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.
If you were to dissemble one of these smoke detectors, what you would find is an infrared LED (light emitting diode) on one side of the chamber and the other side you would find a light sensing device called a photo diode.
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 milligram) 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.
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