Have you ever started packing for a weekend in the backcountry and wondered just how much fuel is left in that canister? Giving it a little shake and guessing will not get you very far. Instead. here are a couple of ways you can get a pretty good handle on just how much fuel is left in all those canister filling up your garage.
Weigh The Canister
By far the easiest way to figure out exactly how much fuel is left is to weigh the canister on a digital scale. Both MSR and Jetboil give you the weight of the fuel (net weight) and the weight of the canister and fuel together (gross weight) to make things easy.
Say you have a small four-ounce canister from MSR and it now weighs 5.5 ounces on a digital scale. That means there is two ounces left of fuel or roughly 50%. A four-ounce canister will give you roughly 10-12 liters of boiling water (unless it’s super cold, then stick to the six liter rule) so you probably have enough fuel left to boil five to six liters of water — plenty for that solo weekend backpacking trip you have planned.
The Float Test
What if you don’t own a digital scale or are already out in the middle of the backcountry and need to know roughly how much fuel you have left? Easy — all you need is some water and some good old-fashioned physics (don’t worry, there will be no calculus necessary).
Place your canister in any still water source (like a pot), making sure to tilt it slightly to get rid of any air bubbles in the concavity underneath. Since a full canister weighs more than an empty one, it will float lower in the water. By rule of thumb, a full canister will sink to where the sides start to round over the top, while an empty canister will have a water level that hits about 1/4 or so up the side. Based on that assumption, you can guesstimate the percentage of fuel left by looking at the water level. If you are using MSR fuel, the company actually prints handy float levels on their canisters.
Transfer Fuel Between Canisters
If you’re like me, you have a bunch of close to empty canisters that you aren’t ready to recycle just yet but can’t exactly take them on a weeks long backpacking trip. The handy G-Works Gas Saver can help.
Resembling your basic canister stove without the burner or pot supports, the device works by screwing the canister you want to remove fuel from to the top and the receiving canister to the bottom. Using the pressure difference between the two, the fuel transfers once the valve is opened. Voilà — one full canister instead of two close to empty ones.
Tip: As the system works on pressure differential, you really need to cool down the receiving canister to lower the pressure compared to the source canister and it can’t hurt to stick the source canister in the sun to warm it up a bit — this will speed up the whole process. And to be sure you aren’t overfilling the receiving canister, a digital scale can come in handy.