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Saturday, February 25, 2017

Gas vs. Alcohol – Which is Lighter?

Ah, the perennial question among those who wish to save weight:  Which is lighter, gas or alcohol?

I've visited this question before, but now that super ultralight (SUL) canister gas stoves are available, it's time to re-visit the subject.
The BRS-3000T, the world's first super ultralight (SUL) canister gas stove – a mere 25 grams!

Heavy?  Or (Ultra) Light? 
What do I mean by "SUL?"  How do we classify canister stoves?  Well, here's my schema:
Canister Gas Stove Weight Classes
(Less Than or Equal To)
Ounces Grams
Heavy
 4+
113+
Moderate 4 113
Light 3 85
Ultralight (UL) 2 57
Super Ultralight (SUL) 1 28

Basically, if a canister stove weighs less than an ounce (28 g), that's SUL in my book.  If a stove weighs more than a quarter pound (113 g), that's heavy.  I think that's a fair and reasonable categorization, given the state of the art.

But, Hikin' Jim, uh, isn't this post about comparing gas to alcohol?  All you've done so far is talk about gas.

The Math
Ah, yes, quite right.  So, for a given trip, which is lighter, alcohol or canister gas?  Let's do some math.  If you don't like math, then by all means skip down to the discussion, but in order to really feel like I'm on solid footing here, I need to go through the details.

First, for the purposes of this discussion, I'm going to assume that our gas stove and our alcohol stove set ups weigh about the same.  In the case of the BRS-3000T, that would be 25 grams.

Second, I'm allocating 30 g for the alcohol container and 100 g for the gas canister.  Those may not be representative of all containers and canisters everywhere, but let's just go with it for now.

For this example, assume the individual is going to boil 500 ml of water twice a day.  Based on that assumption, I allot 40 ml of alcohol or 20 grams of gas per day.  Both of those could be overstated, but these are round numbers for comparative purposes.

The specific gravity of alcohol is about 0.8, so our 40 ml/day of alcohol is going to weigh about 32 g/day.

I've laid out an example in the below chart, based on a 5 day trip.    By the way, this spreadsheet is available for download if you want to run your own numbers.  See the "Get your own danged spreadsheet" section of this post.
Alcohol Calcs Alcohol Set Up (grams) Gas Set Up (grams) Alcohol Set Up (ounces) Gas Set Up (ounces) Gas Calcs
Stove Weight 25 25 0.9 0.9
Container Weight 30 100 1.1 3.5
Fuel/Day (ml) 40 20 Gas grams/day
Spec Gravity 0.8
Fuel/Day (g) 32
Days on Trail 5 0.0 0.0 100 Total Gas Weight (g)
Fuel Weight 160 110 5.6 3.9
Total Weight 215 235 7.6 8.3

Discussion
Now, the "conventional wisdom" is that a trip shorter than X days is going to be lighter with alcohol but that all trips longer than X days are going to be lighter with gas, the presumption being that since gas is more calorically dense, one would eventually overcome the heavier weight of the steel canister in which the gas is housed.  There's some disagreement on exactly what "X" is, but this is the basic argument, that a trip longer than X days will be lighter with gas.

But is it really true?  Well, take a look at the bottom line, total weight, in the chart above.  For a trip of 5 days, alcohol is just slightly lighter, by 20 g, than gas.  Ah!  There you go. So, for anything beyond that, gas will be lighter, right?

Well, maybe not.  Look at the far right column.  Do you see the cell labeled "Total Gas Weight"?  Look at that number there, 100 grams.  So, let's see, one more day than five would be six days, and on the sixth day, we'd add another 20 grams, our daily allotment, giving us a total of 120 grams of gas... But our canister only holds 110 grams of gas.  Oops.

Therein lies the problem with the "conventional wisdom".  Canister gas is only sold in lots of roughly 4, 8, or 16 ounces (about 110g, 220g, or 450g; brands vary).  The problem with the "conventional wisdom" is that on day six you have to size up to the next larger canister, and... alcohol becomes lighter again.

Now, are my numbers for every person for every trip?  Probably not, but in general the rule should hold:  Gas will be lighter after a certain number of days – until you have to size up to the next larger canister.

Here's how it maps out, in the chart below.  Negative numbers  in the difference column mean that gas is heavier.  Positive numbers mean that gas is lighter.

Empty canister weights are as follows:
110 g size = 100 g empty
220 g size = 150 g empty
450 g size = 210 g empty
Days Alcohol Set Up (grams) Gas Set Up (grams) Difference (grams) Alcohol Set Up (ounces) Gas Set Up (ounces) Difference (ounces)
1 87 235 -148 3.1 8.3 -5.2
2 119 235 -116 4.2 8.3 -4.1
3 151 235 -84 5.3 8.3 -3.0
4 183 235 -52 6.5 8.3 -1.8
5 215 235 -20 7.6 8.3 -0.7
6 247 285 -38 8.7 10.1 -1.3
7 279 285 -6 9.8 10.1 -0.2
8 311 285 26 11.0 10.1 0.9
9 343 285 58 12.1 10.1 2.0
10 375 285 90 13.2 10.1 3.2
11 407 445 -28 14.4 15.3 -1.3
12 439 445 4 15.5 15.3 0.2
13 471 445 36 16.6 15.3 0.2
14 503 445 68 17.7 15.3 2.0
15 535 445 100 18.9 15.3 3.2
16 567 445 132 20.0 15.3 4.3
17 599 445 164 21.1 15.3 5.4
18 631 445 196 22.3 15.3 6.6
19 663 445 228 23.4 15.3 7.7
20 695 445 260 24.5 15.3 8.8
21 727 445 292 25.6 15.3 9.9

