Gas usage

[under10kg]

I am doing a 12 day walk soon. I am a bit unsure on how much gas to take for boiling 2 cups of water a day?
Any guide to use?

[photohiker]

What type of gas stove are you using?

Some are highly efficient, and others are highly inefficient.

Have a look here: https://adventuresinstoving.blogspot.co ... tboil.html

[Warin]

I am doing a 12 day walk soon. I am a bit unsure on how much gas to take for boiling 2 cups of water a day?
Any guide to use?

Measure your own stove/pot.
Put 2 cups of water in your pot.
Measure the stove+gas canister weight.
Set up the stove, pot and any wind shield and boil the water.
Measure the stove+gas canister weight.

The difference in weight gives you the amount of gas used for 2 cups of water.
I'd do the measurement early morning when things are cool - gives a more realistic (larger gas use) measurement.
You should find that operating the stove at ~ 30% of maximum gives you the least gas usage .. takes a little longer but less gas.
Any shielding will help. And not going to a full rolling boil will help too. As will having a cover over the pot.

By performing the measurement you know what your usage is, remembering that if it gets colder it will take more gas.

[crollsurf]

Ball park I'd be thinking 2 small cannisters, 1 at a food drop around half way, otherwise 1 big one. If you run out it's no deal breaker, you can always soak that dehydrated stuff and eat cold.

[Orion]

Conservative approach is to assume something like 40% heating efficiency and 10°C starting temperature for the water. Then you'd need about 120g for 12 x 2 cups of water. So a single 100g canister could leave you short. You might make it but you're probably better off with a large canister and enjoy the extra fuel (make an extra cup of tea now and again). Two 100g canisters is a little heavier than one 230g canister but the redundancy is safer.

[under10kg]

Thanks guys. An extra cup of tea or miso soup now and then would be worth it I feel.

[Lamont]

Adventures in stoving. I heartily second that website. He has calculations etc, everything you could want there and very accessible.
Watching water boil there has never been so interesting! I bought my Amicus after his preview.

[wildwanderer]

As mentioned above when testing keep in mind the big difference in gas usage (or mixture usage) between a early morning brew at 0 degrees celcius and a middle of the day usage at 15 degrees celcius.

[crollsurf]

Oh and don't do what I did once and leave your stove screwed into the cannister overnight. You may wake up without any gas. In which case, take 1 cannister/day

[Franco]

On several posts of this type I pointed out how by changing water temperature/burner/pot size one can expect differences of 30-40% and sometime over that.
For some reason i never got a positive response out of that so my thinking is : why bother asking if that sort of difference is irrelevant.
(short version : you need to test the EXACT set up you intend to use , including expected air and water temperature)

[Warin]

As mentioned above when testing keep in mind the big difference in gas usage (or mixture usage) between a early morning brew at 0 degrees celcius and a middle of the day usage at 15 degrees celcius.

Big? err no .. not unless the water is frozen e.g. snow.

The specific heat of water says it takes 2,092 joules to heat 500 ml (2 cups) of water 1 C.

So 20 to 100 C is 80 degree increase = 80 * 2092 = 167,360 joules.
So 10 to 100 C is 90 degree increase = 90 * 2092 = 188,280 joules.

So that is a 12.5% increase in energy .. this neglects losses (say 10%?) call it 15%? That is not what I'd call big.

[Mathematicians will simply do ((8/9) -1 )*100 .
0 (must be in liquid state) to 15 change? ((100/85)-1)*100 = 17.7% .. say 20%?

There is more to be lost/gained by pot diameter vs volume and shielding.The above should give you an idea of what happens with ambient temperature changes ... with a reasonably efficient stove to pot set up. If you have no shielding and a small diameter pot .. then your losses will be large .. ? say multiply the above % by 1.3 ... so the above15% becomes 20% and 20% becomes 26%. Could be more! Or less.

Bottom Line: Do some measurements. If your worried about a lower temperature .. use colder water.

