Photoelectric vs Ionization Smoke Detectors: What Are The Differences?
Posted: Tuesday, January 06, 2009
by Stephen Davies
Houston Fire Safety
You know what a smoke detector is, but did you know there are different types of detectors? In today's article, we'll explain the differences between two of the most common smoke detectors used in both commercial and residential fire alarm applications. Photoelectric Smoke Detector
Simply put, a photoelectric smoke detector senses smoke using a specially designed optical chamber. In fact, the photoelectric smoke detector is often referred to as an "optical" smoke detector.
How does a photoelectric smoke detector work?
In its non-alarm, idle state, the LED emits a small beam of infrared light to the receiver. When this invisible beam of light is partially blocked or scattered by smoke particles, the circuitry changes state (voltage drop) and therefore sets the smoke detector into alarm condition.
The photoelectric smoke detector is best used in applications where smoldering, thicker smoke might occur in a fire.
Ionization Smoke Detector
The ionization smoke chamber is different in that the detector has a small amount (approximately one miligram) of radiation contained within the unit. This "Alpha" radiation used is called Americium 241 and has been determined to be safe to the environment.
The sensing part of this unit is called the ionization chamber. Within this chamber are electrodes that carry a constant amount of current running between them.
How does an ionization smoke detector sense smoke?
As already stated, in its normal state, the electrodes maintain a steady amount of current. When smoke enters the chamber, it reduces the ionization in the radioactive alpha particles between the electrodes. In turn, this also reduces the electrical current, thus setting the detector into alarm condition.
It is recommended that ionization smoke detectors are best used in applications where very little smoldering occurs. In other words, this type of fire would be more of a flaming, low smoke fire in nature.
How do these two smoke detectors differ in sensitivity and reaction time?
Tests have shown that the ionization detector is quicker to react as it doesn't have to wait until smoke enters the chamber. The downside is that it has a tendency to cause more false alarms than the photoelectric type.
Because of these sensitivity issues, Ionization smoke detectors are being installed less than they used to. I haven't verified this yet, but I've heard that some manufacturers are discontinuing the production of ionization smoke detectors.
I should point out that because photoelectric smoke detectors are a little slower in response, many manufacturers make available, units that have a built-in thermal (heat) detectors inside them.
Can you tell the difference by looking at either one of them?
Most of the time, manufacturers use the same housing for both the Photo detector and the ion. The best way to know which smoke detector you have is by reading the label on the underside of the smoke detector. If it doesn't say specifically which type of smoke detector it is, then you may have to look up the model number for verification.
This Article has been viewed 4,793 times. (Not updated in real-time.)
Top-level comments on this article: (3 total)I appreciate the safety aspects outlined herein. My only suggestion is to make the article more personal and less advertising. Perhaps. "As a person with experience , etc". best wishes.Thanks for the tip Robert, I truly appreciate you taking the time to post.
Actually, I'm very new to writing articles, so I'm still going through a huge learning curve.
If my article appears to be a little too much on the "advertising" side, that probably comes from my past experience as a advertising copy writer.
Seeing as you're a well-seasoned article writer, I think I'll visit some of your articles and see how it's supposed to be done.
Again, thanks for the great advice! ;0)I write to more controversial topics perhaps some of the others who do well on a variety of topics can be more beneficial for you? best of luck, Hope tor read you more.
We have been designing and installing the Smoke Detection based fire alarm systems for more than two decades... where asked for, we give conultancy and advise as well... Ionisation smoke detectors are very dependable and if maintained properly, donot give false alarms... exception being the areas prone to dust storms... if they are not cared for they do tend to give false alarms... Incase of a fire starting suddenly and with very little smoke.... optical smoke detectors fail badly... even in black smoke optical detectors don't work.... yes they can be used where cleaning/maintenance is difficult specially above the false ceilings... i feel some vested lobbies, in the garb of environment, are spreading the word around that ionisation detectors are banned...if you know they are really banned.. please do let me know.... fast detection of would be fire or flash fire can save precious human lives... that is what we should remember and vouch for... ofcourse proper guidance for desposal of old/scrapped ionisation detectors should be provided/circulated... sunil grover.. concept engineers (i) pvt.ltd.
Dear Sir I would like to know which is the best ionization or thirmal smoke detectors
We want your comments! If you can read this, you don't have javascript enabled, so you can't use this comment system. Please enable javascript.