Why CO2 ?

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 GrahamD 02 Aug 2022

Why are those emergency tyre inflator cannisters filled with CO2 rather than compressed air ?

1
 john arran 02 Aug 2022
In reply to GrahamD:

CO2 becomes liquid at the kind of pressure you have in a canister, so it's far more compressible than air could be.

OP GrahamD 02 Aug 2022
In reply to john arran:

I'm surprised that the pressure in a one shot tyre inflator was that high, but it must be something like that.

 LastBoyScout 02 Aug 2022
In reply to GrahamD:

I wondered if nitrogen would be better, but I think it would be more more prone to leaking out of bicycle tyres.

1
 gravy 02 Aug 2022
In reply to GrahamD:

I think it is because the cannisters pre-date the emergency tyre fillers and were designed for carbonating drinks (I think, soda water etc).  There's nowt stopping them from filling them with other gases other than the economy of scale. CO2 is a relatively expensive gas at the moment (it's a by product of ammonia production, you may recall a CO2 crisis recently when the UK fertiliser plants shutdown due to energy prices). Practically I doubt anyone could make an economic cases for filling the cannisters with anything else.

BTW I don't think N2 leaks any different from "air" (which is 80% N2), helium maybe...

Post edited at 10:27
1
 CameronDuff14 02 Aug 2022
In reply to GrahamD:

yeah they can be up at around 800psi. Which considering a diving cylinder can be above 3500psi isn't too crazy

 gethin_allen 02 Aug 2022
In reply to LastBoyScout:

Supposedly nitrogen is used in race car/bike tyres. A friend who was into the car modding thing back in the 90s used to go on about having his tyres filled with nitrogen because the pressure doesn't vary as much over a range of temperatures. No idea if this is true or just a load of rubbish. My thoughts were that air or mostly nitrogen so what's wrong with that.

1
 guffers_hump 02 Aug 2022
In reply to gethin_allen:

Yes that is correct, its worth it for race cars. But for standard tyres you'd have to fill and drain the tyre a few times with Nitrogen just to get all the gas inside 100% Nitrogen so ends up being very expensive.

In reply to gethin_allen:

> Supposedly nitrogen is used in race car/bike tyres.

Or normal road cars if you want a talking point! A neighbour’s son did that on one of his past sports road car which he liked to show off. It was claimed though he did it as the pressures were more stable. As least N in the tyres is silent unlike the modified exhaust he put on that car 🤣!

According to the neighbour it was expensive, partly as only specialised garages do it. Also, any puncture repairs or change of tyres and it of course has to be re done at specialist garages.

Post edited at 11:21
In reply to gethin_allen:

Does the combined gas law apply less to nitrogen than it does to carbon dioxide?

Is it the Vo,volume occupied by the molecules that does it,any chemists around to explain?

 LastBoyScout 02 Aug 2022
In reply to guffers_hump:

> Yes that is correct, its worth it for race cars. But for standard tyres you'd have to fill and drain the tyre a few times with Nitrogen just to get all the gas inside 100% Nitrogen so ends up being very expensive.

Pretty easy to do if you're using inner tubes, though.

 LastBoyScout 02 Aug 2022
In reply to Climbing Pieman:

> According to the neighbour it was expensive, partly as only specialised garages do it. Also, any puncture repairs or change of tyres and it of course has to be re done at specialist garages.

Garage near me does it - always seemed pointless for the average driver, as normal air is (as Gravy mentioned) 78% nitrogen anyway.

Incidentally, the last time I hired a road bike on holiday, the guy supplied it with a CO2 pump, but said if I used it, I needed to tell him, so he could deflate and reinflate with normal air. Seemed to think it was bad for the tubes, but I've never heard that from anywhere else. I've not had a problem with the couple of times I've used it.

 gethin_allen 02 Aug 2022
In reply to Presley Whippet:

> Does the combined gas law apply less to nitrogen than it does to carbon dioxide?

> Is it the Vo,volume occupied by the molecules that does it,any chemists around to explain?


I don't know, it's been a long long time since I did any physics.

 gravy 02 Aug 2022
In reply to LastBoyScout:

Main reason to do it (inflate with N2) is that there is an elevated risk of fire (aircraft, racing cars).  There is a very minor change in deflation and tire ageing (mainly due to rubber oxidization) which could cause a very minor fuel saving but only if you never check your tire pressures. Cost-benefit analysis suggests this is -ve benefit unless your extremely expensive vehicle is at risk of catastrophic fire.

I estimate a roadie tyre loses 0.2g when filled with N2 (vs air), gains 3g when filled with CO2 and loses 6g when filled with Helium (which probably leaks like buggery). Never heard that CO2 does owt to rubber.

 jkarran 02 Aug 2022
In reply to LastBoyScout:

CO2 is heavier than air, that's the only reason I can think of not to leave it in a bike tyre and it's not a real reason outside the weight obsessed world of competitive bike shopping.

N2 in tyres is a nonsense for road use. I guess in formula car tyres the O2 may get warm enough to react with the rubber significantly but I'm sceptical it's of more than theoretical benefit. If anything the main gain would be from bottled gas being pretty dry rather than oxygen free. It's magic beans to someone with a noisy Civic. 

Jk

 wilkie14c 02 Aug 2022
In reply to john arran:

I only ever had to use mine in anger the once, got me home without problem though. Beats stuffing the tyre with leafs and grass like back int old days

 Sam W 02 Aug 2022
In reply to gravy:

> CO2 is a relatively expensive gas at the moment

I was thinking about this for both tyre inflators and sodastreams.  Haven't done any calculations, but my assumption was that as they're fairly small cannisters, the cost of the actual gas is trivial in relation to the other production costs, so wouldn't directly contribute to an increase in final cost

 nniff 02 Aug 2022
In reply to LastBoyScout:

> Incidentally, the last time I hired a road bike on holiday, the guy supplied it with a CO2 pump, but said if I used it, I needed to tell him, so he could deflate and reinflate with normal air. Seemed to think it was bad for the tubes, but I've never heard that from anywhere else. I've not had a problem with the couple of times I've used it.