Notes on the above chart:
  • For days one through four, alcohol is about 5 to about 2 ounces lighter.
  • On day five, it's less than a one ounce difference between the two, but alcohol is still a bit lighter.
  • But on day six, the amount of weight saved by using alcohol increases to 1.3 oz.  Why?  Because we had to size up to the next larger canister.
  • By day seven, you're at basically a break even.
  • For days eight through ten, finally gas is actually lighter.
  • On day eleven, alcohol goes back to being the lighter weight option again.  Why?  Because we had to size up to the next larger canister.
  • Finally, starting on day twelve, gas is always lighter, increasingly so, for the remainder of the three week period that is mapped out in the above chart.
You will note in the above that the progression is not straight line (non-linear).  It jumps up disproportionately because the weight of a canister does not increase steadily.  The first 110 grams of gas requires 100 grams of canister weight (about a 1;1 ratio).  The next 110 grams only requires 50 additional grams of canister weight (about a 2:1 ratio), and then the next 200 grams only requires 60 additional grams (about a 4:1 ratio).  Incidentally, I think we can see here that it's going to generally be better to carry one larger canister rather than multiple smaller canisters for a given amount of gas.

Summary
  • Trips of one to four days have reasonably good weight savings when using alcohol.
  • Trips of five to eight days don't show a whole lot of difference either way.
  • Trips longer than eight days will generally see better results with canister gas, with the exception of day eleven where alcohol will be lighter.
Concluding Remarks
Now, "your mileage may vary" (YMMV) as they say.  The number of people, type of cooking, conditions, and the specific stove set ups will affect these calculations.  But regardless of the specifics, it's going to be along these lines.  Don't just assume that trips longer than X days will always be lighter on canister gas.  Factor in when you will have to size up to the next larger size canister.  And of course, my underlying assumptions in terms of how much fuel will be needed per day may not apply to you.  The best course of action is to run your own numbers based on your situation, which leads me to say:

Get Your Own Danged Spreadsheet!
Don't like my numbers?  That's perfectly understandable.  Use your own numbers.  You can download my Excel spreadsheet, which is in xlsx format, and then you can run any number of different scenarios.  Cells in which you need to enter values into are highlighted in yellow.  You need to enter the following:
  • Stove weight, once for alcohol and once for gas.
  • Container weight, just for alcohol.  Gas container weight will be calculated for you based on how much total gas your trip requires.  
  • Fuel per day, once for alcohol and once for gas.
  • Days on trail.
Once you enter the above values, the spreadsheet will calculate the total weight for both alcohol and gas.  The results will be displayed in both metric (to the left) and English (to the right) units.  I hope it's helpful to you.

Does It Matter?
Does any of this matter?  Aren't the weight differences too small to care about?  Maybe.  It depends on the individual.  There are gram weenies that count every gram.  There are ounce counters that will save every ounce they can.  And then there are people for whom a few ounces really don't matter much either way.

However, considering how excited people get over a stove that weighs about an ounce and a half (FMS-300T) vs. a stove that weighs about one ounce (BRS-3000T), I'm thinking that there are people that will be interested.  Regardless, the spreadsheet is here if you want to use it.  Whether or not it matters, well, that's up to you.

Tailoring to the Trip
Let me leave you with one last thought: Sometimes starting pack weight isn't the only issue.

What do I mean by that?  Well, recall that you have to burn more alcohol per day to boil the same amount of water.  Your pack weight will fall faster with alcohol.  With gas, your pack weight decreases more slowly, and you've always got at least a quarter pound lump of steel in the bottom of your pack – the canister.

If you're planning a trip where you've got a big first day, then of course you want to minimize your first day's weight.  But if you're planning a trip where there's a rough go in the middle or end of the trip, you might actually want to choose alcohol since your pack may be lighter – at the time that you do the rough section – despite having a higher initial starting weight.  If you're going to have big climb, a tricky scramble, or a XC route toward the end of the trip, it might make sense to bring alcohol even if it were heavier at the start of the trip.

Thanks for putting up with all the math,

HJ

2 comments:

  1. Great post! The boredom associated with a cold/dark/wet PNW winter drove me to perform a similar assessment with my gear last month. I have a slightly heavier alcohol stove set up than you used in your example, and thus concluded that my "flip" point in weight efficiency between alcohol and canister systems was at 3-4 days. Practically speaking, they're pretty close, especially if you have a partial canister sitting around. Ultimately, my choice is influenced by factors other than weight, such as number in the party, altitude, temperature, weather/wind, burn-bans, etc. If the sole consideration was weight, I might be using Esbit tabs on a Ti stand, or my Ti fire-box more frequently. I may be a stoveaholic.

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    Replies
    1. Hi, idoc.

      No such thing as too many stoves. ;)

      Ultimately, my choice is influenced by factors other than weight, such as number in the party, altitude, temperature, weather/wind, burn-bans, etc.
      Yeah, and I think what you're saying is completely appropriate. Weight is one more thing to throw into the mix. At least for me, it shouldn't be the only consideration. I probably won't ever take the little BRS-3000T on any hike that matters. I'm willing to "pay" an ounce or two penalty to have a stove that is reliable, stable, and works well.

      HJ

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