From MVS Chandrashekhar, University of South Carolina
Editor’s Note: MVS Chandrashekhar is Professor of Electrical Engineering at the University of South Carolina. In this interview he explains how smoke alarms work and why they sometimes trigger an alarm for no reason.
Why do smoke alarms go off even when there is no smoke?
The most likely reason smoke alarms go off unexpectedly is because the batteries are not changed often enough. Most of the sensors you might think of will increase the strength of the signal when they detect what they are supposed to. However, most smoke alarms are designed to go off when their electrical current drops. That’s because smoke in the air reduces electricity. When your battery runs out, the current flowing through your sensor will also decrease. And so you can get a false positive.
People often change the battery when they move into the house and never touch it again. The battery should be changed at least every six months, but most of us don’t. Only when the smoke alarm goes off do you check it, make sure you are not dead, and then possibly change it.
The second most common cause of false positives is that your smoke alarm is too close to your bathroom. When you shower hot, the steam from the hot shower can cause false alarms in some cases. The steam from the shower can block the flow of electricity, as can smoke. Anything heavy in the air can cause this to happen. You want the smoke alarm to be near the kitchen as smoke is common when cooking. So open the window if you don’t want it to open by accident.
Another common cause is the volatile organic compounds in paint or other chemical treatments in the house. It’s the stuff that keeps your paint wet but also lets it dry once it’s on the wall. Some of them can also trigger these alarms. It just depends on how the sensor is set.
Most of the new sensors are pretty well matched. They were nowhere near as good at sensitivity 10 years ago.
How do smoke alarms work?
There is a small source of americium-241 radiation inside your smoke alarm, which is a by-product of nuclear fuel. It emits alpha particles that can be thought of as small spheres. These little balls come out of the source and hit air molecules to break them apart.
In this case, some fragments will be charged positively and others negatively. And these two oppositely charged are attracted to the negative and positive battery terminals in the smoke alarm battery. We call this movement of charged particles an electric current.
When smoke gets into the area where this break-up is taking place, it blocks the movement of the charged particles and reduces the current. This reduced current is interpreted by your sensor to mean that there is smoke here.
Is there a better smoke alarm?
A newer type of smoke alarm is based on the photoelectric effect. For this Albert Einstein received the Nobel Prize. When light hits something, it creates electricity – it is very similar to a mini solar cell. The engineers found out how a light source can be made sensitive to smoke.
The light can shine and you get electricity. However, when smoke enters, the light is otherwise diffused or blocked in some way and this changes the amount of current flowing.
When properly set, you can interpret this change in current as the presence of smoke. Again, you can get false positives because organic compounds are often very good at absorbing infrared light. In a way, it is similar to americium-241 smoke alarms. The photoelectric ones are likely to be more energy intensive. As a result, your battery life may not be that good. But hey, you should still change it every six months.
MVS Chandrashekhar, Assistant Professor of Electrical Engineering, University of South Carolina
This article is republished by The Conversation under a Creative Commons license. Read the original article.