[chris (OP)]

Shirley has been in touch in response to a recent item on the Naked Scientists podcast concerning the best scientifically-informed way to rid a car of a fart.

Shirley's solution:

I have a friend whose husband and Great Dane used to fart mightily in their car. First we struck matches to save ourselves from asphyxiation which worked depending on the density of the farts and the number of the available matches. Then we served them both bran daily for breakfast with excellent results.

Anyone able to improve on that?

[RD]

Charcoal dog-biscuits are supposed to be a cure for that



https://en.wikipedia.org/wiki/Charcoal_biscuit

[timey]

Can you imagine the look on her husbands face... (chuckle)

[alancalverd]

Lighting a match in a fartfilled car is likely to "blow the bloody doors off" (subtle cinematic quote). It's cheaper and safer to open the bloody windows.

[chris (OP)]

Lighting a match in a fartfilled car is likely to "blow the bloody doors off" (subtle cinematic quote). It's cheaper and safer to open the bloody windows.

What concentration of flammable gas would need to exist within the car to be explosive? There must be a threshold concentration to make a reaction propagate. What is that, and is there a formula for calculating it?

[Colin2B]

What concentration of flammable gas would need to exist within the car to be explosive? There must be a threshold concentration to make a reaction propagate. What is that, and is there a formula for calculating it?

Not aware of a formula, usually done by experiment. Lower explosive limit (LEL) for methane is 5%, so you need at least 5% methane mixed with air in the car. Not sure what upper limit is for methane i.e. concentration at which explosion cannot occur, but I expect it is above the limit of breathability 

[chris (OP)]

you need at least 5% methane mixed with air in the car

5%! Phew, that's a LOT of farts.

[alancalverd]

It's quite a serious problem in practice. Every year a few people ignite on operating tables, though explosions are mercifully rare, and now and again I have the memorable experience of calibrating an x-ray system in a veterinary theatre alongside a recovering mastiff or Great Dane. We take all sorts of measures to eliminate ether, chloroform and other explosive anesthetics but the sight and smell of a big dog with flames coming out of its arse can make a lasting impression.

[Colin2B]

you need at least 5% methane mixed with air in the car

5%! Phew, that's a LOT of farts.

Yes, if you want the whole volume of gas inside the car to go up, but as Alan points out the local concentration near the source can hit the LEL - usually the same as the lower flamability limit.
Even small volumes can be quite powerful. In our explosives lectures we were treated to a demo of a milk bottle containing lab gas + air, covered by steel tube for protection and a lab stool over the top. When the mix was ignited the stool seat split in 2 (very memorable) and when the steel tube was lifted we saw a pile of powered glass - no lumps!

[Tim the Plumber]

It's quite a serious problem in practice. Every year a few people ignite on operating tables, though explosions are mercifully rare, and now and again I have the memorable experience of calibrating an x-ray system in a veterinary theatre alongside a recovering mastiff or Great Dane. We take all sorts of measures to eliminate ether, chloroform and other explosive anesthetics but the sight and smell of a big dog with flames coming out of its arse can make a lasting impression.

How did you explain the burns to the owner?

And what was the initial ignition source?

And !!!!!!!!!!

[Colin2B]

How did you explain the burns to the owner?

I'm hoping he will write it up for posteriority

[chris (OP)]

Even small volumes can be quite powerful. In our explosives lectures we were treated to a demo of a milk bottle containing lab gas + air, covered by steel tube for protection and a lab stool over the top. When the mix was ignited the stool seat split in 2 (very memorable) and when the steel tube was lifted we saw a pile of powered glass - no lumps!

How did they ignite the mix?

[alancalverd]

How did you explain the burns to the owner?

That's a job for the nurse at the reception desk.

Fortunately an ignited fart does little damage to an intact animal. Exploding human bowel gas occasionally turns up in a coroner's court, but "misadventure" covers a multitude of God's little trials.

[PmbPhy]

Shirley has been in touch in response to a recent item on the Naked Scientists podcast concerning the best scientifically-informed way to rid a car of a fart.

Shirley's solution:

I have a friend whose husband and Great Dane used to fart mightily in their car. First we struck matches to save ourselves from asphyxiation which worked depending on the density of the farts and the number of the available matches. Then we served them both bran daily for breakfast with excellent results.

Anyone able to improve on that?

For the man there's a product called Beano which works to prevent gas. See: http://beanogas.com/

For dogs it might help to get the dog on a better higher quality diet (no Beano for the dog though). See: https://www.petsbest.com/blog/human-medicine-for-gassy-dogs/

As for a match causing an explosion - Won't happen. The concentration after expulsion of the gas is far too low to allow for an explosion. I used to worry about leaving the gas on the stove with no pilot light. It would bleed gas for a while and leave that bad smell. However after a while I realized that the concentration is far too low even after I smell it to cause an explosion.

People often confuse what they see on TV or in movies for real life. You might see a  show where someone leaves the gas on for a few minutes and then there's a spark and the entire house blows up. There s a show called Myth Busters on TV (or there used to be) in which they tried to reproduce such an explosion. They were unable to do it. The best that they were able to get was a muffled "pop" which was far from the explosions one sees in movies. Then there's the inevitable "throw a cigarette in a puddle of gas" and watch it ignite and possibly blow up the gas station. The problem with that is that the end of a lit cigarette is not hot enough to cause ignition unless its already flaming, which rarely, if ever, happens after initially lighting it. Give it a try with a small puddle and see what happens. Its very educational.

[Colin2B]

Even small volumes can be quite powerful. In our explosives lectures we were treated to a demo of a milk bottle containing lab gas + air, covered by steel tube for protection and a lab stool over the top. When the mix was ignited the stool seat split in 2 (very memorable) and when the steel tube was lifted we saw a pile of powered glass - no lumps!

How did they ignite the mix?

Wax taper fastened to a stick. Lecturer stood behind a Perspex screen on the bench and fed the taper through the gap between stool seat and the tube - about 12cm. Later in the course we built electrical igniters which are safer to use.


Methane is particularly nasty if you get the mix right. Known as firedamp it was regularly responsible for coal mine explosions before the Davey lamp was invented.
It was also responsible for the explosion in the underground valve house at Abbeystead in 1984. The explosion killed 16 visitors, eight instantly, caused the concrete roof to fall down on to the group, and destroyed the steel mesh floor throwing some of the victims into the water. There was suspicion that one of the group may have lit a cigarette although, as Pete says, an already lit cigarette is unlikely to ignite the gas.
An interesting example of the power of a vapour explosion was that at Buncefield in 2005. Technically a fuel-air explosion (unleaded petrol was the fuel) it was heard 120miles away and registered 2.4 on the Richter Scale - and it woke me up! Cause of ignition was never established, but electrical equipment was suspected.

[UrgentHomeworkHelp]

Just open the windows and let the fart out instead of lighting it up inside the car with the windows closed. It's also much safer this way.

[chris (OP)]

Just open the windows and let the fart out instead of lighting it up inside the car with the windows closed. It's also much safer this way.

That's not necessarily going to work: https://www.thenakedscientists.com/articles/questions/how-do-you-rid-car-fart