[rcaffin]

Summer time: 30 g per day (butane/propane) for the TWO of us. That applies up in alpine regions as well. I have got that donw to 22 g/day, but it took some trying.
Winter time in the snow: I allow 60 g per day for the TWO of us, but if we can find water rather than melting snow I will be bringing some back. There's a safety factor here.
These figures recorded from the last 10+ years.
Mind you, I usually have a little extra fuel in summer alpine regions as well - it can get COLD.

Cheers
Roger
PS: good windshield and pot lid and low-medium power only. And I usually do dinner inside the tent as well.

[Orion]

As mentioned above when testing keep in mind the big difference in gas usage (or mixture usage) between a early morning brew at 0 degrees celcius and a middle of the day usage at 15 degrees celcius.

Big? err no .. not unless the water is frozen e.g. snow.

I find that field conditions (not just water temperature) make a significant difference. I suspect the heat losses due to lower air temperature and air movement are very consequential. At home I get 70% efficiency from a particular stove/pot arrangement without doing anything fancy. On walks in the summer I estimate the efficiency with the same stove/pot averages very roughly 50%.

[Warin]

As mentioned above when testing keep in mind the big difference in gas usage (or mixture usage) between a early morning brew at 0 degrees celcius and a middle of the day usage at 15 degrees celcius.

Big? err no .. not unless the water is frozen e.g. snow.

I find that field conditions (not just water temperature) make a significant difference. I suspect the heat losses due to lower air temperature and air movement are very consequential. At home I get 70% efficiency from a particular stove/pot arrangement without doing anything fancy. On walks in the summer I estimate the efficiency with the same stove/pot averages very roughly 50%.

The air movement might be a large part of that.
How much shielding is there?
At home .. try a household fan providing a consistent breeze and see how much the efficiency changes?
Shielding can provide a reduction in that breeze loss and also radiation losses to the ambient temperatures.

Calculating radiation and convective losses breeze enhanced gets too vague due to estimating the conditions, not to mention more complex.

[Orion]

The air movement might be a large part of that.
How much shielding is there?
At home .. try a household fan providing a consistent breeze and see how much the efficiency changes?
Shielding can provide a reduction in that breeze loss and also radiation losses to the ambient temperatures.

Calculating radiation and convective losses breeze enhanced gets too vague due to estimating the conditions, not to mention more complex.

Yes, it's complicated. One could do endless experiments varying the multitude of variables. I did some tests on a day where the indoor temperature was a little bit lower than previous tests I'd done (13°C vs 23°C). There was no apparent difference in efficiency. But there was also zero wind indoors so the effect would be muted compared to cooking outdoors. For what it's worth I use a windscreen and usually cook in the vestibule.

Another factor is the stove setting. The fuel efficiency is very sensitive to the output power, so much so that most stove boil test conclusions are suspect because this isn't taken into account. Outdoors I probably turn the stove up higher than is optimal sometimes. That isn't going to cut the efficiency by 30% but it's another factor.

Indoor tests give you an indoor result. Outdoor tests are difficult to control. What you really need isn't testing, it's experience. Lacking that, you poll people on the internet for advice.


I was in the same place as the OP recently. I boil about 4 cups per day but my trip was 6 days long, so the math was the same. I thought I might have been able to squeak by with a small canister but thought better of it. I was glad I did. It was much nicer not to have to be a fuel miser. And on my trip I encountered a couple who had been accidentally sold watered down metho and so couldn't cook. I was able to boil them a bit of water without any worry.

[rcaffin]

watered down metho
I have also heard of shady operators refilling half-used gas canisters with water, in Kathmandu (Nepal). That would definitely be a problem!

Cheers
Roger

[Orion]

At least that would work for a while. Pretty nasty surprise at the halfway point. You'd probably think there was something wrong with the stove. I've had it happen twice where stoves would simply stop working once the canister got down to a certain level.

On the other hand, it might be a funny practical joke to play on a friend.

In the case of the metho it was clearly an unfortunate and rare accident. The seller had nothing to gain and in fact gave that couple $600 worth of services as compensation for their trouble.