Smoke Detectors: What is the difference between a Photoelectric Smoke Detector and an Ionization Smoke Detector?

This article written by: Mike Glover, Security & Fire Alarm Operations Manager @ Hiller Systems Virginia.

Hiller Systems - Your premier fire protection and security systems contractor in Virginia for all of your fire protection and security systems needs, presents a brief explanation of the difference between photoelectric and ionization smoke detectors and smoke alarms.

At Hiller Systems, we actually get this question a lot.  The short answer is that they detect different types of particles that are released during different times of a fire's life cycle.  Which smoke detector or smoke alarm is best for you?  Well, that depends on the application you intend to use the detector for.  Before you can decide which type of detector you should use, you should first probably know a little more about the way each one works.  Here is a brief outline designed to give you the knowledge you need to properly identify the correct type of detector for your particular application.

Ionization Detectors - This type of detector uses an ionization chamber and an additional source of "ionizing radiation" also known as "alpha radiation" to detect smoke.  Inside an ionization detector is a small amount (usaully about 1/5000th of a gram) of americium-241 radioactive material.  During normal operation and without the presence of smoke or fire particulate matter, the ionization chamber sends a very small electrical current through the ionization chamber.  The detector goes into an "alarm" condition when particulate matter from a fire enters this chamber and disrupts the normal electrical current flow through the chamber.  Once this is detected, an "alarm condition" is generated by the unit. 

Photoelectric Detectors - This type of detector operates on a different principal.  They also have a smoke sensing chamber, however instead of sending electricity across the chamber, the photoelectric detector sends photoelectric beams of light across the sensing chamber.  Any smoke particles that are present in the chamber will instantly begin to reflect the light and change the pattern by sending the light 90 degrees from the direction it was intended.  The redirected/reflected light is then picked up and sensed by the photoelectric sensor in the detector and that condition then triggers an alarm condition on the unit. 

For smoke "alarms" (units made and sold for residential and consumer use) the unit activates an internal sounder that alerts people of the fire/smoke condition present. In an industrial or commercial application where the detector is part of a larger fire alarm and detection system, the detector creates an alarm condition and notifies the Fire Alarm Control Panel, which in turns activates and evacuation signal via the notification circuits which can be either chimes, horns or in some cases voice notification of the alarm condition.

Because of these different detection technologies, ionization sensors are more likely to detect smaller, less visible fire particles, like those produced by flaming fires, sooner than photoelectric sensors. However, photoelectric sensors are more likely to detect larger, more visible fire particles, such as those produced by smoldering fires, sooner than ionization sensors.  Remember, these detectors look very similar in body type and color.  Determining which principal a detector operates on is not always easy to tell...

Can you tell which detector is an Ionization detector and which one is a Photoelectric Detector?

So which unit is best for you?  Well again, that depends on the application you intend to use the detector for.  Hiller Systems strongly recommends contacting a factory authorized representative of these type of units and consulting with a professional to help you select the correct detector for your application.  Remember, whether you are selecting a smoke alarm for your house or a smoke detector as part of a larger fire alarm and detection system, the best choice is always to let the professionals help you!  Learn more about Hiller Systems at our website or by clicking the about us page of this blog.  If you are still determined to go it alone...here is an informative video produced and published by NFPA (the National Fire Protection Association), that may help you!

Hiller Systems Virginia can help you no matter how large or complicated your fire protection problem is!  Be sure to contact us for more information or to schedule a hazard analysis. See our line card for more information on the manufacturers we represent and the systems and services we provide.