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I nested my 110g gas canister inside my stainless steel mug for a hike. I used it about halfway through for lunch, and when I was done put it back in the way I had packed it (upside down). Now it is stuck.

My guess is that the canister was at the cooler outside temperature, but the mug had expanded during the cooking. After I put the canister back inside the mug, the mug shrank around it.

The threaded connection for the gas canister is facing top-down in the mug.

How can I safely get the stuck gas canister out of the steel mug? Any ideas?

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    Was the mug designed to be able to hold a 110g canister? If yes, why would it have been so snug that the canister is now stuck? If no, how did you insert the canister without issue and then remove it in the field effortlessly in the first place? – Gabriel C. Dec 12 '19 at 15:39
  • Theoretically you now have a gas canister protector in the form of your mug. Unless the mug has sentimental value you could leave it there until the canister is empty. – speciesUnknown Dec 13 '19 at 10:13
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Soak the mug in a sink of hot water to warm it up quickly. Be prepared to pry the gas cartridge out quickly, perhaps using small screwdrivers or needle nose pliers. The thermal mass of the gas in the cartridge will help slow it heating. Some shaking may help.

Then, don’t do that again;)!

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    @bob1 Not really. The proposed answer relies on heating up the mug quickly while the canister stays cool (because it is thicker metal and full of gas). There is no way to quickly cool the canister while leaving the mug warm. – Martin Bonner supports Monica Dec 10 '19 at 14:13
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    It helps if you cool everything before starting to heat the mug - then there's more thermal difference between the two. Heating the outer part (the mug) is still required (and use hot water, to avoid spots of over-heating). – Toby Speight Dec 10 '19 at 15:30
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    @MartinBonnersupportsMonica: If the canister still contains compressed gas which can be vented quickly, the rapid expansion of the gas will cause it to cool quickly. (You may have noticed this with compressed air canisters, or CO2 cartridges for paintball guns or soda siphons.) Of course, you can only do this once, and it may not be a big enough effect depending on how much gas is present. – Michael Seifert Dec 10 '19 at 20:50
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    "Then, don’t do that again;)!" Hang on! Repeatability is essential for scientific discovery... – Don Branson Dec 10 '19 at 21:34
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    If the exposed base of the cylinder has a hollow in it, as mine all do, filling that with crushed ice will slow the rate at which the cylinder warms and expands as you heat the mug. I'd start with everything as cold as reasonable, and use water as hot as reasonable, where reasonable is taken to be the range of storage conditions for the gas cylinder (this is cautious, deliberately) – Chris H Dec 11 '19 at 15:32
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The coefficient of expansion of mild steel is about 12. The coefficient of expansion of 304 stainless steel is about 17. This means the cup expands more with heat than the cannister, so warming the whole assembly should loosen it.

If it was a porcelain, glass, stoneware or other pottery cup, the advice would be the other way around. Coefficient for porcelain is about 4 and of regular pottery is about 5.

Use of lubricant such as vegetable oil is also a great idea.

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Put a few drops of oil into the mug where it meets the canister. If the oil can seep past there, into the mug, add as much oil as possible. See whether you can twist the canister out. If not: Chill the whole assembly. Get tools to handle the mug and canister. Heat the mug in very hot water, and pry it out. Twist as you pry.

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    Speaking from experience, I would say that the oil idea is a bad one. The only thing it accomplishes is to make the subsequent attempts harder. – Peter Taylor Dec 11 '19 at 11:18
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    Yeah... spray some WD40 on the connecting spots and afterwards throw the cup away or use it for shoe polish only. – Peter - Reinstate Monica Dec 11 '19 at 15:08
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How did this happen?

Yes, the mug was warm. But also, the gas canister got colder. It is stored in liquid form, but you use it in gaseous form. That means as you use it, the liquid gas must boil. To transition from liquid to gaseous state, the liquid gas must absorb heat energy equal to its latent heat of vaporization. It steals that energy by chilling (taking heat from) the remaining liquid and container. Quite colder; you can often see a "frost line" at the liquid level.

(If you're wondering, yes, this tech can be used to make air conditioners. That’s how they work.)

Then, once nestled into the mug, the gas canister warmed up to ambient.

Getting it loose

Start with the whole assembly in the freezer. Give it 6 hours for the liquid gas to get quite cold. Then, pop the mug quickly into the warmest water you can bear. The mug will heat up quickly and expand. The liquid gas is a substantial thermal mass that will make the canister take longer to warm up. You should have a short window of opportunity to separate it.

If not, try again. Just don't handle it in a way that would make it worse!

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Duct tape a loop of twine or a shoe lace to the visible face of the canister. Then pull the canister out.

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You can use hot glue sticks to pull it out. Warm a couple of glue sticks from one end and stick to the visible area of the canister. Let it cool down. Now the glue sticks will act as a handle to the can. You can pull the can out slowly. If glue gets removed, try again a couple of times. You can also use this method along with putting the mug in hot water for a few moments.

Search for "hot glue dent puller" videos on youtube for more information.

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