Smoke Detectors 4/11/22

Science with Richard Bleil

Do you want radioactive isotopes in your house?  I sure hope so.  In fact, I hope you have them already.  Eesh, I feel like this is the start to a middle school essay. 

Having brunch with a friend, we got to talking about smoke detectors.  She is a good “sounding board” for me, because she lets me talk about science and technology like the geek that I am and keeps me grounded about what most people do and do not know about science.  For example, I was talking about how smoke detectors work, and cut myself short thinking most people already know.  She told me that she does not.  So, let’s talk smoke detectors.

First and foremost, let me point out that there are a variety of types of smoke detectors.  Some, for example, detect heat.  The smoke detectors I want to talk about are the traditional little effective but inexpensive common smoke detectors.  Like the one that I set off by taking a particularly steamy shower a few months back, or the one that woke me up in the wee hours as my house filled with smoke from an actual unexpected house fire.  Just to put this to rest, it was an electrical fire, and small enough that there was no harm done, save a lot of smoke and a lost night of sleep. 

First, let’s talk about radioisotopes.  The element in my smoke detector is Americium (symbol Am).  A radioisotope is an unstable element, meaning it “decays” into something different.  If you want to know what is in your smoke detector, look inside the cover (that you open to replace the batteries) and look for the radioactive symbol.  Mine has small print identifying that the radioisotope is specifically.

As it decays, it gives off radiation, but there are three types of radiation.  Americium gives off “beta rays”, which is in essence a stream of electrons.  In that little component with the radioactive symbol, they have the beta-emitter on one side, and a plate on the other which is part of a circuit.  As the electrons hit the plate, the circuit detects this as current (because it is), and the circuit reads this constant current as a “closed circuit”, like a switch that is in the “on” position.  In the switch, all that’s happening is the wires inside of the switch are allowed to make contact allowing current to flow, “closing” the circuit.

When smoke gets into the smoke detector, and into this component specifically, it will interrupt this flow of electrons (or water droplets as I discovered).  The electrons will hit the small smoke particles instead of the plate, so the plate will no longer be receiving this current.  The circuit will detect this lack of current as what is called an “open”, like turning off the switch, or cutting a wire in the circuit.  When this happens, a secondary circuit will be triggered, which sounds the alarm.

There are a couple of things to remember about smoke detectors, which should now make sense.  First, you have to keep them clean.  This mechanism cannot detect between smoke from a fire, smoke from my terrible cooking, or water droplets from my decadent shower.  All it knows is that something is blocking the flow of electrons.  This can also be dust, so be careful with anything particulate that heads towards the detector.

As I said previously, this works because the radioisotope providing the electrons is decaying.  There is not a lot of radiation given off by these detectors, so they are safe to be around and to handle, but don’t open the component with the radioactive symbol.  But because the radioisotope is decaying, it will eventually become Thallium.  As this occurs, the radioisotope will become less concentrated and more contaminated, resulting in weaker and weaker beta emissions.  This is why you must pay attention to the life span in the information sheet that comes with the detector.  If your detectors are too old, or if you do not know how old they are (such as in a house you just purchased), go ahead and replace them.  Typically, the life span of a new smoke detector is roughly between eight and ten years. 

Be sure to get plenty of them as well.  Every hallway, and every room should have a smoke detector, and consider the type of smoke detector to get based on where it will be placed.  Look at the characteristics of the ones you’re purchasing.  Some are slower to sound than others, which might sound bad, but in a kitchen, for example, that might be perfect, or get a heat-based on for the kitchen instead of smoke depending on your cooking skill.  And get at least one carbon monoxide (or combination) detector as well.  Every combustion process (such as gas heaters, stoves, water heaters or, of course, cars) emit some level of carbon monoxide, which is odorless and deadly.  It’s a very light gas, so it will rise.  Place the carbon monoxide detectors high, especially where the gas might get “trapped” and accumulate.  For example, I have one at the top of the stairs from the basement, above the door that is kept closed, placed on the side of the basement.  If you have an attached garage, it would be good to place one high in there as well.  Consult with an expert if you have questions or concerns (I myself work with ADT).  Good luck and stay safe!


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