Detecting fire: Smoke but not just smoke – CPS lecture 5

By Penny Carmichael, on 8 November 2012

- Article by Abigail Mountain

Having attended every one of this year’s CPS lectures so far, I have to say that all of them have brought up issues often unthought-of yet so important to our day-to-day lives. Continuing this trend, last night we saw Dr John Shaw from Tyco telling us about Detecting fire, Smoke but not just smoke. Being such a vital piece of equipment, we’d be foolish to overlook the complexity that goes into designing that little white box, and forgive it for occasionally interrupting our morning toast.

 

False alarms, delayed reaction times and the ability to detect any type of fire are all factors that must be taken into account. Aside from our usual Bunsen burner-type flaming, there is also smouldering and pyrolysis to consider, each of which produce different gases and types of smoke. After introducing the most common forms of smoke detection (ionisation chambers, optical scattering), the focus of the lecture move to gas detection. Most real fires generate lots of CO2, water vapour and CO; the first two already being in the air in variable concentrations over time. On the other hand, the amount of CO around us is very minimal and particularly constant. This makes it a very good gas for fire detection – typically 30ppm being the safe threshold for alarms. But is there anything else we can detect? Well, our noses can sniff out a fire extremely quickly. Personally, I love the smell of burning wood, but is there a specific gas or two that we can pick up faster than it takes for CO detector to alert us of the danger?

 

Dr Shaw and his team found some research that showed for your usual pyrolysis we get a load of different gases produced, and at varying amounts as the temperature is increased. But mostly not a lot of any particular thing. However these tests were not proper fire tests and there may be something else we could look at for early stage nuisance fire detection. IR/FTIR analysis would not be adequate simply because the C-H and C-C peaks don’t reach any intensity high enough to detect. Ion mobility spectroscopy was able to distinguish between different types of fires, but how useful that is is questionable, because in the end we just want to be able to tell the difference between any type of fire and cooking or steam. Tyco looked at the gas chromatography-mass spectroscopy (GC-MS) of controlled reproducible fires and although they were able to identify specific molecules, they found that the data collected wasn’t as reproducible as the fire… Nothing was found to be in any appreciable concentration for every single fire and so it was concluded that new gas sensors for fire detection are not an advisable route to go down. Of course, the big negative result of the study is actually a good result for the industry as it was prevent any more time being wasted on research in this area.

 

I suppose this shows just how sensitive our noses are to small changes in concentrations of gases, but we mustn’t rely on them – we can’t have a man sitting in every room 24/7 waiting for that familiar charring smell. Instead, the devices we see on the ceilings around us at work and in the labs are often complex pieces of equipment. More than one sensing technique are often combined, utilising complicated algorithms to determine when the alarm should sound. The goal for companies such as Tyco is still to be able detect fires at very early stages and to have as few false alarms as possible. The sheer number of different things that can ignite makes this a very difficult puzzle.

 

Well I hope this has rekindled your fire awareness. Perhaps a little check of your alarm at home isn’t the worst idea in the World. Next week we have Dr Christoph Salzmann from our very own department giving us an insight into the material chemistry of ice!