Anyone who fills up a car at a petrol station will be familiar with the signs
banning smoking and, more recently, the use of mobile phones. It is pretty
obvious that the naked flame of a burning cigarette is a real hazard, but is a
mobile phone really going to blow you into the next world? What is the science
behind the story?
Fire
Chemically speaking, a fire is an
exothermic oxidative reaction, which is to say that a substance, the fuel, is
persuaded by heat to react with oxygen, with the production of further heat.
Three components are therefore necessary to start a fire: fuel, a source of
oxygen, and a source of heat. Assuming further supplies of fuel and oxygen, the
heat given off is then sufficient to maintain the fire.
Fuel
Some substances can be persuaded to burn more easily than others. This is due
to the need to convert the fuel in its original form to a gas or vapour, in
which form it mingles more readily with the oxygen molecules. Any solid will
melt at a sufficiently high temperature to form a liquid, and similarly any
liquid, when heated further, can be turned into a gas or vapour, but each of
these transitions requires further energy. Unless this energy is supplied by the
fire it will go out. This helps to explain why flammable gases make good fuels,
and why it is harder to get a coal fire going than a petrol fire.
A source of oxygen Air is 21% oxygen, and, as everyone knows, that is quite sufficient to
maintain a fire. Fire extinguishers work by either denying the fire access to
this oxygen, by covering it with a layer of foam, powder or inert gas, or by
cooling everything down to below the ignition temperature. An atmosphere
enriched with oxygen (compressed gases, fumes from liquid oxygen, nitrous oxide)
is correspondingly more dangerous. Oddly enough, a fire partly starved of oxygen
can still be dangerous: instead of completely burning to carbon dioxide, it
burns to carbon monoxide, also a deadly poison, instead.
A source of heat It is with this point – how much energy in the form of heat do we net to
ignite the fire? – that we have the most difficulty.
The properties of a fuel What makes a substance flammable is its readiness to form a vapour.
The flash point of a liquid is the temperature at which sufficient
vapour is released to allow ignition. This is pretty low for petrol (-43 °C)
but this obviously does not mean that spontaneous ignition will occur at that
temperature. However, it does mean that at any temperature above this, a
certain amount of heat applied to even a small amount of the vapour may start
the conflagration.
The spontaneous ignition temperature of a fuel is much higher than
the flash point. For petrol it is somewhere between 200 °C and 300 °C.
Petrol dripping onto the engine casing may not catch fire; petrol dripping on
the hot exhaust very likely will.
The vapour density is another property to consider. A vapour that is
lighter than air will dissipate readily in an open space, but petrol vapour is
heavier than air, so it will drop to the ground and spread. The puddle of
petrol on the ground may be small but the area of danger is much greater. This
is particularly dangerous in the enclosed space of a garage.
The properties of an ignition source Heating a fuel takes energy. It need not be much: only a few molecules of the
fuel/air mix need to be raised to the ignition temperature in order to start the
fire. A burning match can do it when close to the mixture. A spark caused by
striking metal on metal, or metal on mineral, could do it if it is within the
vapour cloud.
However, when all is said and done, the temperature rise is the important
factor. Distant sources of energy, such as the radiation from the sun or a
mobile phone transmitter, need to be much greater, if not hugely greater, to
achieve the same effect. Can a mobile phone achieve this?
The properties of a mobile phone A mobile phone contains a battery which delivers current. Sometimes it even
illuminates the screen. Surely somewhere within there is sufficient energy to
cause ignition. Perhaps unfortunate circumstances may allow sparking inside it?
All tosh. All complete rubbish. It is certainly true that sparking may occur
inside mains-powered equipment, particularly across the contacts of the mains
switch as it is opened. It is also true that such equipment may contain hot
spots. The cathode-ray tube of a conventional TV set needs a heater, and very
high voltages to accelerate the beam. Any component failure which causes a short
circuit could result in a large current and a hot spot. It is easy enough to say
that petrol vapour should not be mixed with the inside of a TV set, but the
situation is fundamentally different with a mobile phone. The voltages are low
to start with, some 5 volts instead of 240, and the currents much smaller, so
the overall power much less. The power radiated by a transmitting mobile phone
is of the order of 1 watt, which is dissipated in the huge volume of atmosphere
reaching to the transmitter. Can it cause local heating? Well, have you ever
noticed the mobile getting hot in your hand? Have you ever noticed your ears
burning? Well, have you? If you worry about 1 watt raising the temperature of
the petrol forecourt, then start worrying about your 60 watt headlights instead.
Has a mobile phone ever caused a catastrophe on a petrol forecourt? There is no recorded case in the UK, in the mainland of Europe or in the
United States. It is even doubtful if the few cases from the Far East which
circulate on the internet have any bearing on reality. One of them relates to an
Australian episode in which the official report unequivocally denies the
involvement of a mobile phone in the explosion.
Conclusion Unless you strike two mobile phones together to make fire, or dunk one in
petrol and set a match to it, the chance of coming up with the jackpot is
probably greater than that of setting fire to a petrol station. I don’t think
I’m the original what-me-worry kid, but in this instance I shall carry on not
worrying.
Copyright PHP Harris 2003
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