RadioBanter - View Single Post - CB Radios, Cellphones and Gasoline Vapor Ignition

March 19th 04, 10:41 PM

(The Captain) wrote in message . com...
Dave Shrader wrote in message news:Xu36c.33004$po.292953@attbi_s52...
John Michael Williams wrote:

SNIP

However, the first radios transmitted
sparks, so in principle it should be possible to
transmit near a long wire separated by a small gap from
ground or another wire and get a small spark. So, I
decided to try an experiment.

SNIP

There is one other potential source for a spark that you did not
investigate.

A make/break contact in a switch causes sparks when opened. The US
Military specifies special shielded switches for their explosive, gas
vapor, etc., environments.

So, it is possible that pressing the PTT or the ON/OFF switch causes the
necessary spark. Remember the Apollo ground fire. A switch/spark caused
an oxygen explosion.

Actually, anyone who has worked in the offshore oil industry will be
familiar with the concept of intrinsic safety. This requires that no
electronic instrument shall be able to ignite a mixture of air and
inflamable vapour or gas. All handheld radios used on rigs are
intrinsically safe, making them far more expensive than the standard
variety.

I very much doubt that cell phones are buit to intrinsicly safe
standards, and under those circumstances I would certainly not feel
safe near someone yacking while filling.

So, an interesting querstion is; does your phone conform to UL
requirements for intrinsic safety? And if not, why are you using it
in an area where an explosive gas air mixture is possible?

Cap

Actually, a former maritime safety engineer Emailed me
about this. However, he could not locate the law or regulation
which defines "intrinsic safety". If you can find a law
or regulation governing operation of a transmitter around
a gas pump, please post it.

I have no idea how UL testing would pertain to a battery operated
device incapable, itself, of electrocuting anyone. However, the
battery eliminator which I have (but did not use in the
experiment I described) is UL approved.

On the safety issue, the same engineer also told me he was able
to create visible sparks with a 100 W transmitter, holding
the antenna near a piece of metal. However, for the following
reason, I suspect the sparks were because his transmitter was
earth-grounded:

The handheld CB I used had a completely insulated rubber antenna
and of course had no ground connection. I replaced the rubber
antenna with a telescoping metal one. I then keyed the transmit
button (as above) in the dark, while trying to get a spark by
bringing the tip near a 1 m x 1 m aluminum 1/4 in plate (ungrounded).
I could see nothing, although touching the metal caused the CB's
power out bar to indicate a drop in power. The plate should have
been an effective AC ground at ~27 MHz.

So, neither induction into a wire nor electrical direct contact
seems likely to make a visible spark, with a 5 W CB transmitter.

I would only expect a 100 mV or so spark anyway, which would be
hard to see.

So, I'm not convinced that a cell phone could cause a spark, either.

I agree that key closure sparks might be possible internal to
the device, and that neither it nor a cell phone would be likely
to have been designed to suppress a flash from such a source.

However, the issue I have tried to address here is a spark from
the RF, not from generic electrical causes. I don't doubt that
gasoline vapor is inflammable in a generic sense.

John

John Michael Williams