Welcome again to the wonderful but sometimes weird world of wireless comms, written by Joel Young, CTO of Digi InternationalThis is another one of those puzzlers for me. I've noticed recently that the number of posted signs prohibiting cell phone usage around gas pumps in the US has gone down. Often times they seem to disappear when the pumps are upgraded or replaced. So what's the deal?
Well, let's look at what we know: Gasoline really only burns in its gaseous state, not in the liquid form. This is why you may have heard someone say that you could extinguish a cigarette in a bucket of gasoline. DON'T try this.
For a while the liquid gasoline will most definitely extinguish the cigarette and the path the cigarette has to take to get to the liquid will have it move through gasoline vapour mixed with oxygen - a deadly combination.
In order for gasoline vapour to ignite, it also needs oxygen and a heat source. The gasoline vapour / air mix must be in the range of 1.4 percent to 7.6 percent. That tells me that your ignition source, on a calm day, will need to be a few inches to no more than a few feet away from the liquid gasoline (I haven't done the calculations so it is just my swag).
Once we have the right mix of gas vapour and air, we need an ignition source that can provide the proper ignition temperature. For this mix of gasoline vapour and oxygen, we need at least 495 degrees Fahrenheit (280 degrees C).
The only practical way you get that kind of temperature is a spark or flame. This is why smoking a cigarette while pumping gas will put you in the category of the fools.
This is also why we are told to keep ourselves well grounded while we get in and out of our car so that we don't create one of those nasty static electricity sparks while pumping gas.
The problem here is that none of this relates to cell phone usage. In this case we must ask - what are we afraid of?
A cell phone draws the most current when transmitting and much of this power going to send those little signals out the antenna at someplace up to 2100 MHz in Europe. This is all low frequency stuff, meaning that these electromagnetic waves won't really do anything in free space unless you have a liquid and really high power (in the hundreds of watts) like a microwave oven.
The other path is moving a wire or electrical conductor through the cell phone transmission to generate a current. Unfortunately, we again need a lot of power to get a voltage high enough for a spark. So that rules out raw emissions of the antenna, leaving other sources for a spark - namely where the battery connects to the phone.
Fortunately, some really smart people have studied things like this and there are many guidelines for operating electronics in environments where flammable gases are present. You will see this referred to as Class 1 Division 1 or Intrinsically Safe. You can do your own research here but in general, you want to keep the voltage under 30 volts and the current under 300 mA.
Fortunately, modern cell phones have batteries less than 10 volts - most are closer to 6 volts and are also in the safe current range.
That wasn't true in the analogue bag phone days; however, so I believe these cell phone safe signs are a throw-back to the old analogue days.
So, unless you are going to do something stupid like remove you cell phone battery and short it out within a few feet of filling your gas tank (which would start a toxic fire), you should be fine. Just make sure to keep yourself well grounded, because static electricity is probably your biggest safety risk.
(Picture - No gas, under Creative Commons Attribution Licence)
--
Joel Young, VP of Research and Development and CTO at Digi International,
has more than 22 years of experience in developing and managing data
and voice communications. He joined Digi International in June 2000 and
in his current role he is responsible for research and development of
all of Digi's core products.
Prior to joining Digi, Joel was VP of Sales & Marketing at Transcrypt International where he was responsible for sales, marketing, and product development for all information security products. During his tenure at Transcrypt, he also served as VP of Product Development and VP of Engineering where he was responsible for engineering, research and product development for wireless communications products, cellular telephony, wireline telephony and land mobile radio, data security and specialized digital radio products.
He also served as District Manager for AT&T Business Communications Services where he was responsible for the creation and implementation of voice processing and network database strategies, including deploying new voice processing platforms into the AT&T switched network for private network and other outbound calling services.
Once we have the right mix of gas vapour and air, we need an ignition source that can provide the proper ignition temperature. For this mix of gasoline vapour and oxygen, we need at least 495 degrees Fahrenheit (280 degrees C).
The only practical way you get that kind of temperature is a spark or flame. This is why smoking a cigarette while pumping gas will put you in the category of the fools.
This is also why we are told to keep ourselves well grounded while we get in and out of our car so that we don't create one of those nasty static electricity sparks while pumping gas.
