Increasing usage of Optical alarms: any concerns?

[Gel]

I note on another newsboard concern being expressed by some CFS staff
at response times of Optical (aka Photoelectric) alarms compared to Ions.

Some have tested in their fire houses the 2 types side by side,
and were alarmed (excuse the pun) at the relative slowness
of the opticals to react.
Has anyone heard of concerns as a matter of interest.?

Clearly both have performance characteristics best suited to particular fire types,
& indeed combined Ion/Photo alarms do exist.

I believe BRE or was it FRS did some testing on this issue some years back but believe
it's findings are not in public domain, though should be!!

[wee brian]

Most of the safety concerns about choice of alarms relates to criticisms of Ion technology.

Theres much concern about the fact that ions will miss some types of smouldering fire altogether.

NIST did a lot of work on this a while ago. Some countries are dropping Ion technology altogether. I would be a bit worried if a few tests in a fire house were used to set a brigades advice to the public.

Whilst I would be worried, I wouldn't be suprised.

[Gel]

Thanks wee brian.
I think other countries dropping is primarily driven by the strong
green lobby in some mainland European Countries.
However WEEE regs which finally came into force on Sunday may
have a future bearing; alarms are currently exempt.

Other board comment:
The trouble is they are generally less sensitive full stop. I have
done some experiments in our fire house. I placed one of each
detector (both XXX incidently) and lit a small waste paper bin fire
in the corner of the room. The ionisation alarm activated after about
10-15 seconds. The optical alarms took between 2.5 - 3.5 minutes.!!!  
(this experiment was repeated 10 times using new alarms every time).

Totally unacceptable.

I appreciate that this was a small fire and it was not growing, however a fire is a fire. This gave off alot of smoke and the neutral plane was about a metre from the floor before the optical detectors activated. Still ample smoke to kill someone!

I just do not think that battery powered optical detectors are up to the job. ( I have never encountered a problem with an optical head on a 5839 part 1 system)

[kurnal]

A few years ago I attended a fire in a modern semi, they had mains interlinked ion alarms. The fire started in the kitchen at night where a tumble drier was in use on economy 7. The whole house was filled with thick black smoke from the smouldering fire but the detectors did not operate at all.  The family all had black soot streaming from their noses and fortunately the smoke eventually woke the father. Although they were taken to hospital fortunately there were no long term health effects.  I was called in to investigate the fire and the smoke alarms. It was as Wee B suggests "The wrong type of smoke" for an ionising detector. The alarms worked fine with the right kind of smoke.
Hence the general advice for a mix of opticals in rooms and ionising in corridors and staircases.
Disposal of ionising detectors is going to be an increasing problem in the future and this is why many countries in europe are moving away from them. But there are alternatives and the multi sensor intelligent detectors with dual chamber optical ( work on both obscuration and light scatter) in conjunction with a heat detector working on rate of rise- if the temperature increases at a fast rate the sensitivity of the opticals is also increased. Some add in CO detection as well. This is the way forward, and both eliminates false alarms and beats  ionising detectors for speed of response in all smoke conditions.

[wee brian]

Gel

The countries that are dropping Ions are doing it because of concerns about their sensitivity.

Ions are very sensitive to certain types of incident, but they can be completely blind to others.

In the scheme of things I dont think it's that big a deal but dont go telling people that Ions are better than opticals. Unless you are well insured of course.

[John Webb]

The BRE work (which I did!) is in the public domain in that it was work for the Home Office Fire Research and Development Group. Cath Reynolds was their contact and she published a summary on behalf of FRDG. This was circulated to fire brigades, I understand. She is now in charge of DCLG's fire research and may still be able to supply copies of this publication.

The work however is nearly 20 years old, and I'm not certain quite how comparable today's detectors are with the ones available then.

That study gave an indication that with small fires with low energy, the optical detectors were slow to respond. It was thought this was due to the laberinthyn entry to the detecting chamber, needed to keep out light, which also needs more energy to push smoke through into the chamber compared with the ion detectors. This was based on the fact that such detectors in a wind-tunnel with a gentle flow of air showed very little delay in responding to changes in smoke level.

[Gel]

Interesting John; I'm sure some more recent work was done in last 5 years.
|Done at Cardington my grey matter is telling me if that's a clue.

If anyone should have any knowledge pse do post details of where to find.

[John Webb]

Gel,
Work you may be thinking of was looking at mains powered domestic detectors, completed in Jan 2000, I believe. Contact Nigel Smithies at BRE (01923 664000) or via www.bre.co.uk

The work on comparing smoke detector standards where problems of getting optical detectors to alarm was published in 1997, so it's more recent than I remembered. The summary Report can be downloaded from the www.communities.gov.uk website - go to the 'fire and resilience' section and find summary reports.

[Gel]

John, thanks again; found the 97 data at

http://www.communities.gov.uk/pub/360/781997ComparisonofdomesticsmokealarmsensitivitystandardsPDF191Kb_id1125360.pdf

in case anyone else interested.

[Jason Miller]

Another problem is that people do not generally have all the available facts when they buy so they may be buying heat detectors to put in hallways because they heard that detectors "always go off when you burn toast" or because one type looks nicer than another.

Also, positioning is normally hopeless in homes. I'm in a rented holiday home at the moment with 4 detectors and all are tucked into corners and apexes where they would be very unlikley to operate in a fire - none are linked.

There are still plenty of hurdles to get over in home safety.

Jason

[Uncle Dave]

Gel,
Work you may be thinking of was looking at mains powered domestic detectors, completed in Jan 2000, I believe. Contact Nigel Smithies at BRE (01923 664000) or via www.bre.co.uk

John, I think Nigel has left BRE.

Dave

[John Webb]

Uncle Dave,
It's possible - he was there a year or so ago when I dropped in to BRE - but if Gel tries the number above they should be able to find someone who knows something about it.