bishbosh Posted January 26, 2007 Share Posted January 26, 2007 You need the phase diagram for each gas. CO2 - HERE Argon - HERE. I haven't looked at the detail, but basically, the combination of pressure and temperature inside the bottle will result in one remaining in the liquid phase whilst the other is gaseous. Cue chemist who actually knows what he's talking about.... Quote Link to comment Share on other sites More sharing options...
rtbarton Posted January 26, 2007 Share Posted January 26, 2007 You need the phase diagram for each gas.I haven't looked at the detail, but basically, the combination of pressure and temperature inside the bottle will result in one remaining in the liquid phase whilst the other is gaseous. Cue chemist who actually knows what he's talking about.... That's my point exactly - so how can the CO2 get out as a gas along with the argon? Quote Link to comment Share on other sites More sharing options...
Astro_Al Posted January 26, 2007 Share Posted January 26, 2007 But if one is a liquid, won't it just sit at the bottom? How can they come out as a 'mixture' if one is a gas adjacent to the valve at the top of the bottle, and one is a liquid at the bottom of the tank? Al. Quote Link to comment Share on other sites More sharing options...
bishbosh Posted January 26, 2007 Share Posted January 26, 2007 Looking at the two diagrams. CO2 is on the saturation line (i.e just about to vaporise / condense) at 50 bar and 15 degrees C. Argon is gaseous at 50 bar down to minus a lot - better phase diagram HERE. So my guess is that the cylinder needs to be above 15 degress for both to be gas or that there is some clever mechanism inside the bottle that forces the liquid out ahead of the gas into a vaporiser????? Dunno - I'll ask my Dad - he was a chemist once upon a time. Quote Link to comment Share on other sites More sharing options...
dollythelw Posted January 26, 2007 Share Posted January 26, 2007 they are held together by love - obvious really c'mon guys! spread the luuuuuuuurve Quote Link to comment Share on other sites More sharing options...
Jon White Posted January 26, 2007 Share Posted January 26, 2007 they are held together by love - obvious really c'mon guys!spread the luuuuuuuurve You great tart Jez!!! You'll be encoraging the whole world to paint their argoshield bottle pink to "spread the luuuuurve" next!!!! Quote Link to comment Share on other sites More sharing options...
rtbarton Posted January 26, 2007 Share Posted January 26, 2007 Looking at the two diagrams. CO2 is on the saturation line (i.e just about to vaporise / condense) at 50 bar and 15 degrees C. Argon is gaseous at 50 bar down to minus a lot - better phase diagram HERE.So my guess is that the cylinder needs to be above 15 degress for both to be gas or that there is some clever mechanism inside the bottle that forces the liquid out ahead of the gas into a vaporiser????? Dunno - I'll ask my Dad - he was a chemist once upon a time. But at over 50 bar the CO2 will be liquid, the pressure in the bottle is c210 bar! Quote Link to comment Share on other sites More sharing options...
Astro_Al Posted January 26, 2007 Share Posted January 26, 2007 Also, there is no way that the cylinder stops working at 15 degrees C - my garage has no heating (like most, I guess?) and it's sure as hell colder than that in the winter, but welding is just the same... ??? Al. Quote Link to comment Share on other sites More sharing options...
simonr Posted January 26, 2007 Share Posted January 26, 2007 The reason the CO2 does not liquefy is the same (ish (for the know-it-alls)) as when you add salt to water and it raises the boiling/freezing points. The phase diagram for a mixture will be quite different (depending upon the ratio) to either of the pure gasses. In fact, the diagram morphs from one to the other depending on ratio. There will still be a pressure / temperature combo which will liquefy the mixture. It will probably be lower than the temp for pure argon and higher than pure CO2. Also, to increase the pressure by 30 bar, you will need a temperature rise of about 40 degrees. Although this sounds a lot, as the bottle is filled, it's temperature increases. It could easily have a mean temperature of 40C by the time it is full. Leave it outside on a cold day - and it drops to 200. At 210bar, the temperature will have risen by about 14C. I had a particular liking of physical chemistry at school. Sadly, the person setting the A level questions had a similar liking for Inorganic! Si Quote Link to comment Share on other sites More sharing options...
rtbarton Posted January 26, 2007 Share Posted January 26, 2007 The reason the CO2 does not liquefy is the same (ish (for the know-it-alls)) as when you add salt to water and it raises the boiling/freezing points.The phase diagram for a mixture will be quite different (depending upon the ratio) to either of the pure gasses. In fact, the diagram morphs from one to the other depending on ratio. There will still be a pressure / temperature combo which will liquefy the mixture. It will probably be lower than the temp for pure argon and higher than pure CO2. Si Thought that might be it - so for fractional distalltion to work you have to up the pressure and/or decrease the temperature of the mixture much more than you would to liquify the pure gasses independently?? Quote Link to comment Share on other sites More sharing options...
simonr Posted January 26, 2007 Share Posted January 26, 2007 Thought that might be it - so for fractional distalltion to work you have to up the pressure and/or decrease the temperature of the mixture much more than you would to liquify the pure gasses independently?? A good reason for not storing mixtures of gasses in liquid form is that the composition of the vapour (proportion of each gas) is temperature dependent. It is also different to the proportions of liquids in the mixture. It is this difference that makes fractional distillation possible. If you are interested, there is a reasonable description of the whole thing, including fractional distillation here Quote Link to comment Share on other sites More sharing options...
