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#121
March 21st 04, 07:56 AM
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Guy Macon http://www.guymacon.com wrote in message ...
See my post titled Intrinsic Safety I did. Thanks. It appears that the laws involved pertain to manufacturers and commercial operations. So, I think none would govern use of a cell phone while gassing up. I once wired a farm house, but that was before the NEC included the concept, I think. John John Michael Williams |
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#122
March 21st 04, 08:28 AM
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Hi Jeff.
Thanks. Comments below. Jeff Liebermann wrote in message . .. On 17 Mar 2004 12:02:15 -0800, (John Michael Williams) wrote: 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. There were few spark transmitters mounted in automobiles. They were just too inefficient, big, and clumsy to be functional. Transmitters in vehicles really didn't start until tube type transmitters became popular. The problem was that the typical mobile radio used a dynamotor (motor-generator) combination to generate the necessary high voltages. With the radio and dynamotor mounted in the trunk of the vehicle, there was a good chance that gasoline fumes would accumulate in the trunk of the vehicle and be ignited by the spark from the dynamotor commutator. See the photo of the 80D at: http://www.telmore.com/ka1nvz/old_tw...ola/49-59.html The dynamotor is the black cylinder near the handle. The 140D was twice as big and heavy. Ships around the turn of the 20th century transmitted morse code by spark, I think. Back in the 1960's, my 1960 Ford Falcon had an assortment of Motorola 80D and 140D radios in the trunk. I experienced a small explosion in the trunk ignited by the dynamotor. I had filled up the gas tank at the local gas stop (for 19 cents per gallon). Warm weather caused it to expand and leak vapour into the trunk. Key the transmitter, the dynamotor starts, sparks, and boom. I then attached a 1.2 m monopole antenna to an oscilloscope. This antenna has a Schottky hot carrier diode and impedance matching resistors builtin. Lovely. A harmonic generator. Any reason you want lots of harmonics? Shottky diodes or any other non-linear device, do not belong in antenna matching circuits. It's home made, but it's probably as good as any other wire about that long. Wrong. Optimum for CB is either a 1/4 wave monopole (102 inches) or two of them to form a half wave dipole. So, first conclusion: To get even a 1 V spark would take a wire at least 9 m long, all somehow kept within 1 m of the transmitter. Thus, it appears it is not feasible to create a hazardous spark with a CB at a gas station. Find a 4 watt flourescent light. Attach a 1/4 wave antenna to your 5 watt CB radio. Transmit. Hold lamp in hand and touch the end of the 102" antenna. It will light when you talk. (Note: With AM modulation, you only get 5 watts when you yell into the microphone. Without modulation, you only get about 2.5 watts of RF). Interesting idea. I would have thought that a tube would require more V than a neon lamp to get started. I'll try it if I can find a lamp. What you seem to be suggesting is that I simply connect the lamp to the 1/4 wave receiving antenna, right? Why introduce my hand? For ground on the other lamp contact? I don't see the point of attaching a long wire to the CB, because they don't come with long bare wires. Clearly, I could get a good spark by attaching a wire to the CB batteries, and avoid all the RF stuff! Now, ask yourself what voltage is required to light the flourescent lamp. In order to get a spark, you need to generate enough voltage to ionize the air between the contacts. That's about 20KV/inch. If we eliminate the antenna, 5 watts of RF into 50 ohms will generate: P = E^2 / R E = 16v rms E(peak) = 1.4 * 16 = 22 volts The gap necessary to create an arc with 22 volts is: 22V / 20,000V/in = 0.001 inches Kinda small, but given a microscope, a 1 mil spark gap will arc. Of course the VSWR protection circuity in the transmitter will instantly shut down the transmitter when it arcs, but that takes a few millisec. 