Sunday, September 14, 2014

Which is Lighter – Alcohol or Gas?

The "prevailing wisdom" seems to be that canister gas is lighter than alcohol on longer backpacking trips.  On the surface, this would seem to make sense.  From a chemistry standpoint you have to carry roughly twice as much alcohol (assume 50/50 methanol/ethanol) as gas to produce the same amount of heat.  Even though alcohol containers are lighter than gas canisters, you have to carry so much more fuel, at some point the greater amount of fuel you carry for an alcohol set up will exceed the weight of an equivalent canister gas set up.  Or so the thinking goes.  But is it true?
Time to head into the backcountry.  Uh, hope your heavy pack doesn't ruin your trip.  But what's really lighter?
So, which is actually lighter – alcohol or canister gas?  Well, let's "do the math" and see what we come up with. I'll do three comparisons of gas stoves against alcohol as listed below.
  1. Conventional canister stove vs. alcohol
  2. Jetboil canister stove, typical use, vs. alcohol
  3. Jetboil canister stove,  "gram weenie" (minimalist) use, vs. alcohol
I'll cover conventional canister stoves vs. alcohol in this post.  I'll cover high efficiency stoves (i.e. a Jetboil) in future posts.

Conventional Canister Stove vs. Alcohol
OK, so what do I mean by "conventional" canister stove?  I'm talking about just a regular upright canister gas stove and a plain pot, the kind that does not have a heat exchanger.  In other words, just an ordinary gas stove with a plain pot.  Examples would include an MSR Pocket Rocket and a Snow Peak Gigapower.  Let me put up the numbers first, and then I'll walk you through them.  For the sake of brevity, I'm going to limit my projections to 14 days.  I'm assuming that most people don't do trips longer than two weeks without re-supplying.  I have numbers if you're interested that go out to three weeks. I'll discuss how to request those numbers later on in this post.

An ultralight conventional canister gas stove
Here are the numbers:
Number of days on trail 1 2 3 4 5 6 7 8 9 10 11 12 13 14
ml of alcohol per day 50 50 50 50 50 50 50 50 50 50 50 50 50 50
Total ml of alcohol 50 100 150 200 250 300 350 400 450 500 550 600 650 700
Specific gravity 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8
Total grams of alcohol 40 80 120 160 200 240 280 320 360 400 440 480 520 560
Fuel bottle weight (grams) 18 18 32 32 32 45 45 45 45 45 45 63 63 63
Stove system weight 60 60 60 60 60 60 60 60 60 60 60 60 60 60
Total grams (bottle + fuel + stove) 118 158 212 252 292 345 385 425 465 505 545 603 643 683
Weight carried end of trip (grams) 78 78 92 92 92 105 105 105 105 105 105 123 123 123
Number of days on trail 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Grams of gas per day 20 20 20 20 20 20 20 20 20 20 20 20 20 20
Total grams of gas needed 20 40 60 80 100 120 140 160 180 200 220 240 260 280
Actual grams of gas carried 110 110 110 110 110 220 220 220 220 220 220 330 330 330
Canister weight (grams) 100 100 100 100 100 140 140 140 140 140 140 240 240 240
Stove system weight 60 60 60 60 60 60 60 60 60 60 60 60 60 60
Total grams (can + fuel + stove) 270 270 270 270 270 420 420 420 420 420 420 630 630 630
Weight carried end of trip (grams) 250 230 210 190 170 300 280 260 240 220 200 390 370 350
Number of days on trail 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Start of Trip Difference (grams) 152 112 58 18 -22 75 35 -5 -45 -85 -125 27 -13 -53
Start of Trip Difference (ounces) 5.4 4.0 2.0 0.6 -0.8 2.6 1.2 -0.2 -1.6 -3.0 -4.4 1.0 -0.5 -1.9
End of Trip Difference (grams) 172 152 118 98 78 195 175 155 135 115 95 267 247 227
End of Trip Difference (ounces) 6.1 5.4 4.2 3.5 2.8 6.9 6.2 5.5 4.8 4.1 3.4 9.4 8.7 8.0
Average Difference (grams) 162 132 88 58 28 135 105 75 45 15 -15 147 117 87
Average Difference (ounces) 5.7 4.7 3.1 2.0 1.0 4.8 3.7 2.6 1.6 0.5 -0.5 5.2 4.1 3.1

OK, so let's talk about the numbers.  These numbers are for a solo stove user.  Use of a single stove by multiple persons would require an adjustment to the above numbers.

