Thread: refill your disposable canisters

There are 3 main gases we use for cookers. n-butane, Isobutane and Propane. They all have different properties and we use them in different situations.

The boiling point of the gases are:
n-butane = -0.4C
Isobutane = -11.7C
Propane = -42C

As you increase the gas temperature above the boiling point of the gas the pressure inside the canister rises. At 20C n-butane is still pretty harmless and can be contained inside a plastic Bic lighter, you could not do this with 100% Propane. Have you ever seen one of those green steel gas canisters that Coleman makes? That's 100% Propane. That is what is needed to contain that gas.
So n-butane is no good for our needs and 100% Propane is no good due to the heavy steel vessels we would need to contain it. What we use is a mixture of Isobutane ( a chemically rearranged n-butane which lowers the boiling point) and Propane. A lot of gas canisters don't list the mix ratio anymore but it's around 80/20.

LPG in New Zealand is typically 60/40 Propane and Butane.

What happens when you fill a camping canister with LPG and your pot of boiling water overflows onto the canister below? Or you use a windshield around your cooker? These canisters are not designed to withstand the pressures of the higher ratio of Propane. You are literally risking your (or whoever is near the stove) life doing this guys. Youtube has some examples of what a camping canister explosion looks like - Do you really want to risk it?

The only way you should refill your camping canisters is with the same blend of gas it originally had in it. Since most manufacturers don't list the blend anymore the only real way to minimise the risk is to transfer from the same brand to the same brand eg. MSR to MSR. It's generally cheaper to buy gas in the larger bottles so this really the only reason to be doing this, or you want a custom amount inside a canister instead of two small ones you could take a 3/4 full larger can and save on the weight of the can material.

The correct way to do the gas transfer is to weigh your empty canister first and record the weight. Say it was a 230g canister and the empty canister weighed 150g. You would fill the canister until it weighed 150+230=380g this is the safe amount. Do not overfill. Put your donor canister (of the exact same blend of fuel) in warm (NOT hot) water and your receiving canister into an ice bath. Once the donor canister has warmed up you invert it and using the adapter you start to fill the receiving canister which is still in the ice bath. Let it fill for a minute or so and then weigh the receiving canister. Repeat until you reach your safe fill value. If you exceed the safe fill value vent the excess off outside or use a stove to burn it off.

I can post some links to further reading and a suitable adapter if anyone is interested to learn more. I would recommend any of you that have filled your canisters with LPG to vent them off.