Smoke Alarm

Smoke Alarms are required in all buildings, residential and commercial. This page is about 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) early enough so you have time to escape.

Smoke comes in a wide array of variety. The type depends on what is burning. Pretty much all types of smokes are dangerous to humans when inhaled in large quantity. Some are very dangerous even in small quantities, although those are only possible in commercial buildings 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 Amazon.com to check out your alarms—note that I’m an Amazon affiliate and will get a small commission if you purchase a Geiger or other items from Amazon after following this link) count how many nuclear atoms have decayed, with a pretty high precision. This works because when ionized particles hit another particle, they lose part of their ionization. When those particles hit the nuclear radiation detectors, they generate less electricity than fully ionized particles.

As a result, the Nuclear Radiation Detector on the side where the tube is perfectly air tight will always know of the right level of radiations. On the other hand, the detector on the other side will be disturbed by any particle in the alarm’s surrounding atmosphere. If that atmosphere currently contains smoke, the smoke particles enter the tube and are hit by nuclear emissions which then won’t reach the Nuclear Radiation Detector with as much energy. Thus the counters of the air tight and the non-air tight tubes get what looks like different levels of radiations and once that difference is too large, it sounds the alarm.

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 whole room, on the ceiling or high on a wall. The beam is either checked on a receiving alarm (a second device) or reflected and then checked in the emitting device. This allows these alarms 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 as 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 and many are invisible to us.

Smoke detectors detect such 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. The 10 years limit is also the lifetime of the smoke detector. Electronics ages and after a while does not perform as expected. For that reason, we want to replace older devices. Also within 10 years it is very likely that newer devices will be performing better, so having newer versions is likely going to help you avoid the fire an survive.

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 area.

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 by smoke detection.

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 are other factors at play, though. Newer homes have better:

  • Escape routes;
    • 100 years ago, many windows were too small for people to escape through them efficiently;
    • Ladders were not provided at higher floors, now  a two stories home is expected to include escape ladders in all second floor rooms; also you’ve certainly seen ladders out of tall buildings in cities like San Francisco and New York (we often see them in movies and bad guys use them to escape the police. Not the exact intend, but I guess that proves that it works.)
    • Commercial buildings have more escape routes and one way doors that will sound the fire alarm when opened
    • Businesses and school are expected to have fire drills once or twice a year to train people on how to behave in case of a fire; in some places we also get earthquake drills
    • All businesses must clearly show a map with all available escape routes; you’ve certainly seen those in hotels as well
    • Many businesses also have lights that will indicate the best routes from where you are; in general it indicates the closest exit
    • All businesses must have sprinklers so in case of a fire they can extinguish it in an automated way without the need for hand held extinguishers; some homes also have such systems in place, most particularly in their kitchen
    • Multi-stories apartment complexes have to have sprinklers, it is the safety of everyone in the building which is at play, so stopping a fire as soon as possible is an incredible advantage
  • Materials used to build homes are not readily flammable, we use flame retardants which gives us more time to escape;
  • Flame retardant materials are also used on furniture, again giving us more time to escape;
  • We use inflammable materials whenever possible; for instance we used asbestos for isolation because it does not burn; asbestos is now illegal because it causes cancer
  • Electricity is also a strong factor, although many fire start because of a spark or a melting item powered by electricity, in the old days when we used candles to see and fire to warm our houses, changes of a fire was much greater
  • This is probably less of a factor in the US, but we had that problem (and still have it) in most Old World cities: homes were built very close to each others. As a result, many homes would burn when one caught on fire. Newer construction code prevents building homes to closely to each others. In Europe, also, we build using much more concrete than wood now. Concrete just doesn’t take on fire.

The large drop in fire deaths since 1915 is due to all of these advances, not just fire alarms. However, statistics show us that homes without or with a dysfunctional alarm system have 1.18% deaths compared to homes with a working alarm: 0.53% deaths (from 2009—2013 statistics).

In other words, smoke alarms are not the only factor giving you a chance to escape a fire, but I think that doubling your odd of survival for the price of a smoke alarm is more than worth it.

You Said Nuclear Radiation?

Yes. The smoke detectors have a small amount of Nuclear Material in them. It is normally shown on the device, that is, you are asked not to temper with the alarm to avoid risk of radiation. Really, one device has such a small amount of radioactive material that you are not at risk.

However, by purchasing many detectors and collecting the radioactive material, putting it all together, you can actually end up sick due to high radiation exposure. This has been done in 1994 by David Hahn who built a breeder reactor at age 17. The reactor never reached critical mass, so it was never functional. He was discovered only because he was stopped by police for a different reason and as they search his car they found some of the radioactive material. It was serious enough that the EPA clean the home of radioactive materials.

In 1999, at the University of Chicago, during the Scavenger Hunt, Justin Kasper and Fred Niell created a breeder reactor that was able to create small amounts of plutonium.

In 2008, Taylor Wilson created a nuclear reactor that achieved fusion. This was in a fully controlled environment, though. Within 4 years, Taylor created a detector that governments can use to detect proliferated nuclear materials.

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