Why does a BIC lighter have separated fuel chambers?

November 27, 2008 5:58 PM

In a cheap ol' plastic BIC-type lighter there's a divider that separates the fuel chamber into two sections. Why does this exist?

If the lighter works on the fuel (butane?) being turned to gas, why does the liquid fuel need to be separated? I don't think there needs to be an "emergency reserve" of the fuel on the other side. What gives?

posted by toomanyplugs to technology (18 comments total)

I think it's more to reinforce the body of the lighter. One without that divider would be much more easily crushed.
posted by scruss at 6:08 PM on November 27, 2008 [1 favorite]

My first thought is that enables fuel to be available no matter the angle at which it's held. But that is purely off the cuff without much thought. Thinking more as I type, it might be to simply add some structural integrity to the lighter.
posted by Frasermoo at 6:08 PM on November 27, 2008

Hey, no having fun in Askme!

Yes, the divide is most likely for structural integrity.
posted by Brockles at 6:14 PM on November 27, 2008

I think the divider also allows you to keep using the lighter slightly longer when the fuel is low. If a tiny bit remains, it's twice as high in a half chamber than it would be if spread across the whole bottom surface of the lighter.
posted by macrone at 6:15 PM on November 27, 2008 [4 favorites]

It's not actually a divider. You can move the fuel from one side to the other just by tipping it. I think it's just a physical reinforcement of the casing.
posted by pompomtom at 6:47 PM on November 27, 2008

I don't know why it is there, but it has nothing to do with fuel usage, as it is the gas phase of butane in a lighter that escapes through the valve and burns.
posted by TedW at 6:52 PM on November 27, 2008 [2 favorites]

Check the menu on the left for everything you ever needed to know about the Bic lighter.

But...it doesn't actually answer the question, so I'll nth the structural thing and also suggest that it helps you get a few more smoke's worth of juice out of the thing.
posted by turgid dahlia at 6:58 PM on November 27, 2008

structural integrity yes, but more to the point it is likely to allow them to use less plastic in the body of the lighter and/or make it a less naturally strong shape, while maintaining structural integrity.
posted by sfenders at 8:27 PM on November 27, 2008

I'm sure it helps structural integrity, but a smaller container would be stronger... They could make the standard size without a divider but it would hold more gas (bad for sales) or half fill it (looks bad, bad for sales), or fill the void with plastic and have a nice bid satisfying shape without it containing too much gas. A physically small container would be tough to grasp and use.

The size/proportion of containers is often chosen for reasons other than the most efficient size. To be easier to grasp or to appeal to the purchaser. For example soft drink cans are not the shape that minimizes the amount of aluminum; the most efficient shape is not the most appealing.
posted by Quinbus Flestrin at 9:35 PM on November 27, 2008 [1 favorite]

The size/proportion of containers is often chosen for reasons other than the most efficient size. To be easier to grasp or to appeal to the purchaser. For example soft drink cans are not the shape that minimizes the amount of aluminum; the most efficient shape is not the most appealing.

I dunno, i think drinking pop out of a sphere would be kind of cool, but it would make for more inefficient packing. in the box.

As far as the divider goes, it could be that it's just a cheap way to manufacture the things too. It's a very simple design that might be easier to stamp out then more elaborate supports. And I doubt they're too worried about saving plastic. It's not that expensive.
posted by delmoi at 10:29 PM on November 27, 2008

I wasn't thinking of a spherical can -- a squatter cylinder would use less material.

As for the plastic, I agree it's cheap; it's the amount of gas they are limiting. The sooner it runs out the sooner you will buy another.
posted by Quinbus Flestrin at 10:36 PM on November 27, 2008

I wasn't thinking of a spherical can -- a squatter cylinder would use less material.

Whoops, I should have previewed: a squatter cylinder would use less material than the current shape. (A sphere obviously uses the least material, though it has other problems as you rightly point out.)
posted by Quinbus Flestrin at 10:40 PM on November 27, 2008

Just a side note from a design engineer (who doesn't have a lighter to hand as he doesn't smoke), it's most likely structural integrity and by the way plastic is actually quite expensive per tonne, so it's best to make parts strong through shape and not mass.
posted by sdevans at 3:09 AM on November 28, 2008

Agree, it could well be primarily about limiting the amount of gas it takes to make the thing appear full.

But I'm not convinced saving material doesn't count to the designers: Although whatever plastic they use isn't that expensive, neither are disposable lighters. From a web search, at least some of them seem to be made from Derlin acetal polymer. You can buy a 113 cubic inches of it for about $10 retail. The same volume of butane would cost maybe 50 cents.
posted by sfenders at 3:20 AM on November 28, 2008

...soft drink cans...

That jogs a memory. I read a convincing argument somewhere that soft drink cans and bottles are cylindrical because the contents are under pressure and the round container is best suited to resisting that pressure without deforming. I'm thinking that the divider could facilitate the lighter's flattened shape by both resisting crushing and by tying together the sides of the body to resist outward pressure from the gas.
posted by jon1270 at 4:32 AM on November 28, 2008

jon1270 is on the right track. A spherical container is the best for pressure; strongest shape with the least amount of material. Next best is a round tube with domed ends (think of a scuba tank). Flat sides, like in the standard cheap lighter, are the worst. Tie it together with the divider and you have a much stronger container, both to resist pressure and to resist crushing. Plus, because people generally don't want butane escaping, they have to be pretty massively over-designed so as not to rupture during extreme conditions, like the hottest possible car in summer or getting run over.

So a spherical lighter would be ideal. Except not so great in a pocket. I have seen tubular lighters but those have all been thicker than the conventional sort, again not so great in a pocket. I tried searching to find cross section pics of the oval BIC lighters but was unable. Does anyone know if they have a center divider, too? I've only ever seen them in solid colors. The oval shape should be somewhat better than the flat sided sort. But maybe the BICs are cleverly shaped to feel big while minimizing plastic and butane so you replace them more frequently.
posted by 6550 at 10:21 AM on November 28, 2008

I was going to say "It's not that much pressure.." but then, this being mefi, I did some research...

Butane boils at -0.5c -which is why your bic won't light well on a winter day, unless it's been in your pocket and is warm.
In theory you could bust it open once it's cold and pour the butane out, and it would sit there as long as it's below freezing.


At 25c, the vapor pressure of butane is about 2.4 atm....
At 45c (summer arabian desert) - the vapor pressure is about 4.3atm...
At 64c (wal-mart parking lot in texas in summer, windows up) - 7.1atm...

So I'd buy the pressure argument.

I buy the "to avoid crushing" argument the most though... it just seems, obvious.
posted by TravellingDen at 1:25 PM on November 28, 2008

so you can't squeeze it and break the reservoir.
posted by gonzo_ID at 1:45 PM on December 26, 2008