Smoke Alarm

Smoke Alarms are required in all buildings, residential and commercial. This page is about on the history, functionality, and current requirements of smoke alarms.

How Does It Work?

Ionization (Modern Alarms)

Modern Smoke Alarms, first invented in the 1930s, use quite advance science. It has to detect that your house contains smoke (see What Is Smoke? below).

Smoke comes in a wide array of variety. The type depends on what is burning. Pretty much all types of smokes are dangerous when inhaled in large quantity. Some are very dangerous even in small quantities, although those are only possible in commercial building that carry certain types of materials and safety guidelines for companies handling  such materials are quite stringent.

In mot cases, the way we currently use to detect smoke in our residential environment is by counting large particles in the atmosphere. This is done using a nuclear beam.

Your smoke alarm has:

  • A small chamber in the center which contains a small lump of radioactive material;
  • Two tubes going in opposite directions from that chamber; and
  • Two Nuclear Radiation Detectors at the end of those tubes.

One of the two tubes is perfectly air tight, the other has a very small hole which allows smoke particles to enter.

The Nuclear Radiation Detectors (you can even get your own Geiger Counter on to check out your alarms) can count how many nuclear atoms have decayed, with a pretty high precision. Only, the full amount of available radiation reaches the detectors only if undisturbed by matter.

Therefore, the Nuclear Radiation Detector on the side where the tube is perfectly air tight will always know of the pure level of radiation. On the other hand, the detector on the other side will be disturbed by any particle in your close by atmosphere. If that atmosphere currently contains smoke, the smoke particles will enter the tube and will be hit by nuclear emissions which then won’t reach the Nuclear Radiation Detector. Thus the counters of the air tight and the non-air tight tubes will get different levels of radiation.

Fun Facts: This method to detect smoke was discovered by Swiss scientist Walter Jaeger who was rather unsuccessfully working on a poison gas detector. One day, after the poison gas was again not detected, he started smoking. To his surprise, a meter on his detector started moving (that version was not yet attached to an alarm system.) So it detected his cigarette smoke. Not too long after, another Swiss scientist, Ernst Meili, found how to detect gases with a better ionized chamber in his device. This detection method was used in mines to detect dangerous gases and allow for workers to escape as soon as possible. It took another 12 years for these detectors to make it to the US where they were only used in major commercial and industrial facilities as these first smoke alarms were very bulky.

Optical (Photoelectric—Older Smoke Detectors)

Before we understood how to handle ionization and later radioactive material and especially to miniaturize Nuclear Radiation Detectors, we extensively used an optical mechanism to detect smoke.

Smoke from wood fire is composed of rather large particles (see What Is Smoke? below) and a beam of infrared light generated by an incandescent light bulb is affected by such large particles as the particles absorb some of the photons. A reader of the beam can easily tell us whether there are too many particles in the way since photons will be missing. At that time we can sound the alarm. Newer versions use ultraviolet light and the light is most likely generated by an L.E.D. component which last much longer than incandescent light bulbs.

Note that even though this is an older technology (The first automated smoke alarm was invented 1890 by Francis Robbins Upton, who was an associate of Thomas Edison,) it is still in use today. Actually, some alarms use both methods of detection, Photoelectric and Ionization, in order to eliminate some false alarms and to at times detect smoke earlier or better. Also very large rooms have a better chance of being properly monitors by having the beam go throughout the room, on the ceiling or high on a wall. The beam is either checked on a receive alarm (a second device) or reflected and then checked in the emitter device. This allows these alarm to check the room along their whole distance instead of just one spot with smoke alarms we are used to in our homes.

What Stage of a Fire Are Smoke Alarm Detecting?

A Ionization Alarm better detects the flaming stage. This means it detects the fire early on and that’s why we most often want to use this type of detectors. It is also better at detecting that stage than the smoke stage.

The Photoelectric Alarm better detects smoke. This means the combustible already burned quite a bit of material. This is why we switched to the newer method once it was available in miniature form in the 70s (compared to the bulky versions of the 50s and 60s.) At that time, an alarm cost around $125 a piece and it was not a good as today. Electronics costs have decreased dramatically and I guess we can use more automation to create such devices further decreasing the costs.

What Is Smoke ?

Smoke are small particles that, when hot, are light enough to float in our atmosphere. As they cool down, they fall back on the ground as they become heavier than our atmosphere and thus gravity wins.

Smoke particles, at least those that are dangerous to us, are very small. If you’ve been by wood fires before, you probably saw some large particles floating around. Those are called cinders. Smoke is similar, only the particles are way smaller.

The smoke detectors thus have to detect very small cluster of atoms.

Battery Requirements

Since 2014, a battery powered smoke alarm in California must come with a sealed 10 year lithium battery. That battery is sealed meaning that it can’t be replaced. (It has been found that many people would not replace their battery often enough hence the longer period and less worry for that amount of time solution.)

Life Improvements as a Result

Although many people are still not following the code (there is no direct enforcement in an owner occupied dwelling,) most do. Also all rentals have to have working alarms and since the owners are responsible for the possible consequences, rentals are much more likely to be up to date in that arena.

To give you an idea, some research was conducted in 2013 and we found out that the penetration was 96% of US homes, but a staggering 30% of the alarms that are installed are not functional (missing batteries, disconnected, too old, etc.) In effect, only about 67.2% of homes are well protected.

Yet, the number of people dying in a house fire has decreased dramatically in the last 100 years. In the early 20th century, only 9 people out of 10 would get out of a fire alive (10% would perish—from 1915 statistics). In comparison, in the early 21st century, the odds are 99 people out of 100 which is much better (only 1% perish—circa 2016.)

There may be other factors at play, though. Newer homes are likely to have better escape routes than 100 years ago. We can see that homes without an alarm or a dysfunctional alarm have 1.18% deaths compared to homes with a working alarm: 0.53% deaths (from 2009—2013 statistics). Although with a good alarm you reduce your death risk by half, even without an alarm we have improve dramatically since 1915. In other words, smoke alarms are not the only factor, but I think that doubling your odd of survival for the price of a smoke alarm is more than worth it.

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