Mounting fire / smoke alarms with magnets

[Franly]

I am building a detachable smoke detector that I'm planning to install with magnets. I am concerned about the safety using this method, because of my knowledge of how magnets react to / affect heat and smoke. I wouldn't risk having the alarm detach itself during a fire, or the magnet to interfere with the smoke. Can someone here explain to me whether this would be a viable solution or not?

 

[Baluncore]

Welcome to PF.
The magnet will release when the temperature of the magnet, or the magnetic material the magnet is attracted to, exceeds the curie point. The plastic case will have melted long before that will happen.
https://en.wikipedia.org/wiki/Curie_temperature

Might the presence of the magnets prevent the detector functioning? The magnetic field will be largely restricted to the magnetic path of the attachment, so it is unlikely.

Photoelectric detectors sense smoke early, while ionisation detectors sense the product of later flame.
There should be no problem if you use a photoelectric detector as they should be immune to magnetic fields.

Ionisation detectors may be effected since the path taken by a charged particle will be influenced by magnetic fields. But the balanced ionisation chambers will probably cancel any magnetic distortion. Depending on jurisdiction, a smoke detector may be prohibited if it has a replaceable battery, or employs an ionisation detector.

 

[berkeman]

To minimize potential problems, just be sure to use a magnet that has a closed magnetic path when attached to the ferrous metal surface. Horseshoe magnets (regular or cylindrical) have this property, for example.

BTW, not many walls have ferrous metal in them (unless this application is for a ship). Are you intending to attach this detector to some metal appliance like a refrigerator or dishwasher?

Cylindrical Horseshoe Magnet:

https://www.imagnetshop.com/661-large_default/19-ima731.jpg

 

[anorlunda]

To fulfill its mission, the device should give the alarm long before the temperature becomes hot enough to reach the curie temperature, or hot enough to melt the plastic. After the alarm is given and the people (hopefully) evacuate, nobody cares if the smoke alarm keeps beeping as the structure burns.

 

[Franly]

Welcome to PF.
The magnet will release when the temperature of the magnet, or the magnetic material the magnet is attracted to, exceeds the curie point. The plastic case will have melted long before that will happen.
https://en.wikipedia.org/wiki/Curie_temperature

Might the presence of the magnets prevent the detector functioning? The magnetic field will be largely restricted to the magnetic path of the attachment, so it is unlikely.

Photoelectric detectors sense smoke early, while ionisation detectors sense the product of later flame.
There should be no problem if you use a photoelectric detector as they should be immune to magnetic fields.

Ionisation detectors may be effected since the path taken by a charged particle will be influenced by magnetic fields. But the balanced ionisation chambers will probably cancel any magnetic distortion. Depending on jurisdiction, a smoke detector may be prohibited if it has a replaceable battery, or employs an ionisation detector.

Thanks, both for the welcome and the thorough answer!

Correct me if I'm wrong, what I get from this is that as long as I use a photoelectric detector, the magnets won't influence the detection performance and the magnets will hold until they exceed the curie point - which means that it will be long after a fire alarm has served its purpose and would have melted anyway?

 

[Franly]

To minimize potential problems, just be sure to use a magnet that has a closed magnetic path when attached to the ferrous metal surface. Horseshoe magnets (regular or cylindrical) have this property, for example.

BTW, not many walls have ferrous metal in them (unless this application is for a ship). Are you intending to attach this detector to some metal appliance like a refrigerator or dishwasher?

I was actually intending to use a flat cylindrical magnet, but didn't know that it would minimize potential problems. Could you please elaborate on this, explaining me what pro's and con's there would be using a "normal" magnet to a cylindrical one?

I intend to have a ceiling plate attached with screws. This plate is equipped with the magnet that the detector attaches to.

 

[Franly]

To fulfill its mission, the device should give the alarm long before the temperature becomes hot enough to reach the curie temperature, or hot enough to melt the plastic. After the alarm is given and the people (hopefully) evacuate, nobody cares if the smoke alarm keeps beeping as the structure burns.

Great point, thanks!

 

[berkeman]

I was actually intending to use a flat cylindrical magnet, but didn't know that it would minimize potential problems. Could you please elaborate on this, explaining me what pro's and con's there would be using a "normal" magnet to a cylindrical one?

The important point is that the magnetic field be confined to the magnet and the ferrous plate it is stuck to, and that the external fringe field is very small. If there are any inductive components in your circuit (inductors, cores, etc.) that use ferrite in their construction, external stray DC magnetic fields can alter their performance and potentially cause the circuit to not work well or fail altogether.

Horseshoe shaped magnets have such a property when they are attached to a ferrous surface. "Horseshoe" magnets come in different configurations, from the traditional U-shaped version to the cylindrical version that I posted a picture of. You can also get cylindrical magnets where the two poles are coaxial, with a center pole surrounded by a peripheral pole:

https://www.magnetics.com/product.asp?ProductID=42

While a "horseshoe" type magnet is not attached to anything, there are still stray fields that can interfere with the operation of inductive circuits that contain ferrite. I don't know if that's an issue in your appliation or not. One solution to that is to add a "keeper" piece of ferrous metal that attaches to the magnet on the product when it is not mounted on anything.

Field when not attached to anything:

https://cpb-us-e1.wpmucdn.com/sites...5/hsmagnet-142sair-e1547587903347-300x300.jpg

Field when attached to ferrous metal:

https://www.kjmagnetics.com/blog.asp?p=why-horseshoes

Compared to the external stray field of a regular bar magnet (yikes!):

 

[Vanadium 50]

To fulfill its mission, the device should give the alarm long before the temperature becomes hot enough to reach the curie temperature

The Curie temperature for iron is 1400F. By that point, I would hope everyone is already out of the building.