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#1
March 17th 04, 10:33 PM
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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. |
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#2
March 17th 04, 11:05 PM
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"Dave Shrader" wrote in message news:Xu36c.33004$po.292953@attbi_s52... John Michael Williams wrote: SNIP However, the first radios transmitted sparks, so in principle it should be possible to transmit near a long wire separated by a small gap from ground or another wire and get a small spark. So, I decided to try an experiment. SNIP There is one other potential source for a spark that you did not investigate. A make/break contact in a switch causes sparks when opened. The US Military specifies special shielded switches for their explosive, gas vapor, etc., environments. So, it is possible that pressing the PTT or the ON/OFF switch causes the necessary spark. Remember the Apollo ground fire. A switch/spark caused an oxygen explosion. I wouldn't call it an "oxygen explosion" but a small fire that grew rapidly due to the pure-oxygen atmosphere. |
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#3
March 18th 04, 02:16 AM
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Better not open your door or have someone else open theirs next to yu then.
That dome light switch might get you. "Dave Shrader" wrote in message news:Xu36c.33004$po.292953@attbi_s52... 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. |
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#4
March 18th 04, 03:21 PM
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On Wed, 17 Mar 2004 21:33:43 GMT, Dave Shrader
wrote: 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. This reminds me of a story... (pause while room clears out). Years ago I was an engineering student working for Cadillac Motor Car. Electronic fuel injection was new then, and there had been a couple of fires in the field that were suspected to have been caused by leaky fuel lines, which were at higher pressure than on the old carbureted cars. There were competing theories, however, as to what exactly was setting off the fire. I got assigned to help the guy doing the experiments to find out. He had a car fitted with a plexiglas hood, topped by a small tower with a high-speed movie camera pointing down for a good look at anything happening in the engine compartment. Two fire extinguishers were arranged to cover the under-hood area, and a fuel vapor sensor was also installed there. The way it was supposed to work was that he would drive and I would operate the hand-pumped sensor, and at the first sign of fire he would trigger the extinguishers. We ran all the tests at the GM Proving Grounds in Milford, Michigan. We tried making leaks in the fuel injection hoses. We had gas spraying all over under the hood, collecting in pools on the hot exhaust manifold. We tried poking holes in the spark plug wire insulation. Nothing, no fire. Finally I got a bright idea, and loosened the ground for the air conditioner compressor clutch. The idea was that this was a big inductor, and if the circuit opened there would be a big spark. Then all we needed to do was get the wire to bounce. We tried swerving from side to side, and driving on bumpy tracks, but no deal. I was *sure* that this spark would do the job, but we couldn't tell if we were really getting the spark. So finally I stood on the hood, holding on to the camera tower, so I could see for myself if there were sparks. He drove down the bumpy road one more time, and I did in fact see a spark: The high-speed movie shows the fire spreading out from it, more and more on each frame. Also on each frame was the back of my head, moving away more and more on each frame, until the extinguishers doused everything. What a rush! Them was the good ole' days..... Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis www.daqarta.com |
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#5
March 19th 04, 02:57 AM
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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 |
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#6
March 19th 04, 04:26 AM
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I read in sci.electronics.design that The Captain
wrote (in ) about 'CB Radios, Cellphones and Gasoline Vapor Ignition', on Thu, 18 Mar 2004: All handheld radios used on rigs are intrinsically safe, making them far more expensive than the standard variety. But do they NEED to be that costly or is that what the market will stand? 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? We are effectively discussing whether there are any grounds for requiring cell-phones, non-intrinsically safe, to be switched off, or not used, at gas stations. So far, the numbers suggest that the hazard is minute and the risk is also minute. There is a relatively new philosophy being applied to safety standards, including UL standards. It's called 'hazard-based', and requires a logical chain of reasoning to justify every provision of a standard. This is likely to result in significant changes to such standards over the next decade or so. Many current standards have 'just growed' over many years, and in some cases no-one knows why a certain provision is included. -- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |
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#7
March 19th 04, 07:23 PM
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#9
March 19th 04, 11:41 PM
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(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 |
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#10
March 20th 04, 02:12 PM
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See my post titled Intrinsic Safety -- Guy Macon, Electronics Engineer & Project Manager for hire. Remember Doc Brown from the _Back to the Future_ movies? Do you have an "impossible" engineering project that only someone like Doc Brown can solve? My resume is at http://www.guymacon.com/ |
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