Estimating Remaining Usage In A Canister Stove
Have you ever been packing for a trip, looked at all of those partially used canisters of fuel you have laying around, and just didn’t feel comfortable taking them because you didn’t know how long they’d last? So you purchase a new canister, use it for a while until you are again not quite sure. Before you know it you have a pile of partially used canisters.
In this post I’ll describe a couple of methods that you can use to determine how much fuel you have left in your canister and how to determine how much you need for your trip. What I won’t do is recommend that you use various adapters to combine or refill your canisters. It’s decidedly dangerous and simply not worth the risk. So before we even get started, I’ll explain what to do with a canister that just doesn’t have enough fuel for a trip.
Nearly Empty Fuel Canisters
When your canister is nearly empty, set it aside or put it in a box labeled “Limited Use”. These are your canisters that really aren’t going to be good for a multi-night trip, but still have a little fuel in them. I like to use these for day hikes, particularly in the winter. I’ll put one in my back for a longer hike and then pull it out on a break to make a cup of hot chocolate or coffee. You only need a cup of water (or milk) and you don’t need to bring it to a boil, so it only requires a couple of grams of fuel. Since you are only out for a day hike, you can even throw a couple into your bag and not worry about the extra weight.
Once it really is too low to even eek enough for a warm cup of coffee, I use the “Crunchit” tool from Jetboil. You use this tool first to safely release any last bits of fuel from the canister and then it punctures the canister itself. You don’t need a special tool though. You can simply attach your stove to the canister and open the valve fully too let out any remaining gas and then just puncture the canister with a screwdriver. Of course when you release fuel from the canister and when you puncture it, regardless of method, be sure to do it in a well ventilated area, away from any source of spark.
Why puncture the canister? There are a couple of reasons. The first is that they canister isn’t really empty. The gas inside a full canister is in liquid form because it is pressurized. As you use up the gas, the pressure within the canister decreases (this is why some stoves perform more poorly as the canister empties). Eventually, it reaches a point of equilibrium with the outside air pressure. At this point there isn’t enough pressure inside the canister to keep any gas in liquid state, and there isn’t enough pressure to push the gas out. And this is what we call an “empty” canister. In reality there is a bit of gas still in the canister.
And having a truly empty canister is what will allow you to recycle it. By puncturing the canister, it is easy visible confirmation that the canister is empty and not under pressure. You can now take it to a mixed-metals recycling facility. And if you happen in be in the Seattle area, you can take to the MSR Repair facility where they will also give you a discount for 20% replacements!
Weighing Fuel Canisters
The best way to determine the amount of fuel left in a canister is by weighing it. To do this, you’ll need to have a precise scale that measures grams. I use an inexpensive Nicewell Kitchen Scale that measures to the nearest 1gram. That is precise enough for our purposes. In my testing, there is more than 1g of variability in boil times, so anything more precise is just unnecessary.
To determine the amount of fuel remaining, you will also need to know the weight of your empty canister. The best way to determine this is to simply weigh one of your empty canisters before recycling. If you don’t have one handy, some canisters list both a gross and a net weight on the canisters. The canister weight would be the difference between these two.
In this MSR example, the medium canister lists a Gross Weight of 374 gram and a Net Weight of 227 grams. The canister weight then is 374-227 or 147 grams. The small canister then is 211-110 or 101 grams.
Below is a table of some of the canisters I have either tested or have looked up from manufacturer specs.
And if your fuel canister does not list Gross Weight or is not in the table of fuels I have listed, know that many of the canisters are actually produced in the same factories and will have very similar weights. See the similarity between the caps in these two canisters? While each manufacturer may have a custom blend of fuels and slight variations of fuel weight, the canisters are coming from the same factory in Korea and so likely weigh the same. If your canister cap looks like this, you can pretty safely just use about 100g for the smaller “4 oz” canisters and about 150g for the medium “8oz” size canisters.
Determining Fuel Consumption: Weight Calculation Method
The next part of the equation is figuring out how much fuel you actually use when you cook. I usually only use my stove to boil water. This makes meal prep and cleanup easy and also makes this exercise much simpler. I simply have to determine how much fuel is consumed to bring my water to a boil.
To do this, you will need your scale along with the stove and pot you intend to use. There is a lot of factors that affect stove performance, including the pot that you use and whether you keep it covered or not. So, set it up just as you will do when you’ll be cooking in the field.
With everything assembled, including your measured amount of water, you’ll need your starting weight. Put the canister (I leave the stove on, but it doesn’t matter) onto the scale and record the weight.
Now, go ahead and place your pot of water onto the stove and ignite. A timer is not necessary, but it can be handy. If you boil water with the pot covered how do you know it’s boiling? You can listen for it, and that is what I usually do in the field. But I also know that my stove/pot combination will bring a cup of water to a boil in a little over 1 minute. So, I watch the timer and remove the lid at about the minute mark and then watch until it develops a full rolling boil.
Once it has reached a boil, turn off the stove and remove the post. Check your final weight and record this as well.
In this example, I was boiling 1 cup of water and had a starting weight of 298 grams. The final weight was 292 grams so now I know it takes about 6 grams of fuel to boil 1 cup of water. If you occasionally boil differing amounts of water, it may be helpful to repeat this procedure for each water volume as the fuel consumption is not linear. This same setup, for example, will bring 2 cups of water to a boil using 9 grams of fuel, not 12 as you might expect if it were linear.
