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Fire Alarm -- Description

Construction of a combination Ion Smoke Detector and a Photo Smoke Detector
The job of a smoke detector is to detect particles in the air, call the particles smoke, and sound the alarm. An Ion Smoke Detector senses smaller particles better than it senses larger particles. A Photo Smoke Detector senses larger particles better than it senses smaller particles.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer






Should the Detector be Photo, Ion, or Both?

By Douglas Krantz

In a smoke alarm, the smoke detector senses particles in the air from a fire and then the alarm sounds. Hearing the alarm, people evacuate.

Particles in the air from cooking or steam from a shower sound false alarms. Hearing the false alarms, people get mad, and after a few false alarms ignore the alarms or disable the smoke alarm.

With false alarms, there's no fire, and when people have lost all desire to react to the false alarms, hearing one they just go on with their lives. But if there is a fire, and people are not reacting to the alarms because of all the false alarms, people can die.

All of this is to say, false alarms are bad.

Major Cause of False Alarms

The smoke alarms are supposed to alert people of a fire, but the smoke detectors inside the alarms detect particles in the air, not smoke or fire.

Whereas the smoke particles are a precursor of real fire, other similar types of airborne particles also activate the smoke alarms. Besides from coming from a real fire, the particles floating in the air can come from:
  • Cooking Smoke
  • Steam from a Hot Shower
  • Dust Stirred Up while Cleaning
  • Spray Paint
  • Sanding Dust
  • Solder Smoke
  • Etc.
The trouble is that to a smoke detector the particles in the air coming from these other sources look the same as real smoke.

For example, a smoke detector has a hard time telling the difference between the airborne particles produced by Cajun cooking and the airborne particles produced by a grease fire. No matter how good the smoke detector, it just can't tell what produced the airborne particles.

If only the smoke detectors were less sensitive.

Smoke Is Not Smoke

The problem is that different types of fire produce different types of smoke particles. A flaming fire tends to produce small particles in its smoke while a smoldering fire tends to produce large particles in its smoke.

A smoke detector or smoke alarm, though, has to be sensitive to both types of smoke.

Sensitivity of an Ion Smoke Detector verses a Photo Smoke Detector

Ion Detector
An ion smoke detector ionizes the air inside its sensing chamber, and looks for changes in the conductivity of air caused by particles of smoke. It specializes in sensing the smaller smoke particles from flaming fires.

To be useful though, the detector also has to also be sensitive to the larger particles coming from smoldering fires. In order to be sensitive enough to the larger particles, the ion detector's overall sensitivity has to be increased, which makes it oversensitive to the smaller particles.

Photo Detector
A Photo smoke detector sends light into its chamber and looks for reflectivity of smoke particles within its chamber. It specializes in sensing the larger smoke particles from smoldering fires.

To be useful though, the detector also has to also be sensitive to the smaller particles coming from flaming fires. In order to be sensitive enough to the smaller particles, the photo detector's overall sensitivity has to be increased, which makes it oversensitive to the larger particles.

Aesthetics and Economics

To most people, smoke is smoke, and a smoke detector is a smoke detector.

If, to most people, a smoke detector is a smoke detector, why would a person spend extra cash to clutter up the ceiling with extra smoke sensing devices, when (they think) only one detector is needed? In other words, the assumption by the NFPA is that no one installs both types of detector in the same room, so any smoke detector sold in this country has to be sensitive to all types of smoke.

Combination Ion and Photo

We have the situation where, just to be sensitive enough to all smoke particles, either type of detector is oversensitive to its own smoke specialty.

Taking this into account, a lot of manufacturers have combined both an ion smoke detector and a photo smoke detector inside the same enclosure.

In reality, both sensors are stand-alone smoke detectors. Each one detects particles in the air and each one activates the alarm.

However, because the ion detector has a photo smoke detector next to it; it doesn't have to be sensitive to large particles, and it therefore doesn't have to be oversensitive to small particles. The same is true with the photo smoke detector, it has an ion smoke detector next to it; it doesn't have to be oversensitive to large particles.

False Alarms

Now we get back to false alarms, and the damaging effects that false alarms do to people's reaction to real alarms.

A combination smoke detector is going to be less sensitive overall, and produce fewer false alarms than either type alone. Fewer false alarms means that people, when they hear the alarm, react more as if there was a real fire.

Isn't that the purpose of the smoke alarm in the first place?




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