The problem here is that none of this relates to cell phone usage. In this case we must ask - what are we afraid of?
A cell phone draws the most current when transmitting and much of this power going to send those little signals out the antenna at someplace up to 2100 MHz in Europe. This is all low frequency stuff, meaning that these electromagnetic waves won't really do anything in free space unless you have a liquid and really high power (in the hundreds of watts) like a microwave oven.
The other path is moving a wire or electrical conductor through the cell phone transmission to generate a current. Unfortunately, we again need a lot of power to get a voltage high enough for a spark. So that rules out raw emissions of the antenna, leaving other sources for a spark - namely where the battery connects to the phone.
Fortunately, some really smart people have studied things like this and there are many guidelines for operating electronics in environments where flammable gases are present. You will see this referred to as Class 1 Division 1 or Intrinsically Safe. You can do your own research here but in general, you want to keep the voltage under 30 volts and the current under 300 mA.
Fortunately, modern cell phones have batteries less than 10 volts - most are closer to 6 volts and are also in the safe current range.
That wasn't true in the analogue bag phone days; however, so I believe these cell phone safe signs are a throw-back to the old analogue days.
So, unless you are going to do something stupid like remove you cell phone battery and short it out within a few feet of filling your gas tank (which would start a toxic fire), you should be fine. Just make sure to keep yourself well grounded, because static electricity is probably your biggest safety risk.
(Picture - No gas, under Creative Commons Attribution Licence)
--
Joel Young, VP of Research and Development and CTO at Digi International,
has more than 22 years of experience in developing and managing data
and voice communications. He joined Digi International in June 2000 and
in his current role he is responsible for research and development of
all of Digi's core products.Prior to joining Digi, Joel was VP of Sales & Marketing at Transcrypt International where he was responsible for sales, marketing, and product development for all information security products. During his tenure at Transcrypt, he also served as VP of Product Development and VP of Engineering where he was responsible for engineering, research and product development for wireless communications products, cellular telephony, wireline telephony and land mobile radio, data security and specialized digital radio products.
He also served as District Manager for AT&T Business Communications Services where he was responsible for the creation and implementation of voice processing and network database strategies, including deploying new voice processing platforms into the AT&T switched network for private network and other outbound calling services.
Comments (12)
Perhap the suspected danger is due to the current trendy use of old fashioned telephone 'bell' ring tones ?
As a lad I was fascinated by the the stream of blue sparks created at the contacts of a DC electric bell !
Posted by Mike Meakin | August 19, 2009 4:35 PM
Posted on August 19, 2009 16:35
The problem is not the battery but the transmitted RF power coupled with any corrosion present in the infra-structure of the filling station. Namely the pump and housing.
Most corrosion will create a semiconductor diode which can act as a detector.
There is also the possibilty of a very primitive RF detector being created where the corrosion creates an insulator and if the wavelength of the loop created by the insulation is ideal then you will get a version of a spark detector as used in early RF detectors.
Static electricty, while a problem, is usually prevalent in cold (dry) weather and nylon clothing.
This is why nurses in operating theatres are usually forbiden to wear nylon underwear, a hang over from when ether was used as an anaethestic and did causes fires.
Posted by Dr Bob | August 24, 2009 2:13 PM
Posted on August 24, 2009 14:13
I always wondered if the real reason for switching off mobiles, is because their transmissions interfere with the petrol pump metering/display systems. Not that I know anything about petrol pump electronics but I suspect they are very low power and maybe therefire susceptible to interference, especially if their EMC certification dates from before the radiated-immunity test jumped from 1GHz to 2GHz?
Comments?
Posted by Dave Randall | August 25, 2009 5:49 PM
Posted on August 25, 2009 17:49
Dave may be right I think. I studied this when I was at my last company which made intrinsically safe gauges to measure the level of underground storage tanks at petrol stations. From memory the standard used was BS5755. In it the ignition power of petroleum vapour was quoted as 2 watts, that is the power that would cause the vapour to ignite. I think that it would be just about impossible to get a spark from a mobile phone's transmission.
I can remember that there was a fuss when CB radios were all the rage, and some powerful CB transmitters used to scramble the pump communications to the point of sale console. I suspect that this is a left-over from that problem.
