How does a smoke detector work? There are two different types of smoke detectors, and they respond to different types of fires. There are photoelectric smoke detectors that respond to smoke from smoldering fires and there are ionization smoke detectors that respond to smoke from flaming fires.
Photoelectric smoke detectors have a chamber that has an LED (Light Emitting Diode) on one side and a photoelectric receiver on the other side. Because light flows in a straight line, the two are positioned so that the light from the LED does not land on the photoelectric receiver. When a fire starts in your house, smoke floats up to the ceiling and smoke particles enter the chamber of the photoelectric smoke detector. These particles float in front of the LED, causing the light flowing from it to scatter. Some of that light hits the photoelectric receiver, which sends an electrical signal that starts the alarm.
Ionization smoke detectors have two chambers. One is open to the air and the other isn’t. They both have a small piece of americium-241 and a voltage meter. Americium is a radioactive element that was first synthesized during the Manhattan project in 1944. It emits alpha particles which are completely safe because they cannot penetrate our skin. In the smoke detector, they electrically charge the air molecules that they come into contact with, and the voltage meter picks up this current. When there is a fire in a room, smoke will float into the open chamber but not the closed chamber. Some of the ions from the americium-241 will attach to the smoke particles instead of the air molecules and this will reduce the electric charge that reaches the voltage meter. The charge in the closed, control, chamber will not change and when there is a difference in current between the two chambers, the device detects the difference and sets off the alarm.
Photoelectric smoke detectors are far better at quickly alerting residents to the presence of smoke. When a fire starts, it usually begins by smoldering and releases large particles that a photoelectric detector is better at detecting. Once the fire gets stronger, the particles get smaller. So, a photoelectric detector is better at telling you that a fire has just begun, while an ionization detector can tell you that one is burning right now. In fact, some countries have banned ionizing smoke detectors. It is possible to get a smoke detector that performs both functions, or you could get both detectors. However, studies show that a photoelectric detector is the better choice.
The first electric fire alarm was invented in 1890, but it worked by detecting heat. As it is smoke and not heat that is the biggest danger with fires, by the time the alarm sounded it was usually too late. The ionization smoke detector was invented by Walter Jaeger in 1930, but he made it to detect poison gas. It wouldn’t work because the concentrations of particles in poison gas were too low and the particles too small to affect the current. It was only when he stopped to have a cigarette underneath it and the meter recognized a drop in current that he realized its potential as a smoke detector. The photoelectric smoke detector was invented by Donald Steele and Robert Emmark in 1972.
In a domestic fire, most people are killed by smoke inhalation and not burns. Smoke inhalation is known as the “silent-killer” because it quite often kills people before there is even any sign of a fire, and often when they are sleeping. Twice as many people die from smoke inhalation as from burns. Smoke inhalation can kill in two different ways. Firstly, it can reduce the amount of oxygen that the body can take in. The particles in the smoke can become lodged in the lungs and prevent the body passing oxygen from the air to the blood and carbon dioxide back. This can lead to asphyxiation. The second way is through poisoning. Many things that burn in a regular house fire are synthetic and they release poisonous chemicals. They can produce, ammonia, carbon monoxide, chlorine, hydrogen chloride, hydrogen cyanide, hydrogen sulfide, and sulfur dioxide. These poisons can work on the body even after someone has been rescued from a fire and medical attention is necessary.
The invention of the smoke detector has saved countless lives and can be likened to the invention of the seatbelt. Statistics show that in three out of every home fire deaths, there was either no smoke detector or a detector with a dead battery. Install a detector and check your battery regularly! And this is what I learned today.
Sources
https://www.nist.gov/how-do-you-measure-it/how-do-smoke-detectors-work
https://en.wikipedia.org/wiki/Americium-241
http://resources.schoolscience.co.uk/Corus/14-16/rad/psch5pg2.html
https://en.wikipedia.org/wiki/Smoke_detector
https://www.medstarhealth.org/blog/smoke-inhalation-fires-quiet-killer