Now that we know how much fuel we need to bring a cup of water to a boil, we can determine how many boils we should get out of a canister by simply dividing our net (fuel in the full canister) weight by the fuel consumed in our boil test.
If I use an MSR 4oz canister, that would be 110 grams of fuel divided by 6 grams of fuel per boil. 110/6 = 18.33. So, I will get about 18 boils from a single canister of stove.
Some people like to write this number somewhere on their can and then just start creating tick marks on the can each time they boil. I don’t like this method because you have to either carry a marker with you in the field or you have to reliably remember when you get home from your trip. And if you do end up deciding to do something different, like boil an arbitrary amount of water, your system is thrown off. Instead, I just record the weight of my canister after every trip.
Now before a trip, I determine how many boils I will need. On a typical weekend trip, I’ll boil for Friday dinner, Saturday breakfast and dinner, and Sunday breakfast. That’s 4 boils using 6 grams per boil, for a total of 24 grams. As long as I take canister with at least 24 grams of NET weight, I’m good.
Looking at the canister in the above picture with a current weight of 210 grams, if I subtract the 150gram weight of the canister itself, I have 60 grams of fuel left. Plenty of fuel for my weekend trip.
The Float Method
You might have noticed on the side of your canister a description of estimating the amount of fuel in your canister by floating it in water, like this MSR fuel canister?
This will be a pretty course measurement, but it does work. And it can work in the field as long as you can find a still pool of water. Just place the canister in the water, tilting it a bit sideways to allow the air bubble formed under the convex bottom escape. The see where the water line is relative to the gauge.
This this example, it looks like the water level is between the 1/4 and 1/2 marks. Since we know this canister holds about 227g of fuel when full, it should be between 56g and 113g. Splitting the difference, maybe 84 grams or thereabouts.
Looking at our scale to validate we get a weight of 224grams. Subtracting out the weight of the canister (~150g) we get 74g of fuel. Our float estimate wasn’t too far off.
MSR Float Weight Confirmation
The Bismar Scale
A bismar scale is similar to a balance scale. You are probably familiar with a balance scale, like an old-time merchant scale where you would determine the weight of an object by placing known, standard, weights, on one side of the scale until it balances out with the weight of the object. Add up the standard weights and you have your object weight. In this type of scale, you have equal length arms on either side of a fixed fulcrum:
With a Bismar Scale, rather that varying the amount of standardized reference weights you add to the scale, you are instead varying the the fulcrum point, creating unequal arm lengths. Using the principles of a lever, you just move the calibrated bar along the fulcrum until balance is achieved. If you’ve ever tried to balance a pole or a stick across you finger, you were creating a Bismar scale!
If you squint your eyes and maybe enjoy another cocktail and bend your imagination a bit, that calibrated bar on the Bismar Scale starts looking like a hiking pole, doesn’t it? Well, you actually have a pretty decent field scale if you calibrate your hiking pole and use the edge of your knife as the fulcrum. But it does require a bit of a setup and poles that aren’t too light or don’t have handles that extend too far.
I happen to have measuring scales on my poles. I find it useful to lay out the scales when snapping pictures of animal tracks or plants, for example, so that I have a scale reference to aid in identification when reviewing photos later.
So the only thing to do then is to calibrate the scale. To do this, find a bottle or something that can contain some water. We’re just going to use the water as a controllable weight. Increasing in 10gram increments and recording where the on the scale the fulcrum balances.
Starting at 100 grams (the empty weight of a small canister), hang your weight from the handle strap of your pole and then balance the pole on a knife edge. Record where on the scale the knife edge is resting once you find the balancing point.
Calibrating Bismar ScaleAdd 10g of water so that you have 110g and redo the procedure. As the weight increases the fulcrum point will move closer to your handle (to the weight). Note that if you have collapsible poles that you will need to also note the pole length when you calibrate as it does make a difference. The longer the pole, the more range you will have for your scale. But if you set them to a length other than what you use when you hike, you’ll just have to remember to change them before you rely on your scale.
In my case I was able to calibrate my pole roughly as:
100 g = 6.6 cm
110 g = 5.6 cm
120 g = 4.9 cm
130 g = 4.1 cm
140 g = 3.1 cm
150 g = 2.2 cm
160 g = 1.7 cm
170 g = 1.0 cm
180 g = .5 cm
190 g = 0 cm
This is just about right to cover the range from an empty small (4oz) canister to one nearly full before I run out of room on my scale (which begins at the base the extend foam grip below my handles. The results should be exponential. The distances should be getting closer together for equal differences in weight as you increase the weight. My results aren’t spot on due to a significant margin of error (it’s hard to get the pole to balance perfectly level) and my impatience. But it’s close enough for rough gauging fuel, so, let’s see how it works out.
I found the rough balancing mark for this canister around the 2.2 cm mark. Looking at my calibration test above, that would put it around 150 grams of fuel. Let’s see how it compares to actual measured weight.
And look at that, 152 grams. Pretty darn close!
If you don’t have scales on your poles, you can simply use a marker to mark the calibration points or even etch them with your knife. If you have the calibration marks and start from an easy reference point, like 100g, it’s easy to remember that each mark is another 10 grams. Alternatively, you can write out your calibration on a piece of paper and just place that somewhere in your pack for reference.
This is a fantastic party trick by the way. Impress your friends around the campfire by weighing the contents of their fuel canisters.
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