The first line for each type of stove is the length of the trip.  The trip is assumed to be "unsupported".  In other words, you aren't going to pick up supplies anywhere along the way during the trip.
"Unsupported" trips occur in areas where resupply is infeasible.  All supplies must be packed in.
Now, we need to know the total weight for each set up.  The total weight is the sum of the weight of the fuel + the weight of the stove set up + the weight of the fuel container.  Assume for these purposes that we use identical pots.

In the Alcohol section, I go through some gyrations to calculate the weight of how much fuel I think I'll need.  We can argue exactly what is the correct value for the specific gravity of SLX denatured alcohol (which is what I've been using lately), but the important point is that the weight of alcohol is double that of canister gas.

Now, look at the two stove set ups.  I'm allotting 60g each for both alcohol and canister gas.  I include the weight of a full windscreen with the alcohol stove set up.  I consider a windscreen essential to the efficient burning of alcohol.   Now, can you find a canister stove that is lighter than 60g?  Certainly.  And you can also find an alcohol set up that is lighter than 60g.  For these purposes, let's assume that the two stove set ups weigh the same.

The container weight will vary with the amount of fuel that we use.  More fuel will require a larger container or multiple smaller containers.  In this comparison, I'm going to assume that people are going to take full canisters at the start of the trip, a fairly standard practice.  In the "gram weenie" (minimalist) comparison, I'll use partial canisters to see what kind of weight savings I can obtain.  See Appendix I for a list of the specific sizes and weights of the containers used.

At this point we've got the total weight of each set up for each length of trip, from 1 to 14 days.  But that weight is the weight you show up with at the trailhead at the start of the trip.  Recall that during the trip, you'll be burning the fuel.  The weight you show up at the trailhead will not be carried throughout the trip.  So, below the "total grams" line I show the weight at the end of the trip after you've burned the fuel.

Next, I show the difference in weight between alcohol and gas at the start of a trip.  Negative numbers indicate the weight savings if gas is used.  Positive numbers indicate the weight savings if alcohol is used.  Notice that for trips up to 4 days in duration, alcohol is lighter but that on the 5th day, gas is lighter.  This seems to fit the "prevailing wisdom" that gas is lighter for longer trips.  But now notice that on a 6 day trip, alcohol reverts to being lighter.  What happened?  Well, on a six day trip, we use 120g of fuel.  A small canister only holds 110g of fuel.  We switched to a 220g canister, and now we're carrying more weight in both fuel and container.  As the duration of a trip lengthens, we again see weight savings for 8 through 11 day trips.  A 12 day trip shows up with alcohol being lighter again because we had to switch to a 220g canister and a 110g in order to have enough gas.  And so on.

But these are starting weight differences.  The next set of rows shows ending weight differences.  Now here's something a bit startling:  alcohol set ups are always lighter by the end of the trip.  Why?  Well, container weight.  Take a look at Appendix I.  With a 110g size gas canister, for example, the canister weighs almost as much as the fuel.  That steel canister is still with you at the end of the trip. With alcohol, the weight is almost all fuel.

Lastly, there is one final set of rows in my comparison table:  Average difference.  Here, I'm averaging the starting and ending difference in weight between alcohol and canister gas.  Notice here that in only one instance, at eleven days (highlighted in green), is canister gas lighter on average than alcohol.
Preparing Ramen with mixed vegetables and turkey jerky on an alcohol set up.

Now, ending light may not always be your chief goal.  When is you pack heaviest?  At the start of a trip, when you've got all your food.  So, having a lighter stove set up at the start of the trip is actually a good thing, and therefore a gas set up might be in order.  However, that said, it's only on longer trips, 10 and 11 days long, where anything even close to real weight savings occur, about 1/4 of a pound.  The start of trip weight savings when using a gas stove on longer trips are fairly modest, at least with conventional stoves.  I'll discuss results with a high efficiency stove (i.e. a Jetboil) in two future posts.

