FLASHOVER is something dreaded by firefighters. It is the point at which the temperature in a room has risen so far that everything inflammable ignites spontaneously. In days gone by, when houses were draughty and thus cooler, and rooms ...
FLASHOVER is something dreaded by firefighters. It is the point at which the temperature in a room has risen so far that everything inflammable ignites spontaneously. In days gone by, when houses were draughty and thus cooler, and rooms were filled with furniture made from natural materials that were slow to burn, it could take 15 minutes or more for a fire to reach the point of flashover. Now, though, buildings are better insulated and furnishings are stuffed with hydrocarbon-based foams. In these conditions flashover can happen within three.This means that whereas firefighters once had time, after they had arrived at a blaze, to scout it out before it had taken hold, they are now confronted with situations that could flashover at any moment. This difference can be deadly.Ironically, modern fireproof suits may have made the situation worse. In the past, a firefighter would have felt the heat building up towards flashover and would thus have been forced to retreat. Today’s fully encapsulated suits mean he must rely instead on visual clues that flashover is imminent, such as flames rolling across the ceiling or a scrumpled-up ball of paper bursting alight as it reaches its flashpoint (supposedly 451º Fahrenheit, according to Ray Bradbury’s novel of almost that title). These sorts of warnings, though, are dangerously imprecise.For the past five years, researchers at the Worcester Polytechnic Institute in Massachusetts, led by Kathy Notarianni, have been trying to understand flashovers—the better to predict exactly when they will occur. The team have carried out a series of test burns that simulated different types of house fires, in wooden structures built to mimic standard American homes. One included an actual sofa, armchairs, television, carpeting, doors, windows and curtains.Dr Notarianni’s team found that high air temperatures alone are not enough to trigger flashover. What is needed is a layer of particularly hot gas just below the ceiling. This radiates heat into the room below, causing flashover even when the air temperature in the rest of the room is relatively low. Older, draughtier rooms let these gases leak out, and slower-burning furniture means there is less hot gas in the first place. Dr Notarianni found, though, that as she added things like plasterboard sheeting to the ceilings of her experimental rooms, gas accumulated more easily, and flashover became more likely.In light of this knowledge the team have devised a system that uses hardened thermocouples and thermal flux sensors to feed data to a computer program which follows the progression of the fire and constantly re-calculates the countdown to flashover. In the early stages of a fire, this program’s predictions vary a lot from moment to moment, but as the point of flashover approaches they become more and more accurate. Within the final, crucial, 30 seconds the prediction becomes very accurate indeed, giving firefighters enough warning to get out without having caused them to evacuate needlessly early.The next stage is to miniaturise the hardware, so that it can be fitted to a firefighter’s helmet. To do that, Dr Notarianni has received a grant from the Federal Emergency Management Agency. There is, though, one further problem to solve. The prototype gives its best results when the sensors are about 30cm beneath the ceiling. But not only are firefighters not tall enough for their helmets to be at that height when they are standing up, they also tend to navigate burning buildings on their hands and knees. Ideally, then, a better way of interpreting what is going on based on temperatures near the floor needs to be developed—though with luck this will only mean tweaking the software. That done, the new flashover detector should liberate firefighters from constantly having to keep an eye on screwed up balls of paper, and thus let them get on with trying to put the fire out before flashover happens.
