FUEL EFFICIENCY TO WEIGHT

> it DID NOT , boil 25L plus of water off a 240ml 220g net/370g gross cartridge , probably it would have boiled 10l if I had been careful

I am hardly surprised, and not sure where you got the 25l figure from.

My experience is that it needs about 8g of gas to boil 500ml of water, using a simple pan.  So, your 220g of gas would boil 0.5*220/8 litres of water, or about 14l.

If we look at the energy content of 220g of, let's say, propane, we'll find that it's 52.5kJ/g, giving us 11550kJ, or 11.55MJ.

Now let's look at how much energy we need to boil water.  Starting at 10C, and raising to 100C, we need 4.2J/gK, so, for a litre, that's 4.2*1000*(100-10)J = 378kJ.

So, at 100% efficiency, we'd expect to be able to boil 30.55l.

That means that you're expecting your stove to be 25/30.55 efficient, i.e. 82%.  Which is asking a lot.  My experience of 8g per 500ml gives about 45%.

You've not stated how you used the stove; far too many users follow the 'speed is good' philosophy so beloved of gear testers (they have no choice as it's the only measure they can easily make), and whack the thing up to 11, which means that most of the heat is wasted up the side of the pan.  A much more efficient approach is to use a moderate flame, and take a bit longer.

Liquid fuels generally have a similar energy density to the gases, so there's not much to choose between them from that PoV.

25L...you are having a laugh arent you?

hence the popularity of jetboils and heat exchange pots along with wind shields.

One thing that CP hasnt mentioned is loss of heat energy to ambient - as the pot and water got hotter, losses to ambient increase and, as the air temp. last weekend was low, this will also have been significant.

> One thing that CP hasnt mentioned is loss of heat energy to ambient - as the pot and water got hotter, losses to ambient increase

Indeed, and another good reason to use a reflective windshield; it does more than shield the wind.

Tom Beasley on backpackinglight.com has done some interesting experiements with fuel use vs stove setting/boil time, which show that you can crank the stove down really quite low without having too great an effect on fuel use (aka efficiency).

The other thing I neglected was that I simply brought the water from ambient to 100C. Strictly, this isn't boiling, but it's perfectly good enough for for our purposes.

CP= Thats some pretty complex maths, your not Johnny Balls are you?

PC= I cant back this up by figures, but from experiance with my jetboil I know that as the presure inside the can decreases, the boil time increases, and while the blurb claims 12L per 100g of gas @sea level, I allow 8L, just to give me that extra margin.

Background reading.

Using a multifuel stove (with different jets for the different fuels), I average 9g for a 500ml boil with gas and 11-12 for petrol and kerosene.

The ETA is good, cant say about efficentcy, but its comparable to the jetboil in terms of performance.  The ability to turn the can upside down might improve performance/efficentcy but not by much. 

Have you tried putting the cannister into a shallow pot of hand warm water, or insulating it from the ground with a silicon pot stand(Available from poundland)? or warming it inside a jacket before you use it?

Also I've had my jetboil for 3 years, and although its starting to get a little rough around the edges it's still a nice little stove that never let me down. Yes I've had damp trips where I've been miserable but I've never had to go without a warm brew soley because of the jetboil.

If it bothers you that much why not switch to a more reliable fuel sauce? there is/was a thread similar to this on the bushcraft UK forum, one of the contributers compiled a comparative list of the gas stoves and their performance/fuel efficency, and the different efficentcys of other fuels.

Anyway, the upshot was that dependent on conditions gas is the best choice for weight/power, but for prolonged trecks you'd probably be better with liquid fuels over gas.

9g/500ml was with gas and an Omnifuel........

Regarding the ETA, it is the heat exchanger on the pot and a good windshield that makes the difference - the stove part is the same as the Express Spider.

But when you compare the additional weight of the pot to the fuel savings it becomes less attractive for anything other than long trips without any resupply.

I agree its worth while, its the reason that I opted for the jetboil in the 1st place.

I normally use 100g cannisters, but yes I have had 10 liters from one can.  Although, like I say I normally allow for 8.  I'm Normally  in the peak district 2-3 times a week, rain or shine, and its done me upto 636m(pouring rain with biting wind on the top of kinder) and down to -5 deg without any problems.

Normally I'm at more modest alt but still no problems.