Why is carbon dioxide often used as a compressed gas?

March 29, 2006 1:42 AM

Why is CO₂ used as a compressed gas to inflate things like lifejackets and bike tyres?

From Wikipedia:
Carbon dioxide is often used as an inexpensive, nonflammable pressurized gas. Life jackets often contain canisters of pressured carbon dioxide for quick inflation. Steel capsules are also sold as supplies of compressed gas for airguns, paintball markers, for inflating bicycle tires, and for making seltzer. Rapid vaporization of liquid CO2 is used for blasting in coal mines.

This suggests it's because of the price, but there must be more to it than that. Compressed air would be perfectly ok for filling tyres, lifejackets and for firing airguns and paintguns for instance. What is it about CO₂ that makes it best for these applications?

posted by edd to science & nature (16 comments total)

Pure oxygen is highly corrosive, so you don't want to use that. Noble gasses generally don't react with anything, but are relatively expensive. CO2 is probably just the cheapest gas that is not very reactive. An actual chemist could probably provide a better answer.
posted by b1tr0t at 2:28 AM on March 29, 2006

Might be because air contains 20% of oxygen, which might not always be safe to use under high pressure, since it acts as an oxidizer (duh).

I was told never to put any grease on the cap of a pressurized canister of oxygen (100%) as this might already cause an explosion when opening.
posted by koenie at 2:29 AM on March 29, 2006

I assume nitrogen is relatively expensive to extract/obtain compared to CO₂ then? Since that'd be a pretty inert but cheap gas one might use.
posted by edd at 2:45 AM on March 29, 2006

Yeah, nitrogen needs to be distilled out of liquid air, which is very energy intensive. CO₂ is much easier, I don't know where it comes from commercially, but I guess you could always just burn stuff.
posted by atrazine at 3:05 AM on March 29, 2006

Most compressed CO2 is by-product of other processes making it a fairly cheap alternative. There are a lot of other alternatives like methane or oxygen that would otherwise be discarded but CO2 is one of the safest that is produced in large amounts.
posted by Jenga at 3:59 AM on March 29, 2006

From a google for "co2 cartridge liquid'

"What's the pressure in a CO2 tank?

As long as there is liquid in the tank, the pressure in a CO2 tank is determined ONLY by the temperature. At room temperature (70 degrees F) its about 853 psi."

From a paintball site

A lot more gas can be stored in the cartridge since its in liquid form instead of compressed gas.
posted by jjj606 at 5:22 AM on March 29, 2006

That said, nitrogen which remains gaseous appears to be considered a superior propellant. See here. That page covers a few reasons why one compressed gas might be used over another, including oxidation problems and costs.
posted by edd at 6:14 AM on March 29, 2006

What jjj606 said. I can give you a good example from the paintball world:

Until recently, most paintball guns were powered by compressed CO2. Now some companies have started offering "High Pressure Air" systems (which offer more stable pressure under changing temperature). But here's the catch: To give as many shots as the old CO2 systems, the new HPA tanks are twice as large, made of carbon fibre, and store air at a whopping 2000psi or more! (which requires special compressors)

The term you're looking for isn't pressure, but density. Have a look at this chart. The big density change at a given pressure (when it liquifies) allows us to cram a whole lot more fluid into a given size tank.

This is also why propane is used more often than natural gas as a storable gaseous fuel. Propane liquifies at a reasonable 200psi or so, whereas natural gas won't liquify even under thousands of PSI (unless you refrigerate it).
posted by Popular Ethics at 6:24 AM on March 29, 2006

To summarize:
1: Cheap byproduct of industrial processes.
2: You can pack a lot into a small volume without high pressure.
3: Relatively non-reactive, non-toxic and non-flammable.

I'd also suggest:
4: It can be produced by a variety of chemical processes for emergency use.
5: It tastes good in soda pop, so there is a huge market demand for both the compressed gas and the equipment to make it.
posted by KirkJobSluder at 7:22 AM on March 29, 2006

Costco now puts nitrogen in my tires.
posted by weapons-grade pandemonium at 7:23 AM on March 29, 2006

Costco now puts nitrogen in my tires.

Given that compressed air is already 70% Nitrogen, I can't imaging that using 100% nitrogen would really be that much better. Most of what else in the tire besides N2 would be O2, which would be about the same size of molecule.

Frankly it sounds like the same kind of price-gouging nonsense like the 'carbon deposit cleaning' they try to sell you when you get your fluids changed.

From the article you linked too:

Michelin officials recommend nitrogen only for tires used "in a high risk environment and/or when the user wants to reduce the consequences of a potential abnormal overheating of the tire-wheel assembly (for example in some aircraft applications)," according to a company statement.

In other words, putting nitrogen in your tires helps prevent your wheels from catching fire, which is useful in a racecar or airplane, which is why it's used it in those sorts of tires. And if you think about it, long-time pressure maintenance isn't important in either, because in a race care the tires are changed every few laps, and on airplanes most of the time the wheels aren’t even used -- and between flights the air pressure is going to be checked.
posted by delmoi at 8:17 AM on March 29, 2006

The big advantage to Costco using nitrogen is they don't need to keep compressed air on site.

Nitrogen is used by race teams because:
A) they have nitrogen handy to run air tools because many places don't have power in the pits and don't allow fuel powered equipment like generators or gas powered compressors.
B) N2 is dry so tire pressures won't change from liquid water vapourising as tires heat.
posted by Mitheral at 9:05 AM on March 29, 2006

We buy compressed nitrogen to clean dust out of computers with. IIRC, it's something like $60 for a 51 in tall canister of it. Not particularly expensive when you consider the canister lasts us a year of (light) use.
posted by shepd at 9:46 AM on March 29, 2006

Costco doesn't charge for the nitrogen. They also rotate your tires, re-balance them, and torque the bolts/nuts free of charge. And they back the Democrats.
posted by weapons-grade pandemonium at 10:02 AM on March 29, 2006

Just to amplify Mitherals comment, it's possible now to buy nitrogen generators rather than keep a dewar of liquid nitrogen around (which is what was necessary prior). These things are esentially an air compressor attachment that filter out all the other gases and deiver a stream of pure dry N₂. So pure in fact (99.999%) that we can use it directly for trace chemical analysis. So offering nitrogen to your customers is now just a bit more expensive than running a regular air compressor.
posted by bonehead at 10:49 AM on March 29, 2006

For emphasis of what jjj606 said: Liquid N2 can't be kept safely in a sealed container. The low boiling temperature makes the vapour pressure at room temperature enormous. It is always stored cold, near atmospheric pressure, with valves to let the gas that boils off escape.
posted by springload at 2:46 PM on March 29, 2006