I bought a CO2 cartridge a while back. I think it is used to fill up tires.
If I puncture the cartridge, will it last a while? Would it maintain pressure for at least 6 months?
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Think of these as a one shot device. Once the seal is cracked, any remaining CO₂ will leak out over time, and you have no idea just how much is left. .
How much time I have no idea, but the problem with it is as you have no idea how much is left in the cylinder (which is as likely as not no where near enough to pump up the tire), you have to carry enough cylinders in case its empty. If your carrying enough in case its empty, you are carrying extra weight.
While I do carry one of these, on long trips (more than 30minutes ride from a road with cell phone coverage), I carry a real pump. Between the risk of multiple punctures and loosing the gas from incorrect application (Don't laugh it happens surprisingly often) I see these are a time saver, not a life saver.
Additional time information:
These cartridges are filled with Carbon Dioxide, which is a relatively small molecule and can diffuse through butyl rubber more easily than atmospheric air can. You can inflate a tyre in seconds with these but it will likely be too flat to ride the following morning. You will have to re-inflate your tyre with normal air, which includes larger molecule that leak out slower.
If you don't use all the CO2 gas in the cartridge, it will leak out over the next ~30 minutes approximately.
Also, these little cartridges are expensive at about 5 pounds/dollars/euros each (for me, you might find them cheaper, but they're not free nor cheap and the costs add up over time.)
There are only three situations where they make sense:
And if you need it again on a ride, then you need a second one-time cartridge. It is easier and cheaper to carry a minipump.
The aluminium cartridges can be recycled in your common metal recycling, so take the empties home and don't litter. Also note - they get very cold when discharging fast, to the point of freezing flesh, so use cloth or only hold the green part.