Monday, August 21, 2017

Interactive Canister Gas Estimation Spreadsheet

I've put together something that I hope will be useful.  It's an Excel spreadsheet that you can customize to fit your particular stove and your particular situation.  You can use Google Sheets if you don't have Excel on your device.

This spreadsheet should work for conventional stoves (e.g. Pocket Rocket, GigaPower, Crux, etc.) as well as high efficiency stoves (e.g. Jetboil).  The download link will be down at the bottom of this post, after the explanations

Headed out for a week? Nice!  Uh, how much fuel do you need?
Purple Lake, John Muir Wilderness, Sierra National Forest, Sierra Nevada, California.
Enter Your Numbers
So, it's relatively straightforward. You, based on your needs and your experience, enter four numbers into the spreadsheet as listed, below.  I'll go through some examples after the list to hopefully help you understand how this works.  It's pretty simple, really.
  1. The number of cups (about 250 ml per cup for those of you who prefer metric) of water you plan to boil per day, on average.
  2. The number of grams of fuel your particular stove burns to boil one cup of water.  If you don't know, I recommend that you use 4 g for conventional stoves and 3 g for high efficiency stoves (like a Jetboil) until you get a better idea of what your stove requires.  You might add a gram (i.e. 5 g and 4 g, respectively) for water temperatures around 5 C/40 F or lower.  Double the estimate (i.e. 8 g and 6 g) if you're melting snow.
  3. The length of your trip (number of days).
  4. Your margin for error.  This is sort of a "fudge factor".  You add a bit more extra gas in case it's windy or maybe you just underestimated how much water you'd need to boil.  As you gain more experience, you can dial this back.
Remember, all four of these numbers are under your total control.  You set the parameter, and the spreadsheet will tell you what size canister it thinks you should bring.  Note that I have the following canister sizes programmed into the spreadsheet:  100, 110, 220, 230, and 450 grams.
Isn't this a great spot?  Say, we did bring enough fuel to have dinner tonight, didn't we?
Duck Lake, John Muir Wilderness, Sierra National Forest, Sierra Nevada, California.
Example 1.
In this example, I'm going to go out for five days.  See the chart, below.  I'm going out with a partner.  Say, on average, that we both boil about five cups per day and that we have a conventional stove like, say, a Pocket Rocket.  We're going to enter all the numbers that describe our trip and our needs in the gray shaded cells.  The spreadsheet will then recommend in the yellow shaded cell what size of canister at a minimum that you should bring based on the parameters that you've entered.

Let's go through the example:
Line 1.  If we both boil 5 cups each, then we'd boil a total of 10 cups per day, on average.  Enter the number 10 on line one in column "B" in the gray, shaded area on line 1.

Line 2.  Since we have a conventional stove, we'll use "4" as the number of grams we expect to burn per cup on line 2.  Now, this assumes that you know the Basics of Stove Fuel Economy.  If you're not familiar with how to get good fuel economy with a backpacking stove, I suggest that you read through the Basics of Stove Fuel Economy.

Line 3.  Next, let's enter then number of days our trip will be.  I said five, but I'm going to enter just 4.5.  Why?  Well, I'm planning to exit on the last day well before supper time.  Really, the last day is only going to be a half day, so I'm adjusting the amount of fuel I think I'll need by dropping the length of the trip by 1/2 a day.

Line 4.  Do not touch line four.  Remember, only enter numbers into the gray shaded cells.

Line 5.  Now, Line 5 is your "fudge factor," your safety margin.  We are NOT going to estimate the number of grams we think we'll need and cut it off exactly there.  No, we're going to add just a bit more in case something goes wrong or we're off somewhere.  Now, if you really know your stove, your usage, and the Basics of Stove Fuel Economy, then have at it.  Cut the margin down to zero if you like.  It's your spreadsheet once you download it, but I'm going to recommend a 10% safety margin until you know differently.

Based on our estimate that we, on average, will boil 10 cups of water per day (line 1) and that with our conventional stove will need 4 g of fuel per cup boiled (line 2) and that our upcoming trip will be 4.5 days long (line 3) and that we want a 10% margin for error (line 5), the spreadsheet determines (line 6) that you need 198 g of fuel.  The spreadsheet then, in line 7, tells you what minimum canister size you should bring in order to have that 198 g of fuel.  In this case the spreadsheet recommends that you bring at least a 220 g sized fuel canister.

Now, notice line 8.  This is the amount of fuel we will have over our estimated need.  We need 198 g but our canister holds 220 g which means we have 22 g of fuel in excess of our estimated need.

The spreadsheet then tells you (line 9) about how many extra cups that excess fuel will give you if you need it.

Lastly, the spreadsheet tell you how many additional cups you could boil per day based on this excess fuel (line 10).

Why might you want to know how many additional cups per day you have the capability to boil?  Well, half way through our last trip, someone gave us some coffee they weren't going to use.  Suddenly, we had enough coffee to have not one but two cups of coffee each morning.  But do we have enough gas?  Hopefully, with line 10 of the spreadsheet, you'll know if you have the gas for that second cup of coffee.

Let's see, it's three more days until we re-supply.  You did bring enough gas for three more days, right?
The Silver Divide, John Muir Wilderness, Sierra National Forest, Sierra Nevada, California.
Example 2.
In this example, I'm going to go out for three days.  See the chart, below.  Again, I'm going out with a partner.  Again, let's say, on average, that we both boil about five cups per day, but this time we're going to be sharing a Jetboil type stove.

Let's go through the example:
Line 1.  If we boil 5 cups each per day, then we'd enter "10" on line one in column "B" in the gray, shaded area.

Line 2.  Since we have a Jetboil stove, we'll use "3" as the number of grams we expect to burn per cup, on line 2.

Line 3.  Next, let's assume our trip lasts a full three days.  Therefore, I enter "3" on line 3.

Line 4.  Do not touch line four.  Remember, only enter numbers into the gray shaded cells.

Line 5.  Let's just stick with 10% as our safety margin.

Now, on line 6, our baseline estimate + our margin for error comes to a total of 99 g.  The spreadsheet therefor in line 7 recommends that you buy a 100 g canister.  Is that cutting it too close?  Probably not.  Remember, that we've already got a 10% margin for error in there.  However, 110 g canisters are typically just as cheap as 100 g canisters.  I personally would just get the 110 g canister just so I have a slightly larger margin for error, but each to his or her own.

But notice line 8.  You only have 1 g in this example of "excess" fuel.  Just one gram.  In other words, if someone gives you coffee or something, you may not have the gas to boil water for it.  This is important!  I wouldn't want to eat a cold backpacking dinner in the evening because I had burned up all my fuel on a second cup of coffee in the morning.  If I don't have the gas to do something, then I want to know, in advance.  This estimation spreadsheet gives you an idea of how much slack you do – or do not – have.

Thanks for bearing with me through all the explanations.  I hope I was reasonably clear.  If it wasn't clear, please use the comments section, below, to ask questions.  There are no dumb questions.  If it's not clear, ask.

Now, the link.  You can download the Excel spreadsheet using Google Docs.  If you don't have Excel on your device, you can open the spreadsheet with the Google Sheets app.

Please let me know in the comments, below, if you have any problems with the link or the spreadsheet.

Thanks for joining me,

The author at Sallie Keys Lakes on the John Muir Trail.
You're danged right I had enough gas!