Cellphones -
a fuel pump risk!

An article in a morning paper (Metro, London edition, Pg2, 23rd April 2002). It read.....

Mobiles a fuel pump risk!

DRIVERS have been warned to switch off their mobile phones at petrol stations because of the danger of them sparking fires. The alert follows three incidents where phones have ignited petrol fumes after motorists answered calls while filling their vehicles. One person was left with burns to the face and another received groin injuries. The NHS health chief called for warning signs at the entrances of petrol stations, adding: 'I urge people to remember to switch off phones and keep them off until they leave.'

Now there are a lot of those "how do we reduce our network loading" pranks, and there have been some real howlers! These surface during school holidays etc. when the networks are straining and the business sector that keeps the networks in the black are battling to even make phone calls.

However, this is not one of them. This is a very, very real danger. However, it appears to be a highly contentious issue!

Ever heard of a chap called Marconi? Thought so. Do you remember why he was so well known? Something about radio? You're right. However, many forget that Marconi's first experiments were conducted with a spark generator and receiver. Ok, so the cellphone is a little more complicated than his spark generator, but the output was still the same thing - radio waves.

We are going to get technical for a paragraph; When one sticks the petrol bowser nozzle in the filler a serious resemblance to a resonant cavity begins to form. Should the nozzle be at the correct depth, the point at which it enters the filler becomes a high impedance point i.e. a place where, in the presence of sufficient RF energy (hey, is the antenna not just a short distance away?), a large voltage could be generated.

So all that is required is a tuned circuit that is near the cellphone's transmitter frequency, and this tuned circuit has a small gap where a spark could happen (in essence a spark receiver), and this gap is filled with a mixture of petrol vapour and oxygen, and..... groin injury - ouch!!!

I have had some people claim that if one does all the calculations regarding signal degradation etc. that one would see that insufficient energy is released to be able to create a spark. Well, this is not true. What no-one appreciates is that there are a large number of factors that have to come into play before a problem (blast) can occur.

One other theory is that the phone can cause a spark on the battery contacts. Sure, when the phone disconnects from the battery on being dropped it could cause a spark. However, it is in the open, petrol fumes rise, and therefore the chance of having a blast is pretty close to nil unless you could get enough fuel vapour to settle on the floor.

However, there is more than enough vapour (especially as the fuel is being agitated when pumped into the tank) at the entrance to the filler. If the user were to "change hands" while talking on the phone, there is a high possibility that sufficient energy would be dissipated into the filler and cause a spark between the bowser and filler edge.

A further transfer mechanism is via the now commonplace overhead bowser hoses. Such hoses have (and always have had) an earth wire running down the middle. If this hose runs close enough to the user's head while talking, then sufficient energy could find its way down to the bowser nozzle. Remember, the energy is transferred by electromagnetic waves, not by direct contact.

A question is arising as to the 'official risks'. I don't think there are any. The problem does not happen often enough to cause any form of research to be conducted. What I do know is every radio ham I know makes it a golden rule to ensure that his radio cannot go into transmit while on a petrol station forecourt. They were among the first to discover the consequences of such actions (and because the frequencies they talk on are generally lower the probability of the situation occurring is a lot higher).

The risk is real - but not enough for governments to worry about. There are just so many factors that have to come in to play for this situation to occur (vapour at the correct density, bowser at the correct position in the filler neck, the correct type of filler neck, the cellphone is actually transmitting, the energy transfer can take place i.e. everything is tuned to the transmit frequency,... the list is endless!). It's the reason there have been so few incidences.

Do I take the chance? I tend not to!

Marc Dekenah