The biggest drawback to gas stoves is those heavy steel canisters.  The best you can do in terms of a fuel to container weight ratio is about 2:1.  With alcohol you can get a ratio as good as 10:1.  An important goal in trip planning when using a gas stove is to minimize canister weight.

Minimizing canister weight
First, never carry two smaller canisters when a single larger one will do.  Your fuel to canister weight ratio is always better with a larger canister.  In cases where you need more than 220g but less than 331g of fuel, you'd actually do better weight wise with a 450g size canister that has had some fuel burned off than you would carrying two canisters (i.e. a 110g size and a 220g size).  If you're planning a trip where you just barely have to move up to the next larger sized canister, you might want to plan a stoveless meal or two or bring a more efficient stove just so you don't have to bump up to that next larger canister.  In short, always strive to bring only one canister, the smallest, lightest one that has enough capacity for your trip.  Try not to have to bring two canisters or to have to move up to the next larger size.
In the interests of saving weight, always carry the smallest sized canister you reasonably can.
HJ, your numbers stink!
Don't like my numbers?  No worries.  You can have my spreadsheets and plug in your own numbers.  Write me at Hikin dot Jim using the domain Gmail dot com and ask for a copy.  Note that there is no "g" in "Hikin".  Hopefully, I won't get inundated here.  I have this in Excel 2010 format.  If you need an earlier format, please let me know, and I'll do my best.

So, in conclusion, with a conventional upright gas stove, gas can be lighter at the start of a trip, but alcohol is always lighter by the end of a backpacking trip.  In terms of the average difference in weight throughout the trip, alcohol is almost always lighter.  If you're looking to save weight, alcohol stoves are a pretty good bet.

I hope all these calculations are of some help to those looking to lighten up,

Thank you for joining me,


Appendix I – Weights
Empty canister weights.  I weighed several different brands of canisters on my gram scale.  There is some variability among brands in both content and canister weight, but I'm going to use the below weights which I think are representative.  I use the word "ratio" below but only list one number for brevity's sake.  Mentally, just include ":1" after each number shown below.
1.  A 110g size canister weighs 100g when empty.  Fuel to container weight ratio = 1.1
2.  A 220g size canister weighs 140g when empty.  Fuel to container weight ratio = 1.6
3.  A 450g size canister weighs 210g when empty.  Fuel to container weight ratio = 2.1

Empty alcohol fuel bottle weights.  I weighed some of the alcohol bottles I have lying around.  There may be lighter versions available, but let's use the below numbers in our calculations.  I use the word "ratio" below but only list one number for brevity's sake.  Mentally, just include ":1" after each number shown below.
1.  A 125ml fuel bottle weighs 18g when empty.  Fuel to container weight ratio = 6.3
2.  A 250ml fuel bottle weighs 32g when empty.  Fuel to container weight ratio = 7.0
3.  A 500ml fuel bottle weighs 45g when empty.  Fuel to container weight ratio = 10.0

Appendix II – Assumptions
  • The first assumption is of course that alcohol stoves will work for your style of stove use.  If you're doing gourmet cooking or cooking for a group of three or more, then an alcohol stove may not work for you.
  • Gas to alcohol fuel weight ratio is 2:1.  In other words, you need to burn twice the alcohol by weight to get the same amount of heat as you would from gas.  Yes, I'm assuming 20g/day for gas and 50ml/day for alcohol, but the amounts are not so much what is important; it is the ratio that is critical.
  • 6 cups (approx 1.5L) of water boiled per day.
  • Water temperature approx. 10C.
  • Very little simmering.
  • Efficient alcohol set up (I used a Caldera Cone from Trail Designs to develop my numbers).
  • Gas stove is used efficiently (turn it down, use a lid, use a wide enough pot, etc.)
  • Stoves are reasonably shielded from wind.
  • Regular canister stove (not a Jetboil, a Reactor, or similar).
  • Plain pot (no heat exchanger).
  • Air pressure approx 900 mBar.
  • The same pot is used for both alcohol and canister gas.
  • Gas and alcohol stove set ups are of equal weight.
  • A windscreen is included in the weight of an alcohol stove set up.
  • No resupply (at least of fuel) occurs during the trip.
A windscreen is essentially mandatory on alcohol set ups like this Bobcat set up from Flat Cat Gear.