GBMUD Posted January 26, 2007 Share Posted January 26, 2007 Going back to Tony's original question Si, am I right to think that the pressure in the cylinder will be the same when the cylinder is half empty as when it is new (and one assumes, full) asssuming that the temperature is the same? Chris Quote Link to comment Share on other sites More sharing options...
bishbosh Posted January 26, 2007 Share Posted January 26, 2007 Nope - can't be as 50% of the gas has gone therefore 1/2 the gas is occupying twice the voulme it used to therefore pressure will be halved if temp is constant..... I think Quote Link to comment Share on other sites More sharing options...
GBMUD Posted January 26, 2007 Share Posted January 26, 2007 Nope - can't be as 50% of the gas has gone therefore 1/2 the gas is occupying twice the voulme it used to therefore pressure will be halved if temp is constant..... I think So that would be to suggest that at any given temperature a smaller volume of gas requires less pressure to maintain it as a liquid? How small an amount of gas would remain as a liquid at atmospheric pressure? Chris Quote Link to comment Share on other sites More sharing options...
rtbarton Posted January 26, 2007 Share Posted January 26, 2007 For gas only the pressure will drop as the gas is used, watch the pressure gauge on an air compressors's receiver go down as you use a spray-gun. Applying the forumlae above, volume is the same, pressure drops so temperature drops, so the reciever will cool down. Conversley it will heat up as air is forced into it when the compressor starts. If there is liquid in the cylinder as in the case of a CO2 bottle, the pressure will stay the same until all the liquid is boiled off. The cylinder will still cool down when gas is drawn off due the liquid giving off its latent heat of vapourisation. Once the liquid is gone, the pressure drops very quickly as the gas is used up, which is why CO2 bottles seem to empty suddenly rather than gradually. PS - the volume of gas in the receiver stays the same, but the mass decreases, Quote Link to comment Share on other sites More sharing options...
rtbarton Posted January 26, 2007 Share Posted January 26, 2007 So that would be to suggest that at any given temperature a smaller volume of gas requires less pressure to maintain it as a liquid? How small an amount of gas would remain as a liquid at atmospheric pressure?Chris No. A big bottle of CO2 will be at the same pressure as a small one. If you empty a liquid gas onto the floor it will all boil away. The PV/T formla only applies to gas in its gaseous state. Quote Link to comment Share on other sites More sharing options...
GBMUD Posted January 26, 2007 Share Posted January 26, 2007 No. A big bottle of CO2 will be at the same pressure as a small one. If you empty a liquid gas onto the floor it will all boil away.The PV/T formla only applies to gas in its gaseous state. That is what I understood then Bish seemed to add confusion. Thanks Roger. Chris Quote Link to comment Share on other sites More sharing options...
white90 Posted January 26, 2007 Author Share Posted January 26, 2007 well I took it back and got another one this one shows 220bar in the cold from being collected. Quote Link to comment Share on other sites More sharing options...
Bull Bar Cowboy Posted January 26, 2007 Share Posted January 26, 2007 well I took it back and got another one this one shows 220bar in the cold from being collected. :lol: Tight Git ! ........ BTW: I get mine from BOC in Marsh Barton ............ yes, sometimes they are a bit light ! Ian Quote Link to comment Share on other sites More sharing options...
white90 Posted January 26, 2007 Author Share Posted January 26, 2007 after 2hrs practice though I called the supplier first he said bring it back tomorrow(Friday) I called Thursday I said I have welding to do he said doesn't matter so I got some practice in Quote Link to comment Share on other sites More sharing options...
white90 Posted January 28, 2007 Author Share Posted January 28, 2007 Update! the replacement cylinder if bluddy leaking from the turn on/off tap! you can see the gas escaping and the tap id freezing up, I've turned it off and left it outside facing the fields incase it lets go. can anyone confirm if I can replace the cylinder at any BOC outlet? as I'll be nowhere near the one I used to get the last two cylinders from tomorrow. Quote Link to comment Share on other sites More sharing options...
pugwash Posted January 28, 2007 Share Posted January 28, 2007 you can definitely use any outlet- i swapped a tank in taunton for Paul C who usually swaps tanks in Yeovil. You will need your account number though so they can phone BOC H/O and get all the details. Either the same tank is doing the rounds, or a leaking tank is quite common, as Nick W had one that did this too- emptied the tank in about half an hour! Quote Link to comment Share on other sites More sharing options...
white90 Posted February 23, 2007 Author Share Posted February 23, 2007 Well after x1 cylinder being light of Argoshield and x1 leaking from the tap I complained to BOC and yesterday was advised I got a complete refund for the exchange cylinder. so a new bottle of Argoshield universal for nowt instead of £40 ish. maybe worth noting if you get a leaky cylinder etc. Call BOC and ask to raise a ticket against a charge on your account. Quote Link to comment Share on other sites More sharing options...
dollythelw Posted February 24, 2007 Share Posted February 24, 2007 leakers are pretty common - especially with the tap type valve (as opposed to bottle key type), get into the habit of shutting down the bottle as soon as you are finished welding, purging when you want to start - that way even if it leaks faster than my wallet with my wife around you wont loose much Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.