The oil companies probably put a blanket ban on any radio transmissions rather than go into the issue in detail.
Posted by Phil Bromley | August 27, 2009 10:23 AM
Posted on August 27, 2009 10:23
The problem is I think more to do with receiving a call, most modern phones vibrate and the motor which does the shaking is a tiny dc commutated motor. There is therefore a very real risk of the brush sparking igniting any stray petrol-air nearby. No need to conjure up unlikely sounding RF based scenarios yet!
Posted by EW | August 27, 2009 11:23 AM
Posted on August 27, 2009 11:23
Here in England quite often a mobile phone mast can be hidden within the sign for a petrol station. The very same garage can have a sign prohibiting the use of mobile phones. Inconsistent or what?
Posted by Jeff Cousins | August 27, 2009 11:26 AM
Posted on August 27, 2009 11:26
Earlier petrol pumps were not screened sufficently and the analogue and GSM mobile phone signals would indeed interfere with the electronics metering in the petrol pump if held nearby.
It had nothing to do with causing a fire or explosion. I can not remember hearing of such a case ever being reported and if it had happened there would have been very strong warnings and serious criminal penaties imposed for ignoring the regulation.
It was all about gasoline companies preserving their profits and this scare story was therefore created. The main body of the article albeit very interesting is not at all relevant.
Posted by D. Spencer | August 27, 2009 11:30 AM
Posted on August 27, 2009 11:30
I belive the danger arises when a mobile phone is dropped and the battery falls out. If the phone is drawing a large current from the battery at the instant it is dropped there is potential for a spark.
Posted by Roy | August 27, 2009 12:08 PM
Posted on August 27, 2009 12:08
If petrol station owners had any real fears of petrol vapour being ignited by sparks, they would ban all vehicles from their forecourts. There is a much greater risk of a stray spark from the ignition system or the high temperature of the exhaust manifold causing vapour ignition than any mobile phone. I think you will find that Jeff has explained the real reason.
Posted by Julian Ben-David | August 28, 2009 9:12 PM
Posted on August 28, 2009 21:12
Actually I think this all stemmed from a hoax email claiming to come from Shell. It discussed several petrol station explosions caused directly by mobile phones. The entire email was a fake, but interestingly I found it quoted verbatim on a county council health and safety website a couple of years ago when I was looking into using DECT phones on our shop floor.
http://www.snopes.com/autos/hazards/gasvapor.asp
In order to get our DECT phones approved by health and safety I merely had to make up some authoratitive sounding bollocks, I can't remember my justification exactly but I remember comparing the millitants thrown out by mobiles versus DECT. Very satisfying, using cod science to counter restrictions enforced because of hearsay.
So, it would seem that despite there being no evidence at all that there is a problem the onus is still on me to justify use of my equipment to Health and Safety because they don't know how to use Snopes. As far as I am aware there has never been an incident of a cell phone actually causing a fire at a petrol station.
Posted by Rich | September 10, 2009 11:57 AM
Posted on September 10, 2009 11:57
I use to work for a very well know mobile phone company and on one of my site visits I meet an engineer that use to work for very well know petrol company.
This question came up.
He explained that if you started to used the brick phone beside the new electronic petrol pumps, the pump would not register thus "Free Petrol". The petrol company spotted this and the signs started to appear.
(This use to happen on the old style electronic one arm bandits)
As we now use GSM and there new metering systems this does not occur.
One day you will be informed that Tesla invented the radio not Marconi
Posted by Tim Asquith | September 15, 2009 2:04 PM
Posted on September 15, 2009 14:04
All the talk about the reasons for banning mobiles phones in garages is now surely out of date. Any mobile phone is a car, (and I assume have gone to the garage in your car to fill up) should be either firmly fixed in its clip on the dashboard or turned off, so you shouldn’t have it in your hand to make a call anyway. Is it time they changed the notice in garages, “Make you mobile call HERE or at home where it safe”
Speaking as someone who uses his bicycle to get to work, I see so many drivers holding mobiles to their ear, although they are looking straight at me, they still can’t see me, It scary.
Drive safely
Steve
Posted by Steve Berks | January 21, 2010 2:09 PM
Posted on January 21, 2010 14:09