22 V is a lot more than I could get with a 1 m monopole: I only got 100 mV peak to peak. It appears my 1 m wires were too short; but, if I use a long wire, the distance from the transmitter will lower the power transfer to some of the wire, won't it? Or, I'll have to move away, into the far field--but that will also lower the power. There's no question 5 W is enough to make a spark of arbitrary size, given an inductor somewhere around, but I don't see where the 50 ohms comes from, if I'm looking for a spark caused by the RF? Notice that this is a voltage phenomenon, and is not dependent upon the power level. Therefore, an antenna that offers a voltage step-up will generate a higher voltage. However as the antenna is in the air and nowhere near a close enough ground to arc, it doesn't matter. If there's gonna be any arcing, it will be between the xmitter output and the base of the antenna. The transmitter antenna is coated with about 3 mm of rubber; I think cell phones are the same way. A spark has to come from the RF, I think. There is a BNC connector, but that implies complete shielding (even flame suppression!) at the antenna base. I have a telescoping antenna intended for a receiver that is bare metal, though. I could substitute it. The typical mobile FM transmitter of the day (1960's) cranked out between 15 and 150 watts. Most were around 75 watts. Run the calcs again for 75 watts and see if the gap is more reasonable (I'm lazy). The next question is how much heat is necessary from the arc to ignite the gasoline vapour. I'll leave that as an exercise for when I have more time to burn. Gotta get back to lying and cheating on my taxes. I think if I can see the spark, it can ignite gas vapor, provided the flame had a path out of the gap. John John Michael Williams |
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#123
March 21st 04, 08:57 AM
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On 20 Mar 2004 23:28:38 -0800, (John Michael
Williams) wrote: Ships around the turn of the 20th century transmitted morse code by spark, I think. The Lusitania, Mauritania, Titanic, and Olypic all ran on coal. No gasoline in sight. Later vessels ran on bunker C fuel oil, which is more like tar than gasoline. I don't think one has to worry about sparks on such a vessel unless it's finely devided coal dust, which finished off the Lusitania in a secondary explosion after the torpedo. Interesting idea. I would have thought that a tube would require more V than a neon lamp to get started. I'll try it if I can find a lamp. Neon lamp needs about 60 volts to light and 40 volts to stay lit. The 4 watt flourescent tube wants at least 90 volts to start, and I think (i.e. guess) about 50 volts to stay lit. What you seem to be suggesting is that I simply connect the lamp to the 1/4 wave receiving antenna, right? Why introduce my hand? For ground on the other lamp contact? Yep. You're the ground. You should be fine with a 5 watt CB and a 1/4 wave whip. The high voltage point is near the tip. However, don't try it with an illegal CB linear. You'll get an RF burn for your troubles. Incidentally, there are cell phone antennas with lights in them. http://cellphones-accessories.com/12stobligcel.html They're LED's which require much less power to light than a 4 watt flourescent bulb. Still, it's kinda interesting. I don't see the point of attaching a long wire to the CB, because they don't come with long bare wires. Clearly, I could get a good spark by attaching a wire to the CB batteries, and avoid all the RF stuff! Exactly. Same with an open relay contact or toggle switch. However, don't foget that you need containment to create an explosion. Sparking the DC inside the trunk is the mostly likely location. 22 V is a lot more than I could get with a 1 m monopole: I only got 100 mV peak to peak. The 22 volts peak is at the RF connector. I'm assuming that if there is a spark gap, it will be in the coax cable or associated antenna connectors. It appears my 1 m wires were too short; but, if I use a long wire, the distance from the transmitter will lower the power transfer to some of the wire, won't it? Or, I'll have to move away, into the far field--but that will also lower the power. Inverse square law. Double the distance, and you get 1/4 the power. For a fix load resistance, 1/4 the power is 1/2 the voltage. However, you'll get vary bad coupling efficiency with such an arrangement. I could grind the near field equations but you'll never get ALL the power (5 watts) delived to your random wire pickup. Think resonance and close coupling if you want to do better. There's no question 5 W is enough to make a spark of arbitrary size, given an inductor somewhere around, but I don't see where the 50 ohms comes from, if I'm looking for a spark caused by the RF? That's the approximate impedance of the antenna as found on a typical mobile installation. Again, I'm assuming that if there is a spark to be found, it will be at the coax ends or connectors. They're all 50 ohms. The transmitter antenna is coated with about 3 mm of rubber; I think cell phones are the same way. A spark has to come from the RF, I think. There is a BNC connector, but