Cycling UK Forum

This is more of a "chat over a cuppa" than an "issue", I've been pondering why tyre inflators use CO2.

I had an N+1 moment earlier this year and a Brompton has joined the family. In "normal" use, if I get a flat I'll just fold it up, get on a bus or the underground and fix it when I get home. As & when I take it travelling though, that won't be an option and the thought of getting the pressure up to the required 7 bar / 100 psi with the little pump the bike comes with seems less than entirely possible. I have therefore acquired a CO2 inflator.

The question I've been leading up to is why use carbon dioxide? The tyres on high performance cars are often filled with nitrogen because those "in the know" say it's more stable than air (AKA Nitrox 21), so why don't bike tyre inflators come filled with N2 rather than CO2?

My only possible explanation (so far) would be that the CO2 cylinders were already available for those fizzy drink makers and some bright spark built a tyre inflator around them.

Anyone know the actual history?

You can't compress nitrogen as much as Co2, the canisters would be a lot bigger.
Will you need a full canister on those little wheels though?

philvantwo wrote:You can't compress nitrogen as much as Co2, the canisters would be a lot bigger.
Will you need a full canister on those little wheels though?

Good point on the compression level, I hadn't considered that.

Everything I've read suggests that a 16g canister does the job adequately without grossly overinflating the 16" wheels. I've yet to test it myself though.

I asked this very question a couple or so years ago.

Basic answer, is that there's a ready supply of CO2 as it's used in the drinks trade for one thing and cheap to produce too.

Mick F wrote:I asked this very question a couple or so years ago.

Basic answer, is that there's a ready supply of CO2 as it's used in the drinks trade for one thing and cheap to produce too.

So my chicken / egg theory wasn't far off, the cylinders already existed for those soda syphon gadgets & someone built a tyre inflator to use them?

Liquid doesn't absorb nitrogen which is why CO2 is used to carbonate drinks. Nitrogen is used to pressurise beer kegs if the cellar is a long way from the bar.

CO2 is cheaper and easier to handle because (weight for weight of gas) the cylinders can be smaller (CO2 gas is usually a liquid in the bottle) and lighter (the pressure can be a lot less).


For example a medium sized CO2 fire extinguished might contain 5kg of CO2 gas (as liquid CO2) and the bottle might have a test pressure of about 100bar, and by itself the bottle might weigh about 5kg too. To store a similar weight of nitrogen might require a bottle of three or four times the volume, at a working pressure of ~250bar (and a much higher test pressure obviously). The bottle is liable to be about ten times heavier.

'Pub gas' these days is often a nitrogen/CO2 mix. The CO2 is there to make the drinks fizzy and because it is cheap. Annoyingly (or perhaps deliberately on the part of the gas suppliers) this renders such pub gas almost 100% useless for anything else; by contrast pure Nitrogen is useful for 'dry inflating' tyres and pure CO2 gas can be used for MIG welding ( of steel).

cheers

Purely out of curiosity (as an uninformed OAP who regards lager as exotic compared to my normal bitter!) Anyone know if it would be possible to inflate a tyre using the ubiquitous nitrous oxide cartridges which litter the gutters of South Manchester ?
Many's the time, stormbound on an isolated midge-ridden campsite somewhere on the west coast of Scotland, that an innertube full of said product might have proved a welcome distraction. Possibly qualifies as another usefully multi-purpose cycle camping item? It certainly seems to work for the yoof of today if the two boxes (each containing 72 emptied cartridges) that I recently spotted dumped in a local car park are anything to go by. I wonder how many of those were inhaled by the actual driver?
The joys of urban cycling..........

AFAICT;

- the same physical size cartridges are used for both CO2 and N2O (so you ought to be able to get an N2O cartridge that fits your CO2 tyre inflator)
- food-grade N2O is sold legally for use in whipped cream products (where the gas presumably doesn't cause the cream to spoil as quickly; no wonder eating whipped cream makes some people happy (briefly)…. )
- similar sized cartridges might contain ~8g of N2O (@60bar) vs ~16g of CO2 (at a much lower pressure).

CO2 is not a particularly good gas for inflating tyres with, because it diffuses through rubber very quickly. The exact reasons for this are complicated; after all, at first sight larger molecules ought to be inherently less likely to wiggle between molecules in a medium like rubber. However if the gas is very soluble in the target medium, the rate of diffusion can be greatly increased. Turns out that CO2 dissolves in rubber, far better than (say) Nitrogen does. CO2 thus diffuses about ten or more times more quickly through rubber.

I couldn't find any figures for N2O diffusion through rubber but if it is anything like CO2 (and it might well be, both are rather polar molecules of comparable size) then N2O is going to diffuse out of your tyres just as quickly. This combined with the higher fill pressure/lower volume means that N2O may be entirely unsuitable for inflating tyres with; when the cartridge first starts to discharge it will do so at several times the rate you are used to with CO2, when used in a similar device. It will obviously only be able to inflate a tyre of about half the volume, too.

BTW it is recommended that N2O cartridges are not exposed to temperatures of more than 50C, presumably because they are in danger of bursting. Such temperatures are easily exceeded in (say) the back of a car on a sunny day. It'll be less bad with CO2 but this can be bad enough; I was once daft enough to leave a CO2 bottle in the back of my land rover on a spring day. Needless to say the sun came out and by the time I got to it, mid-morning, the gauge on the bottle read about 80bar (vs 10-20 bar on a cool day). If I'd left it in the sun all day I think there is a real possibility that the bottle might have burst.

cheers

Surely using hydrogen would make the bike lighter.

st599_uk wrote:Surely using hydrogen would make the bike lighter.

Yes but helium would be less likely to result in spontaneous combustion!

st599_uk wrote:Surely using hydrogen would make the bike lighter.

Especially when complemented by these mudguards: http://freshtripe.co.uk/velo-orange-52m ... ards-700c/

Thanks Brucey for your comprehensive (as ever) response which has assuaged my curiosity and kept me on the straight and narrow! . Not that there was ever any chance of me abandoning bitter as my drug of choice.
Disturbingly further dabbling on the web (I really must get out more) reveals that nitrous cartridges can often be bought more cheaply than the carbon dioxide ones, so despite the differences in volumetric content it's little wonder that they're proving so attractive to today's yoofs.
I see that the empties are recyclable alongside tin cans & the like - it's a shame the little darlings admirable concerns about climate change don't spread to responsible disposal of the nitrous cartridges they're dumping everywhere. I can foresee a cyclist being unseated by a handful of empty cartridges in the near future. (Whoops, slipped into Meldrew mode there. Apologies!)

Brucey wrote:CO2 is not a particularly good gas for inflating tyres with, because it diffuses through rubber very quickly...

This makes it an ideal consumer product: readily available, widely accepted and doesn't last.