Tuesday, September 9, 2014

How Much Canister Gas Do I Need?

Sometimes I see people bringing a lot more gas for their canister stove than they really need.  Now, it never hurts to be prepared, but really? A 450g canister for a simple weekend trip?  That's potentially a lot of extra weight to pack up a mountain.  Of course we don't want to run out of gas, but let's see if we can't do a little bit better in terms of coming up with an estimate of the gas we might need.
The three standard threaded backpacking canister sizes:
(L to R) 450g, 220g, and 110g.  Note:  Some brands vary as to exact amount of gas.

Now, before I give you my estimates, let me say that my estimates are based on my experience and my style of stove use.  You are your own best estimator.  If you're somewhat new to backpacking, then I'd say err on the side of caution and assume that you'll be doing "complex" cooking and select fuel based on that column from the table below.

What about White Gas?
What's that you say?  You don't use canister gas; you use white gas?  Ah.  No worries.  Check out How Much White Gas Do I Need?

Making Your Own Estimates
As for your own estimate based on your cooking style/stove use style, first you need to get a gram scale. Then weigh your gas canister before your trip.  On every trip, keep track of how many meals you cooked and what type of cooking was involved.  After your trip, weigh the canister again.  The difference between the starting weight and the ending weight is the amount of gas you used.  If you've kept track of how much and what type of cooking you've done, you should be able to start estimating your gas usage after a few trips.

Estimates in the Field
You can also make a rough estimate of how much fuel you're using while you're out on a trip.  See my article in Seattle Backpacker's Magazine, How Much Gas Do I Have Left?

Using Less Fuel 
Whatever you do with a stove, keep efficiency in mind.  There are basic things you can do to use less fuel while you're cooking.  I commend to you my earlier blog post, Canister Gas Stoves -- Recommendations and Efficiency.

My Estimates 
OK, now here are my estimates.  Note that when I boil water, the water is normally around 40F/5C to 50F/10C in temperature.  Significantly warmer or colder water may cause you to use more or less gas.  If you're melting snow, you should roughly double the below estimates.

For a solo trip
Jetboil Conventional Canister Gas Stove
(simple cooking)
Conventional Canister Gas Stove
(more complex cooking)
Grams per day 15 20 30
For two
Grams per day 23 30 45

Now, notice two things:
1.  For a simple weekend trip, for one person, there's no need to ever bring a 450g canister.  A 110g canister should do nicely unless you're doing a lot of extra heating for drinks, dish water, bath water, etc.   A reasonably conservative person shouldn't need more than one small canister of gas for a weekend.
2.  I do not double my estimates if a second person comes.  It's usually more efficient to boil more water at a time.  In other words, if you're going to boil a liter of water, do it all at once.  Don't boil 500 ml, empty the pot, and then boil the second 500 ml.  I'm assuming here that you brought a pot large enough for two people.  If not, then you may need to allow for higher gas usage.

Remember that my estimates are just that, estimates.  Always check that your experience is similar to mine before relying too heavily on my estimates.  Particularly in very cold or very windy conditions, gas usage may increase significantly.

I hope you find this post helpful,


Notes and Assumptions
1.  The figures above assume about 6 cups (approx. 1.5L) boiled per day for simple cooking.
2.  Water temperature is assumed to be around 40F/5C to 50F/10C.
3.  The figures in the table above do not include snow melting.
4.  The figures above assume you know how to operate a gas stove reasonably efficiently (turn it down!  Running on high wastes gas.) and that you've taken measures to shield the stove from wind.
5.  The figures above also assume that you have a pot that is wide enough to catch the flame.  Users of tall, skinny pots with a conventional gas stove will generally use more gas.
6.  Air pressure is assumed to be approx. 900 mBar, the air pressure normally found at about 3000'/900m in elevation.  Stove use at lower elevations may require more gas.  Stove use at higher elevations may require less gas.