that implies complete shielding (even flame suppression!) at the antenna base. A BNC connector is quite open but is good for maybe 150 volts of RF. However, all it takes is a sloppy coax connection, with some of the braid wires slopped around near the center pin of the BNC, and you have a potential spark gap. I have a telescoping antenna intended for a receiver that is bare metal, though. I could substitute it. Won't make much differnce. At 0.001" gap necessary for a spark with 5 watts can only happen with a defective installation. I think if I can see the spark, it can ignite gas vapor, provided the flame had a path out of the gap. I beg to differ. The ignition of a gasoline oxygen mixture requires a specific amount of energy to ignite. Anything less will not produce the requiste chemical reaction. Think spark plug heat ranges and glow plugs in model airplanes. I'll grind the numbers if you want, but it's now midnight, I'm tired of waiting for Windoze update, and I'm going home. -- # Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060 # 831.336.2558 voice http://www.LearnByDestroying.com # # 831.421.6491 digital_pager AE6KS |
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#124
March 21st 04, 09:47 AM
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I read in sci.electronics.design that Jeff Liebermann
wrote (in mppp50ho4dr08ahkb3dlbqkcfkp0ih ) about 'CB Radios, Cellphones and Gasoline Vapor Ignition', on Sun, 21 Mar 2004: The gap necessary to create an arc with 22 volts is: 22V / 20,000V/in = 0.001 inches Kinda small, but given a microscope, a 1 mil spark gap will arc. But it takes about 350 V to do so. The relationship between voltage and gap length is very non-linear below about 500 V. -- 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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#125
March 21st 04, 02:45 PM
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Jeff Liebermann wrote in message . ..
On 20 Mar 2004 23:28:38 -0800, (John Michael Williams) wrote: snipped lots of good stuff I think if I can see the spark, it can ignite gas vapor, provided the flame had a path out of the gap. I beg to differ. The ignition of a gasoline oxygen mixture requires a specific amount of energy to ignite. Anything less will not produce the requiste chemical reaction. Think spark plug heat ranges and glow plugs in model airplanes. I'll grind the numbers if you want, but it's now midnight, I'm tired of waiting for Windoze update, and I'm going home. The ignition of a gaseous oxygen-gasoline mixture, or a (potentially more sensitive) hydrogen-oxygen mixture does require a specific minimum amount of energy, which depends on the partial pressures of the oxygen and the fuel, and - IIRR - the partial pressures of any inert diluent gases around. Lesser amounts of energy can induce the requisite chemical reaction, but the reaction will fizzle out, rather than providing enough energy to ingnite the surrounding shell of a gas mixture and produce a self-propagating flame front. The controlling relationship is between the volume of the sphere in which the reaction is first initiated, and the surface area of that sphere - if the intial volume is too small, not enough energy is released to heat the surrounding shell of gas to the ignition temperature. Once you've got the basic idea,the thermodynamics is pretty straightforward. I had to work through the equations many years ago for an experiment intended to monitor the process in which one of the "Dewar benzenes" converted itself to normal - Kekule's - benzene, which is an enormously energetic process, involving about an order of magnitude more energy per molecule than you get out of TNT and PETN. I really didn't want to blast my experimental apparatus to smithereens. When I went through the calculations with my supervisor, he pulled a very long face - the motivation for the experiment had been some unexpected flashes of light seen when a dumb organic chemist had released small drops of liquid "Dewar benzene" into a hot cell, and my calculations made it clear that the flashes of light were just thermal radiation from a hot plasma, rather than fluorsecence from from an electronically excited state of Kekule benezene, which is what my supervisor had been hoping for ... For the difference between Dewar benzene and Kekule benzene see http://www.chemsoc.org/exemplarchem/...enzenering.htm ------- Bill Sloman, Nijmegen |
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#126
March 21st 04, 04:55 PM
