an electrochemcial type CO detector. These detectors are highly effective, as they have a 5 to 10 year lifespan and are highly reactive to exact concentrations of carbon monoxide gas.

Detector Lifespan

Most smoke alarms and combination units have an expiry date printed on them. If your unit does not have an expiration date, we highly recommend it be replaced as soon as possible. Smoke alarms become less sensitive over their lifespan and collect dust, insects and other contaminants that can affect the effectiveness of the unit. Smoke alarms usually have a 10-year life, though that may differ for combination units. Most The detectors have a mechanism built in that will chirp the detector when its lifespan has been reached.


With Ionization smoke alarms containing radioactive materials, it is a good idea to return the unit to the manufacturer upon its expiration. While the radioactive material is generally not harmful and also considered safe to be disposed of in household waste, it’s a good idea to minimize the potential environmental impact. The manufacturer generally takes the detectors back for their internal recycling programs. Disposal information is usually printed on the back of the unit.

Our Services

We carry a large stock of smoke alarms and combination units. Our team can assist with replacement and the regular inspection of smoke alarms and detectors of all technologies. From new installations to replacement after 10 years, our units are a match for every application and will save lives in the event of a fire emergency.

What if my Smoke Alarm is beeping

Smoke Alarm on Yellow Background

What is a smoke alarm?

When it comes to smoke alarms, there is a common misconception. A smoke alarm is an all-in-one device that will sound an alarm when it detects smoke. They are different from the smoke detectors connected to fire alarm systems. You can find smoke alarms in residential homes or apartments. They are also found in sleeping quarters in hotels and other camp-style accommodations.

Smoke alarms encompass different technologies to detect smoke, which are covered here. They sometimes come as combination units that can detect carbon monoxide within the same device. Many of these devices are battery-powered, while others are connected to the 120V supply of the building. If the smoke alarm is a 120V connected device, they can often interface with other alarms to sound throughout the home.

How to maintain a smoke alarm

Smoke alarms, like any piece of technology, should be appropriately maintained. With the proper maintenance, smoke alarms can save lives. The maintenance of these alarms is quite similar, no matter the detection technology or power method. Testing should be completed at least monthly by following these steps:

Smoke Alarm Test Button
  • Press the button on your alarm and keep it pressed down. The alarm should sound. If interconnected, the other alarms should sound as well.
  • If your alarm has no button, it is outdated and must be replaced.
  • Follow the manufacturer’s instructions on testing and maintaining your alarm.
  • Check the expiration date on the side or back of the device. If there isn’t one, replace it.

If the alarm fails to sound or the alarm is past its expiration date, we recommend it be immediately replaced. We also recommend that you blow any dust that may be present within the casing out of the unit every year. This will minimize false alarms and maximize its detection potential.

Regular false alarms can be a detriment to these life-saving technologies, as they desensitize the occupants of the sound. With the alarm regularly sounding, people will automatically assume it a false alarm, which may cost them their life. When replacing a line-voltage or 120V unit, please ensure to hire qualified personnel, such as Nexus Fire & Safety to make sure the device is replaced properly and safely. When replacing interconnected devices, it is important to replace all units with the same make and model, to make sure the interconnection continues to work.

Why is my alarm beeping/chirping?

Smoke alarms are built with fail-safes to provide a warning if something is malfunctioning. For battery-powered or battery-backup devices, the unit will start chirping if the battery is low or missing. For line-voltage or 120V powered, the units will often chirp if line voltage is lost during a power outage. This is used to indicate that the unit is on its battery backup and may power off if power is not restored to it.

All detectors have an expiry date, which is usually 10 years, but as short as 5 years for some brands and types. Many of these detectors will start beeping in regular intervals, once every minute or something of the sort. If there is no expiry date or if the data has passed, we recommend the smoke alarm be replaced immediately.

A beeping or chirping alarm is not just an annoyance, but it is trying to tell you that something is wrong. We recommend immediate action be taken as it is a life-safety device and should be maintained to the highest possible standard. If in doubt as to what the beeping is indicating, consult the manufacturer’s manual, which should be available on their website.

Nexus Fire & Safety carries a large stock of smoke alarms and is ready to assist with replacing any outdated units that you may have. We have electricians on staff that are more than qualified to assist with any new installations or replacements of the devices. Additionally, we can provide design assistance for placement and answer any general fire safety questions you may have.