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Smoke detector phenomenon?

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oucivileng

Sat Mar 28 2009, 05:04AM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

I was setting the spark gap for my new transformers today and my smoke detector went off. I unplugged everything and checked for something burning or the smell of smoke and could find nothing. A few minutes later, I turned the transformers on again and the detector instantly went off. I disconnected and reconnected a few more times giving the air a minute or so to clear but every time I turn it on the smoke detector goes off at the first spark. Also, the detector shuts off almost immediately after I shut the transformers off. I don't see how smoke could have traveled from my floor to the ceiling directly overhead that rapidly or how it could dissipate in such a short period of time.

Can somebody explain what the heck is happening? The detector is battery powered so it's probably not an electrical thing. Ozone couldn't rise that quickly could it? And even if it does, it wouldn't set off the smoke detector, right? The ozone would have to block some of the radiation (is it gamma? i forget) in the detector as it passes through but that doesn't seem very probable to me. Any ideas?

This is actually the second time this has happened. The first time was about a month ago.

aonomus

Sat Mar 28 2009, 05:19AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

There are smoke detectors that have 2 metal plates and measure the capacitance between them, under normal circumstances the capacitance should rarely change, however when foreign particles or ions enter, capacitance changes as charges go to either plate and changes the current across the plates.

Normally the smoke detectors use a radiation source to ionize foreign particles such as smoke to get the current flow, however with various nitrogen oxides and ozone, I think it might just trigger it.

Edit: Also if your smoke detector is close enough, I suppose the RF given off by the coil is enough to trigger the circuit.

Proud Mary

Sat Mar 28 2009, 11:48AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Hiya!

You don't say what sort of detector it is, so I will assume it is the common, cheap ionization chamber type.

In this device, a small alpha particle source - almost always americium nowadays - irradiates an ionization chamber open to the atmospheric air.

As a result, a constant, tiny current - fA - flows through the ionized air between the charged chamber electrodes.

If smoke particles drift into the chamber, they block alpha particles, and so reduce the ionization current flowing through the air between the electrodes.

When the ionization current falls below a specified trigger level, the alarm goes off.

Now in your case, your experiment is releasing massive amounts of ions into the air, sufficient to bring the tiny ionization current in your smoke detector down to its trigger level.

Charged microparticulates - house dust - rising up to the ceiling on convection currents will immediately seek out the charged plates in the ionization chamber, perhaps sticking to them and so altering the chamber current in that way, adding to the effect.

oucivileng

Sat Mar 28 2009, 01:01PM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

Wow. That's pretty cool.

Yeah, it is just one of the cheap, common type. My landlord would not put in anything but the cheapest.

Thanks a lot!

Proud Mary

Sat Mar 28 2009, 01:35PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

oucivileng wrote ...

Yeah, it is just one of the cheap, common type. My landlord would not put in anything but the cheapest.

In some jurisdictions (other countries) the cheap americium detectors have been banned, because of the amount of americium ending up in landfill dumps, and the risk to firefighters from aerosolized americium when the building burns down.

Where they are banned - which they ought to be - a more expensive device must be used, where a laser beam is interrupted by the arrival of smoke particles between the laser diode and a detector. When I say 'more expensive' the real cost of a laser diode in million quantities is only a few pennies, but, of course, as 'environmetaly friendly' detectors there is a special social-economic premium on them.

The real reason americium is used in smoke detectors world-wide is because it is a waste product of the plutonium industry, and this is the most cost-effective (cynical) way for the nuclear industry to sell it off, rather than having to bury it in the ground at great cost. Outside of smoke detectors, this extremely poisonous, very dangerous substance has no other general social use. Smoke detectors are its sole mass-market opportunity.

Herr Zapp

Sun Mar 29 2009, 01:07AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

oucivileng, Harry -

The only thing that doesn't quite fit the "triggered by ionized air from the spark gap" theory is that oucivileng stated that the smoke detector sounded "instantly" when the transformer was turned on.

I'm leaning more towards a "triggered by broad spectrum EMI" explanation.

oucivileng, you could simply tape a piece of Saran wrap over the smoke detector to temporarily isolate it from room air and repeat the experiment. If the detector is still triggered "instantly" when power is applied to the transformer, this provides evidence that ionized air from the spark gap may not be the actual trigger. After this, you could try wrapping the exposed surfaces of the smoke detector with aluminum foil and again repeat the experiment. If the detector is now silent, radiated EMI from the spark gap may be the root cause.

(One other question: Is the smoke detector completely isolated from all other electrical wiring? Some detectors are AC powered with battery back-up, and others are connected into building alarm systems to also trigger a remote monitioring system. If the detector is connected into other wiring in the room, then conducted EMI may also be a potential cause.)

Regards,
Herr Zapp

oucivileng

Sun Mar 29 2009, 01:29AM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

I'll try that as soon as I rebuild my spark gap. It just caught fire.

I guess it was just a little too close to the wood part.

oops.

I should reiterate that it was definitely not burning when the smoke detector was going off. I had shifted some parts around since then.

Yep. It was the EMI causing the detector to go off. The foil seemed to keep the detector from triggering but the plastic didn't.

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