Here’s a way to quickly and accurately measure how much gas is left in a metal fuel canister when you’re on the trail, within a tolerance of about ±1g.
At home, you can quickly measure the fuel in a canister by weighing it on a kitchen scale. A full small fuel canister weighs 200g, and that same canister weighs 100g when empty. But on the trail, you can’t guess the weight of something in that range very accurately just from its heft in your hand.
It turns out that you make some simple marks on a hiking/trekking pole to create an ancient kind of balance scale called a bismar scale (http://www.isasc.org/Tutorial/Scale-Types.html#Bismar). In such a scale, the pivot moves along a balance arm, while the weight and counterweight stay fixed. The moving pivot makes such a scale harder to read, and less useful, in general life but can still be accurate enough for our purposes.
Here, your hiking pole acts as both the balance arm and the counterweight against the canister you want to weigh. All you’ll need to do is scratch some marks on your pole at home beforehand:
This hiking pole has been etched with 11 crosswise marks. The tiny knife is pointing to the leftmost mark, which represents a weight of 100g. The rightmost mark represents 200g, with other marks at 10g intervals between. The 150g mark in the middle, as well as the 100g and 200g marks on the ends are slightly longer than the other marks.
In a bismar balance, the relationship between the marks and weight is NOT constant — you can notice a bigger gap exists between the 100g and 110g marks on the left than between the 190g and 200g marks on the right.
Suppose you’re on the trail, and want to know how much fuel remains in a canister. First, knock any dirt off your pole. Then, slip the canister into your pole’s strap. Conveniently, you can tighten the pole strap to ensure the canister won’t slip out.
The fact that the strap hangs off your pole at a fixed location means that the load you want to weigh is always in a known position. You then just balance the pole on your finger or — if you want better accuracy — the blade of a knife:
Once you’ve balanced the pole, you look down to see to read the canister’s weight:
Here the knife blade is just about at the 110g mark, maybe a hair to the left, so somewhere between 109g–110g. (As noted above, the distance between weights on this scale gets smaller the heavier you go, so you need to keep that in mind when reading the scale.) This result means you’re nearly done with this canister, but still have a bit left for a boil or two.
I happened to take the above pictures on the last morning of a recent hike. Now back at home, let’s check the actual result on a kitchen scale:
The kitchen scale reads 109g, so the hiking pole balance was pretty accurate! Each time I’ve tested this hiking pole scale against a kitchen scale, the weight given by the hiking pole has been within a gram of the weight given by the kitchen scale. That precision gives you a very good sense of how many meals or hot beverages you can prepare with what remains in your canister.
How to mark your hiking pole for use as a scale:
Making the markings at home is simply a matter of measuring a series of known reference weights, and scratching marks at the balancing points. I found that a small, sealable TupperWare container that could be filled with water worked as an adjustable reference weight.
1. Clean any loose dirt off your pole.
2. Start by checking that your pole can accommodate marks at the heaviest end of the range you want to cover. If you have a short pole, the heavier balancing points might end up on your handle. (The collapsible 48” pole shown in these photos is very light, so the balancing marks ended up near the top of the pole, just below the handle, making such a scale just barely possible.) So begin with the heaviest weight you want to be able to measure (e.g., 200g) first. Put the container on your kitchen scale, with its lid next to it. Then slowly fill the container with water until it weighs exactly 200g.
3. Once you’ve got the container of water weighing exactly what you want, seal the container with the lid. Slip it into the hiking pole strap and tighten the strap.
4. Balance the pole on the edge of a knife blade. Take your time. You may have to make very slight adjustments back and forth until the pole holds fairly still.
5. When the pole is balanced, mark the pole at that position: press the pole down into the knife blade, and rotate it back and forth a bit to score the underside of the pole.
6. Repeat steps 2–5 for the remaining weights: 190g, 180g, …, down to 100g.
7. Grab a fuel canister, give the scale a test, then weigh the canister on the kitchen scale to see how well the scale works.
Since you’re calibrating your scale for future use, it pays to make the reference weights as accurate as possible. I found it useful to remove the container of water from the kitchen scale, then re-weigh it to double-check that I really had the weight correct.
You could add numbers (e.g., with a dremel) if you wanted, but I didn’t feel the need for those; I’m the only one who has to know what these marks mean. I’d originally tried writing marks on a sticker or tape on the pole, but the handle on this collapsible pole needs to cover that portion of the pole when the pole is collapsed, and a sticker or tape was too thick to slide under the pole’s handle. Scratches work well enough, and are easy enough to make.
I find that carrying this balance scale with me is reassuring. I like the convenience of gas canister stoves, but have wished there were a way to know how much fuel is left in a canister. This is part vague anxiety, and part practical consideration. If I’m near the end of my last canister, I’d like to know if I have enough fuel for both a hot morning beverage, while still leaving enough for a hot meal at the end of a day. On a longer trip, I may want to check my fuel consumption as I go for peace of mind. Using a hiking pole as a balance can answer those questions, and is just fun to be able to do!
