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I use an MSR Pocket Rocket with which I'm very pleased.  It replaced the usual blue Camping Gaz which lost pressure after boiling up water for one meal.  The MSR seems to maintain a hot flame even when the cyclider is half full.  (Also with being self sealing you can take a full one up into the hills each time and keep the half empty ones for brewing up in the back of the car out of the wind.)

So, here's a poser for you.  Which cyclinder will keep its pressure longer:  the larger 250 sized cyclinders or two of the half sized cyclinders?

 ***THE FOLLOWING SUGGESTION IS PROBABLY INCREDIBLY DANGEROUS AND SHOULD NOT BE ATTEMPTED**** however it does work, (just)
if your canister is getting to the end of its useful life and you are desperate for one last brew, crush the can up a bit,

Rocky - are those the red canisters?

Reiver - I was only playing with various stoves in my garage this morning and found that out then.  It was due to having this muck about with pressurised inflammable gas next to a fully fueled up car that made me consider this original question.

 I've still got a Campingaz Backpacker stove from many years ago with three canisters of gas.  You can't get that size non-resealable anymore and don't want to use them up. 

It's no bigger than a large can of beans and consists of two pans which contain the bits, a handle, burner unit and gas cyl.

> Which cyclinder will keep its pressure longer:  the larger 250 sized cyclinders or two of the half sized cyclinders?

Well, assuming the two cylinders start at the same pressure, and the gas is released in a similar manner in both, then the bigger cylinder will hold pressure for longer.  However, it will also run at a lower pressure for longer...  Think of two water tanks, of the same depth (pressure), but one twice the volume of the other, and with the same sized hole (flow rate) in both.  The bigger tank will empty in twice the time.

Now for the complications...

The cylinders contain mixed gases.  It's possible that the gases will be released from the mix differently between the cylinders.  I can't think why, but I guess it's possible.

If your cylinder is getting a bit feeble, warm it up; body heat, or stand it in a pan of hot water...

captain paranoia wrote (see)

The cylinders contain mixed gases.  It's possible that the gases will be released from the mix differently between the cylinders.  I can't think why, but I guess it's possible.

> Smaller cylinders will cool quicker iun use because of their smaller mass.

That's a good point; I didn't explicitly mention the different boiling points, and was thinking that they'd boil off at the same relative rates between the two cylinders, but the faster falling temperature of the smaller cylinder would affect the relative release rate.

I'm a bit confused here - I assume you're talking about cyclinders which contain liquified propane/butane i.e. you can hear it sloshing around in an un-gas-like manner. 

In which case, so long as there is liquid remaining, the pressure exerted by the gas phase remains the same, assuming the temperature is constant. It is evaporation of the gas (from the liquid) in the cylinder as the gas is released through the burner which causes the cylinder to cool. The gas-filled volume has no effect on the gas pressure if there is liquid present and, as before, the temperature is constant. I would suggest that the larger total mass of the larger cylinder means that the cooling effect of the gas expansion and evaporation will cause a smaller temperature drop in the liquid, thus there will be a smaller apparent pressure drop. Note that when there is less liquid left, it is cooled more for any given amount of gas evaporation than when the cylinder is full - thus near empty cylinders perform less well.

Didn't you guys do physics at school?

Fiona N

Assuming constant temperature and a constant mix of gasses, what you say is true. Much of the discussion above however revolves the effects of changing temperature and changes in the gas mixture, and how these might be affected by canister size.

... also the propane part tends to burn of first as it's boiling point is a lot lower than butane. This causes the last bit of the can to have a much higher butane mixture and consequently creates less pressure.

IsoButane as used by MSR, has a lower boiling point than normal butane and doesn't suffer as badly in normal uk conditions.

Correction, in my previous post I meant to write:

 Assuming constant temperature and a constant mix of gasses, what you say is true. Much of the discussion above however revolves around the effects of changing temperature and changes in the gas mixture, and how these might be affected by canister size

aka barking up the tree. (..excluding the "crush the can up a bit" post )

or even barking up the wrong tree...

> Didn't you guys do physics at school?

Yes, but we've moved on from the simple situations presented in school physics (and chemistry) problems to the more complex real-life situations of gas mixtures that don't behave like simple pressurised gases.  Effectively, you have a fractionating system running rather too hot (well above the boiling points of the individual fractions) to do proper fractional separation.

Try reading the exam question again...

;-)

But once you're above the boiling point of butane (-0.5C, I think) the pressure in the cylinder is the sum of the partial pressures of the two gases which are dependent on their molar proportion in the liquid mixture. I'd post the data table but apparently 'I don't have any personal albums yet' so:

@40F a 30/70 propane butane mix has a vapour pressure of 21.5 psig whereas the pure propane Vp is 65 psig and the pure butane  is 3.1 - so you can see you've got a combined pressure of 0.3 x 65 + 0.7 x 3.1

This relationship is constant with increasing T. But at 30F (i.e. below B.pt of butane) the mixture has a pressure of 15.4 which is pretty much the partial pressure of the propane (100% is 53 psig so 0.3 x 53 --> 15.9)  with virtually no contribution from the butane. So it's only below 0.5C that you'll effectively burn propane without proportionately using the butane.If you're operating below 0C. then a different gas mixture might be a good idea

Wow, that confused a simpleton like me.

So what is the best fuel to use?