The Function of a Smoke Detector

Hunker may earn compensation through affiliate links in this story.
A smoke detector can help warn you of a fire.
Image Credit: Chimpinski/iStock/GettyImages

Smoke alarms are important home safety tools because they increase your chances of escaping from a house fire. Very often confused with smoke detectors that simply sense smoke then trigger a separate alarm system, a self-contained smoke alarm can sense smoke and heat before sounding the alarm contained in the unit. It's the smoke alarm, not the smoke detector, that is found in most homes, and an alarm that's properly functioning gives early warning that makes escape more likely. Having smoke alarms properly installed and regularly checked so they can do their job when needed is crucial.


Video of the Day

Parts of a Smoke Alarm

Smoke alarms have two main components: the sensor and the alarm. Both components play an important role in the overall purpose of the unit, which is to sense fires as soon as possible and alert anyone in the home so they can get out. These components are equally important. Sensing the fire does no good if the alarm isn't loud and functioning. A poor sensor fails to send out the alert soon enough to get people out of the home.

Detection of Smoke and Fires

A smoke alarm's initial function is detecting that fire is present. The way it senses the fire depends on the type of unit it is. A photoelectric model has a light source that goes into the sensing chamber. The light sits at an angle away from the sensor. When smoke enters, it causes the light to reflect onto the sensor, triggering the alarm. These units are best at detecting smoldering fires that start with smoke before eventually breaking into flames.


An ionization model uses radioactive material positioned between two electrically charged plates to ionize the air inside it. This creates a current flow between the two plates. Smoke sets off the alarm when it disrupts the ion flow. These alarms are most responsive to fires with blazing flames.

Photoelectric and ionization smoke alarms both have certain advantages that make them more effective in specific areas. For example, a photoelectric model quickly detects smoke from smoldering bedding or furniture in a bedroom, living room or family room, while ionization units give fast warning of a blaze triggered by flammables in a utility room or a stove fire in a kitchen. Because each type is ideal for a certain type of fire, it's a good idea to have at least one of each in your home. You can also get a dual-sensing model that uses both photoelectric and ionization methods for detection.


Warning Home Occupants

Once a smoke alarm senses smoke, its next job is to let people know about it. That warning can make the difference in whether people in the home have time to escape. Effective units should have an alarm decibel level of at least 85 at 10 feet so they can be heard throughout the home.

Smoke alarms are often located outside bedroom doors. That means the alert sound needs to be loud enough to go through doors and wake up sleeping people in case of a fire at night. Alarms should also have a low-battery notification function that makes a repetitive beep when the battery power is nearly drained.


Testing the Alarm

Smoke alarms do not last forever. It's a good idea to replace them every 10 years to make sure you have reliable detection of fires. Most models have a test function that lets you periodically check them to catch potential problems early. Test your smoke alarms monthly and immediately replace any units that do not respond as they should.

Get new alarms after a decade even if the old models still seem to be functioning. Older units may sound when you press the test button, but that doesn't tell you how well the sensor works. The test response simply tells you that the electronic components and alert part of the alarm still work. You can test the sensor by using smoke alarm test spray, but it's still a good idea to change out any units older than 10 years. The sensor could stop working at any time. Not sure how old your unit is? Look on the back side for a date. That's the date of manufacture, and it's what you should use to calculate the 10-year mark, even if you installed the unit much later. Be sure to properly dispose of old smoke alarms.



Barb Nefer

Based in Kissimmee, Fla., Barb Nefer is a freelance writer with over 20 years of experience. She is a mental health counselor, finance coach and travel agency owner. Her work has appeared in such magazines as "The Writer" and "Grit" and she authored the book, "So You Want to Be a Counselor."