How do bic lighters maintain such a constant flame?

October 22, 2005 2:10 PM

How do bic lighters maintain the same sized flame regardless of the amount of fluid left inside?

Simple physics dictates that if it is a simple pressure system, that as the amount of mass contained in the lighter is used (burned) the overall amount of pressure inside the lighter would decrease, and the flame would get smaller. I have observed that this is the case on several other brands of lighters, but with the bic the flame remainst relativly constant until right before the fluid is completely used up. Do they have some kind of special system? or am i imagining things.

posted by cyphill to Technology (12 answers total)

Response by poster: I am looking at flame size, i have no real way of measuring energy output or anything. I was more noting the difference between the flame size on a bic lighter over its lifespan and the flame size of other brand lighters over thier lifetimes.
posted by cyphill at 2:14 PM on October 22, 2005

The vapor pressure of the butane is the same whether there's a lot of fluid or a little.
posted by notsnot at 2:22 PM on October 22, 2005

The straw connects to the bottom of the butane container.
posted by Citizen Premier at 2:42 PM on October 22, 2005

And I'm guessing that the variation is just too little for you to notice.
posted by Citizen Premier at 2:43 PM on October 22, 2005

The flame does get smaller, right before it goes out. I have seen this many, many times. It doesn't happen as quickly as other lighters, but it does happen. Sorry.
posted by Juliet Banana at 3:28 PM on October 22, 2005

I think notsnot is probably right on this.

It's the vapour pressure in inside the lighter. Once you get only a bit of liquid left you can see the flame size rapidly decrease as it starts to run out of pressure.
posted by alexst at 3:28 PM on October 22, 2005

Oof. Need reading lessons, etc. Disregard above comment.
posted by Juliet Banana at 3:30 PM on October 22, 2005

I'm thirding notsnot. The vapor pressure of the fuel depends mostly on its temperature, which is usually around room temperature; therefore, the flame appears to be a constant size. (Scientifically speaking, the difference between 20 degrees C and 30 degrees C is really not that large—in Kelvin, those amount to 293 K and 303 K, respectively, and the factor of difference between those is small.) The flame rapidly decreases in size once the liquid fuel runs out and the lighter is effectively running on fumes.
posted by jenovus at 4:08 PM on October 22, 2005

So how come a bunch of butane-powered devices I own (mini-torches, refillable lighters, etc) start showing a smaller flame by the time the fluid is only half gone?
posted by five fresh fish at 6:18 PM on October 22, 2005

good question, fff. i agree with the theory above, but also with your experience.

i don't know enough physical chemistry to be sure, but could it be that if there's a mixture of different gases (not just butane, but methane too, for example), then the lighter gases will have higher partial pressures, so will preferentially burn off more quickly. i seem to remember that partial pressures are independent, but never convinced myself it made sense....
posted by andrew cooke at 8:24 PM on October 22, 2005

(so, to clarify the point, initially you're burning butane plus extra methane; later just butane)
posted by andrew cooke at 8:33 PM on October 22, 2005

I don't know.

Does this have to do with a critical flow orifice, in case someone who does know is still watching?
posted by deep_cover at 2:47 PM on October 23, 2005