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John Michael Williams says... Guy Macon http://www.guymacon.com wrote... See my post titled Intrinsic Safety I did. Thanks. It appears that the laws involved pertain to manufacturers and commercial operations. So, I think none would govern use of a cell phone while gassing up. I agree. If Intrinsic Safety rules were applied, automobiles would be banned from gasoline stations. This would not only eliminate the 35 refueling fires that occur every year, but would also eliminate most of the 50,000 traffic accident fatalities... -- 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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#127
March 21st 04, 05:03 PM
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I read in sci.electronics.design that Bill Sloman
wrote (in ) about 'CB Radios, Cellphones and Gasoline Vapor Ignition', on Sun, 21 Mar 2004: For the difference between Dewar benzene and Kekule benzene see http://www.chemsoc.org/exemplarchem/...enzenering.htm Dewar benzene can actually be made? Do you know when it was discovered? What about the prismatic form? I would have thought that was a lot easier to make, if I didn't have a suspicion that that is where simple bonding ideas break down. -- 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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#129
March 21st 04, 09:13 PM
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On Sun, 21 Mar 2004 08:47:34 +0000, John Woodgate
wrote: I read in sci.electronics.design that Jeff Liebermann wrote (in mppp50ho4dr08ahkb3dlbqkcfkp0ih ) about 'CB Radios, Cellphones and Gasoline Vapor Ignition', on Sun, 21 Mar 2004: The gap necessary to create an arc with 22 volts is: 22V / 20,000V/in = 0.001 inches Kinda small, but given a microscope, a 1 mil spark gap will arc. But it takes about 350 V to do so. The relationship between voltage and gap length is very non-linear below about 500 V. I didn't know that it wasn't linear. I just assumed that it takes the same amount of energy to peel electrons off of a single atom (ionize) regardless of gap seperation. A wider gap requires more voltage to ionize more atoms to create a longer conduction path, but the energy per atom is the same. I also couldn't find (Google) any useful references that showed this non-linearity. Unless the heat generated by the ionization contributes to assisting furthur ionization, my seat-o-de-pants physics says it should be linear (for DC). There's also the minor detail of RF excitation versus DC. As I vaguely remember from my 35 years ago college welding classes, TIG welding uses RF to strike the arc because it takes less power/energy/whatever to start the arc. We're allegedly talking about striking an arc across 0.001" with a 5 watt, 27MHz transmitter terminated with a 50 ohm load. If it's non-linear in the opposite direction, the calcs are gonna be no fun. I have everything it takes to test this. Microscope slide, with two sewing pins glued with hotmelt goo and seperated by 0.001". Apply RF and watch through the microscope. I'll see if I can throw something together and post photos (time permitting). Also, I've always been tempted to build a low power, QRP spark gap transmitter. Although the mode is illegal, I suspect that operating spark at below Part 15 incidental emission standards, would be tolerated. Getting the arc to start at such low power levels might require some exotics (i.e. piezoelectrics). This could be the start of something interesting (or disgusting). -- Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060 (831)421-6491 pgr (831)336-2558 home http://www.LearnByDestroying.com AE6KS |
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#130
March 21st 04, 09:33 PM
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John Woodgate wrote in message ...
I read in sci.electronics.design that Jeff Liebermann wrote (in mppp50ho4dr08ahkb3dlbqkcfkp0ih ) about 'CB Radios, Cellphones and Gasoline Vapor Ignition', on Sun, 21 Mar 2004: The gap necessary to create an arc with 22 volts is: 22V / 20,000V/in = 0.001 inches Kinda small, but given a microscope, a 1 mil spark gap will arc. But it takes about 350 V to do so. The relationship between voltage and gap length is very non-linear below about 500 V. In fact the Paschen curve - breakdown voltage plotted against gap - has a minimum at around a couple of hundred volts, and the breakdown voltage starts rising again for very small gaps. The linear right-hand branch of the graph where you might see a slope of 20,000V per inch doesn't extend down to 22V. The theory explaining the conductivity of electricity through gases was worked out around the turn of the last century, and doesn't seem to be all that well known any more. Pity. ------- Bill Sloman, Nijmegen |
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