It leaks out faster than air - a road bike tyre filled with CO2 needs to be pumped up the following day, so a rental returned and then sent out the same day will have a soft tyre by the following morning

In reply to gethin_allen:

> used to go on about having his tyres filled with nitrogen because the pressure doesn't vary as much over a range of temperatures. 

I saw that claim the other day, and my brain went 'huh? PV = nRT'

 jkarran 02 Aug 2022
In reply to nniff:

Does it? It's a much bigger molecule than O2 or N2. I suppose it's by diffusion from high concentration inside the tyre to ppm outside? 

Jk

OP GrahamD 02 Aug 2022
In reply to jkarran:

CO2 filled tyres on road bikes go soft in a day or so.  Surprised me but I can vouch the truth of it.

 gravy 02 Aug 2022
In reply to Sam W:

Definitely costs dominated by other stuff such as cannister and supply chain.

However, before the energy price hike, CO2 was a cheap by-product of the Haber process (essential -ve cost to make, compared with similarly compressed other gases).

Now, the raw CO2 is not so cheap, demand for it for important stuff is quite high and the supply is quite restricted (because of the energy price hike and the lack of production in Ukraine) so availability issue may dominate costs.

 Yanis Nayu 02 Aug 2022
In reply to LastBoyScout:

It’s because the CO2 leaks out quickly.

 Sam W 02 Aug 2022
In reply to gravy:

> CO2 was a cheap by-product of the Haber process (essential -ve cost to make, compared with similarly compressed other gases).

Price of CO2 is interesting.  My day job is in anaerobic digestion (using bacteria to convert organic matter to methane containing biogas).  Lots of sites now 'upgrade' the biogas so it's suitable for use as a natural gas alternative.  A by-product of this process is CO2 and in the past it's often been vented to atmosphere as it's expensive to capture.  With high CO2 prices, that's changing and lots of plants are now compressing it ready for export.

 Hooo 02 Aug 2022
In reply to john arran:

They could also use Nitrous oxide, for the same reason. That would be good for a laugh.

 Philip 02 Aug 2022
In reply to GrahamD:

It's a mixture of reasons. Firstly compressing air isn't as simple as it sounds, to get high pressures you need to dry the air. But air or N2/O2 mixture or just N2 would do. So there is a further reason.

CO2 under enough pressure (~70 atm) becomes a liquid so that 12g of CO in a canister occupies about 10cm^3. At room temperature that's about 6000 cm^3 (density of CO2 gas is ~2g/cm^3).

So you get x600 volume going from liquid to gas. If you wanted to achieve that with an ideal gas (you pV = nRT) then you would need x600 p for 1/600 V. So ~600 atm

So in terms of making a cannister to withstand the pressure, much easier for CO2 than a gas

 Tricky Dicky 02 Aug 2022
In reply to GrahamD:

I did wonder about carrying nitrous oxide cannisters instead of CO2 as they could be used for tyre inflation and/or pain relief in case of emergency..................

In reply to gethin_allen:

> Supposedly nitrogen is used in race car/bike tyres.

 

I generally aim for a 78% nitrogen, 21% oxygen mix in my tyres.

1
 crayefish 03 Aug 2022
In reply to Presley Whippet:

The main reason for using N2, rather than air, is the lack of water vapour in there, more so than the removal of oxygen or trace gases.

Water vapour (and its condensation) is the main reason for the fluctuation in tyre pressure with temperature.  Using N2 is basically like using dry air.

 crayefish 03 Aug 2022
In reply to Sam W:

> My day job is in anaerobic digestion (using bacteria to convert organic matter to methane containing biogas).

I'm fairly sure that most of us here on the forum have a 'day job' in producing smelly hot air from food... the bunch of wind bags that we are.

In reply to crayefish:

Thanks,that makes sense, I had assumed both gases were purr. The Vo values may differ but that is unlikely to be large enough to have any influence on the temp stability,more likely to influence leak rate.

 Rampart 03 Aug 2022
In reply to Tricky Dicky:

>  I did wonder about carrying nitrous oxide cannisters instead of CO2

For a long time I marvelled at the number of punctures people seemed to be getting, based on the little cannisters scattered by the side of the road about town...

 onthebend 03 Aug 2022
In reply to jkarran:

Quote from the tinterwebs: 

"The permeability of a gas through rubber depends mainly on its diffusivity and solubility in rubber.

CO2 has a significantly higher solubility in rubber than O2 and N2, whereas the diffusion coefficients differ not that much.

The result is that carbon dioxide passes ordinary rubber about 5 times faster than oxygen and about 15 times faster than nitrogen"

 JLS 03 Aug 2022
In reply to onthebend:

Who knew! I've never used those co2 canisters. It just seems counter intuitive. Ya'd think a thing made of 3 atom would find less holes it could fit through than a thing made of two atoms. Those school chemistry lessons where wasted on me...

In reply to gravy:

Interestingly,  the reason the cist of energy caused a CO2 shortage was because the UKs main fertiliser manufacturer found they could make more from selling their electricity (bought at a cheaper price) than making fertiliser.  Weird. 

 Marek 12 Aug 2022
In reply to JLS:

> Who knew! I've never used those co2 canisters. It just seems counter intuitive. Ya'd think a thing made of 3 atom would find less holes it could fit through than a thing made of two atoms.

Because solubility has nothing to do with 'holes'?

> Those school chemistry lessons where wasted on me...

Yep.


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