how Smoke Detectors Work?

 

 

 

Smoke detectors save lives. Smoke detectors are an invention which cost almost nothing, in view of their usefulness. In India., you can get one for about R1000.00. Indeed, it is recommended that all homes have one per floor.

 

 

All smoke detectors have two basic parts: a sensor to sense the smoke and a very loud electronic doorbell to wake people up. These detectors can operate with a battery or a 9-volt house current of 120 volts.

 

In this article, we would examine the two most common types of smoke detectors: photoelectric detectors and ionization detectors. In addition, we’d also take a look inside the ionization detector.

Let’s start with photoelectric detectors.

 

Photoelectric detectors

When we walk into a store and a bell rings when the door is passed by, a photodetector is used. Next to the door on one side of the store is a light (either a white light and a lens or a low power laser) and on the other side there is a photodetector that detects light.

 

When we pass the light beam, we block it. The photodetector senses the lack of light and triggers a bell. Imagine how this same type of a sensor can act as a smoke detector. If you have enough smoke in the store to block the light beam, the bell goes off. However, there are two problems here:

 

• The smoke detector has to be too large or smoke has to be too much
• It is not very sensitive.

There has to be a LOT of smoke before the alarm goes off and it would have to be very dense to block the light. It takes a lot of smoke to it.

Therefore, the photoelectric smoke detectors use light in a different way. Inside there is a smoke detector and a light sensor, but they are positioned at angles of 90 ° relative to one another. In this case the detector works after receiving scattered light signals and the light signals are essentially scattered by even a small volume of smoke.

Photoelectric detectors are best to detect the smoke emitted from moderate to large fires, such as a mattress on fire.

 

Ionizing detector

The ionization smoke detectors use an ionization chamber and a source of ionizing radiation to detect smoke. This type of smoke detector is the most common because it is not expensive and better identifies even the smallest amounts of smoke produced by flaming fires.

Within the ionization detector is a small amount (perhaps 1/5000 of a gram) of americium-241. The radioactive element americium has a half life of 432 years, and is a good source of alpha particles.

Another way to talk about the amount of americium in the detector is to say that a typical detector contains 0.9 microcurie of americium-241. A curie is a unit of measure for nuclear material. If you are holding a curie of something in your hand, you’re holding a quantity of material that undergoes 37 billion nuclear transformations per second. Generally, this means that 37 billion atoms in the sample are decreasing and therefore a particle emitting nuclear radiation per second, such as an alpha particle. A gram of radium produces about one curie of activity.

 

Ionization Chamber

The ionization chamber is very simple. It consists of two plates with a voltage, with a source of ionizing radiation, thus:

 

The alpha particles generated by the americium have the following properties: they ionize the atoms of oxygen and nitrogen from the air in the chamber. “Ionize” means to “knock an electron”. When you pluck an electron from the atom, we get a free electron (negatively charged) and an atom without an electron, so that atom is replaced by a positive charge. The negative electron is attracted to the plate with the positive voltage and the positive atom is attracted to the plate with the negative voltage (opposites attract, just like the magnets). The electrons in the smoke detector sense the small amount of electrical current, which is precisely the movement of electrons and ions towards the plate.

When smoke enters the ionization chamber, it disrupts this current and the smoke particles attach to the ions and neutralize them. The smoke detector senses the drop in current between the plates and sets off the alarm.

 

Word of advice: Speaking of alarms, these are perhaps the best friend man has (they seem to have replaced dogs) because crime rates in the country are at an all time high and even if you can’t afford security service company or high tech security equipments, you could have a couple of alarms installed at your place for just a couple of bucks and that could prevent burglary to a huge extent.