Quantity of CO2 in disposable cylinders

[Kev888]

CO2 cylinders are usually filled by weight. This is a good way to describe the amount of gas because it doesn't matter at what pressure the cylinder is at, the amount of liquid CO2 should be the same for a given weight.

Supposedly 1kg of CO2 will expand to roughly 560 litres at atmospheric pressure on a warmish day. I've seen this calculation used several times to calculate volume of gas available from pub gas or fire extinguisher type cylinders.

However, I notice the disposable welding gas cylinders are sold as (for instance) 390g = 60L. This doesn't compute; 390g should give over 200L. All I can think of (assuming they are quoting at atmospheric pressure) is that the weight given for these is total, rather than just the gas, but then that would make them quite light and wussy.

Does anyone know? What am I misunderstanding/missing here?

[BenB]

They weight more like a kg. For sure 390L is the contents. No idea why the discrepancy. I recently moved garages and moved a number of those 390L CO2 and argon bottles and can confirm they don't weight 390g.

[BenB]

I guess they might be quoting the volume at the transition (boiling) point rather than at room temperature?

[Kev888]

Ah thanks for that. Oh well, if 390grams is the contents then out goes my only idea for why this should be; its a mystery to me! I suppose they could be quoting at different pressure/temperature than atmospheric, but why they'd want to do that I've no idea - in welding the gas flows out to cover the welding, rather than builds up in some vessel. Most odd.

[normanh]

Guys an easy calculation for gas volumes - covers all gases is that the mol weight of any gas will occupy 22.4 litres at normal temperature and pressure NTP. Thus CO2 mol weight equals 44, 44g will give 22.4 litres.

Norman

[Kev888]

Yes, give or take some assumptions about room temperature and atmospheric pressure, thats my understanding too. Which is why I'm (still) confused as to how the 390g disposable cylinders are often quoted as giving only 60L instead of about 200L. Ben thinks this refers to the contents, not the whole cylinder which was my only real guess at the cause, so I'm at a loss to explain it; I was hoping someone could.

[vacant]

There's a well defined STP "standard temperature and pressure" (0C) for gases that has been around for years. Sticking to the standard eliminates ambiguity. Normanh referred to "normal temperature and pressure" (20C), but I don't think any physicist would use that?

[Kev888]

Yeah, it could be that they're quoting at something else, but it would have to be 'very' different to make the difference between 60L and 200L for the same amount of gas. Perhaps they do consider some kind of pressure/resistance of the welding torch, rather than as it exits into the air to create the shroud. I suppose that could make sense, the flow meter (litres per minute) would be measuring gas in the supply line.