Naked Science Forum

Non Life Sciences => Technology => Topic started by: neilep on 30/09/2007 21:06:34

Title: How Does A Smoke Detctor Detect Smoke ?
Post by: neilep on 30/09/2007 21:06:34
Dear Peeps,

Hopefully ewe have a few of these in your house:

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...and hopefully ewe have live batteries in it too !!!

A few questions about smoke detectors ..if I may ?

How does it detect this ?

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What is doing the detecting  ?


Why do I need to change it every few years ?

Why does putting one in the kitchen always make it go off ?.
..even it's it's NOT smoky !!..but perhaps a little steamy !!!


I'll be alarmed if ewe can answer ALL the questions...!!

hugs et les shmishes

Neil
xxxxxxx

Title: How Does A Smoke Detctor Detect Smoke ?
Post by: paul.fr on 30/09/2007 23:37:45
If you take a look inside a normal smoke alarm, you'll see a loud speaker, a battery, electronics, and a silver coloured cover. Underneath this cover is about 0.1g of a radioactive element called americium. This is a very heavy element with a nucleus that loses helium atoms, and these high-speed helium atoms are what we call alpha particles. We can detect and count alpha particles with a Geiger counter - very click represents one alpha particle hitting the detector. Our smoke detector (with the cover removed) gave off around 2000 alpha particles every second, which makes it a really quite radioactive source.

But what role does this radiation play inside a smoke detector? The electronics of a smoke detector consists of two metal plates separated by air. This means that an electric current can pass through most of the circuit but is forced to stop when it reaches the gap because air is a good insulator - that is, it can't carry an electric current because electrons can't move through it very easily.

This is where the radioactive americium comes in. The high-speed alpha particles fly into the gap and knock off electrons from air molecules. These free electrons fill in the gap and allow an electric current to flow through it. When the circuit is complete, the alarm does not sound.

In the event of a fire, tiny smoke particles move into the gap and mop up the free electrons, which stops the current flowing and breaks the circuit. The electronics can detect this change and sound the alarm. We can see this by burning something like a leaf near the smoke alarm.

Thankfully most people never experience a real house fire, but setting the alarm off while cooking sausages or burning toast is a much more common occurrence. Although there's no fire in these situations, they still produce lots of small particles, which soak up the electrons in the gap and break the circuit.


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Written by Dave Ansell


(whoever he is !)
Title: Re: How Does A Smoke Detctor Detect Smoke ?
Post by: syhprum on 10/12/2018 09:45:30
.1 gram, micro or nano gram surely
Title: Re: How Does A Smoke Detctor Detect Smoke ?
Post by: Petrochemicals on 10/12/2018 11:43:40
Sadly david hahn has now departed.

https://en.m.wikipedia.org/wiki/David_Hahn
Title: Re: How Does A Smoke Detctor Detect Smoke ?
Post by: Colin2B on 10/12/2018 12:45:43
.1 gram, micro or nano gram surely
Donít understand what you are saying
Title: Re: How Does A Smoke Detctor Detect Smoke ?
Post by: alancalverd on 10/12/2018 13:14:59
 A gram of americium 241 emits 130,000,000,000 alpha particles per second.

About 0.3 microgram is entirely enough, and allows for small quantities of smoke detectors to be scrapped as nonradioactive electronic waste.

David Hahn, inter alia, generated neutrons by using Am241 sources to activate berryllium substrates, with a view to initiating a chain reactionin his garden shed.
Title: Re: How Does A Smoke Detctor Detect Smoke ?
Post by: Petrochemicals on 11/12/2018 00:01:57
A gram of americium 241 emits 130,000,000,000 alpha particles per second.

About 0.3 microgram is entirely enough, and allows for small quantities of smoke detectors to be scrapped as nonradioactive electronic waste.

David Hahn, inter alia, generated neutrons by using Am241 sources to activate berryllium substrates, with a view to initiating a chain reactionin his garden shed.
Apparently he used 1000 pounds worth of lithium plus more

Quote
Hahn diligently amassed radioactive material by collecting small amounts from household products, such as americium from smoke detectors, thorium from camping lantern mantles, radium from clocks, and tritium (a neutron moderator) from gunsights. His "reactor" was a bored-out block of lead, and he used lithium from $1,000 worth of purchased batteries to purify the thorium ash using a Bunsen burner.[3][4]

Hahn posed as an adult scientist or high school teacher to gain the trust of many professionals in letters—and succeeded, despite misspellings and obvious errors.[citation needed] Hahn ultimately hoped to create a breeder reactor, using low-level isotopes to transform samples of thorium and uranium into fissionable isotopes.[5]

That is some boy scout. Not sure how well this approach would pass to provide for your immediate needs outdoors.