85 replies to this topic

[Shadyapex]

ntsqd,

 

Very interesting.  You are you saying that when his CO2 tank is filled to the 100 psi rated pressure it goes from a gas to a liquid inside the cylinder?  Wow, I had no idea that such a low pressure could transform CO2 gas into a liquid.  Very interesting.

 

Thanks for insight,

 

Ps...I just re-read the Outback specs on the cylinder charging...they fill to 100psi 'cold' [no mention of temp] and then state that at 70 F the pressure is 837 psi....not certain how all this folds into the possible phase change for gas to liquid.

This is the safety warning from Outback Air, and I think it's worth reading as summer cab temps can reach 160F and it also addresses the phase change differences at different temps/pressures.

.
CAUTION! – Please Read This Carefully!

. A CO2 cylinder is filled with liquid CO2 by weight. At the time of fill the temperature of the charge is extremely cold and the pressure is around 100 psi.
. When a fully charged CO2 cylinder warms up to room temperature (70 F), the pressure inside the cylinder increases to 837 psi.
. When the same cylinder reaches 87.9 F the entire charge becomes a gas no matter what the pressure. A fully charged CO2 cylinder at 87.9 F will have an internal pressure of approximately 1100 psi.
. At 120 F the same cylinder will have an internal pressure of nearly 2000 psi. This cylinder at 120 oF now has an internal pressure greater than the marked service pressure of a cylinder that is properly filled, not overfilled.
. At 155 F the same cylinder will reach a pressure of 3000 psi, a pressure great enough activate the safety vent on the valve, venting the CO2 charge.

I fill mine in the fall when ambient temps are low so that by summer when I've been exploring the desert all winter the charge is lower. Hopefully this will avoid filling my cab with CO2 on a hot day, it has so far and I've been doing this for about 5 years. Your mileage may vary.

[Shadyapex]

In skimming the recent posts I think a very important aspect about CO2 tanks was missed. The reason that you can get a LOT more tire fills out of a CO2 tank than a comparable sized N2 tank or air tank is rather simple. Phase change due to pressure. N2's phase change pressure isn't reached in a normal tank so even at 2500 psi it is still a gas, but CO2's is and it goes to a liquid state. Change that gas to a liquid and you can get a whole lot more volume of CO2 in the tank than you can get N2 in the same tank. Propane also phase changes to a liquid in the tank, but propane filled tires is too scary to think about. shudder......

Boy I sure found that out the first time I used my CO2 tank. I didn't understand that the CO2 in a full tank was actually in liquid form and that I had to air up with the tank in an upright position. I had the tank, prone, behind the driver's seat and re-inflated my tires. The air chuck got very, very cold. But I persevered and aired up wishing I had a pair of gloves handy. Afterwards I closed the tank valve and headed down the road only to have the **** scared out of me and my brother when the air hose exploded, sounded like a rifle shot right behind me. Apparently the hose had filled with liquid CO2 and exceeded the strength of the hose. I found out why when chatting with the folks at the welding supply shop where I get the tank filled. I was buying a new hose, of course. The tires did seem to air up very quickly that day though.

[Wallowa]

Shady,

 

More interesting information, thanks Shady.  Remember that releasing any compressed gas through a restricted orifice [venturi] drops the temp in directly relation to the speed of gas going through that restriction in your cylinder control valve.  SCUBA regulators can freeze open if purged for long periods in cold water, which is usually deep water.  Even though tank air is "dry" the freeze is caused by the minute amount of moisture still in the compressed air.  Not good. 

 

When you filled a tire it probably is done at 'full-tilt-boogie' and for an extended time depending on volume needed.  The gas going through your control valve accelerated the flow and froze the gas.  Oh yes, the gas when super- cooled will go to a solid phase [ice] and this might have happened inside the hose.  Then from solid to liquid after your closed the valve as the hose heated up and then the resultant gas phase pressure exceeded the burst pressure of the hose design.

 

Lastly remember once filled a tank regardless of the cylinder pressure can only deliver a volume of gas equal to the original volume of gas compressed into the cylinder....87cf @ 100psi will still be 87cf at 2,500 psi if you heat the cylinder.  More pressure no more volume.  The tank is a rigid container the tire is not so increases in temp will increase tire volume and pressure; like wise reducing ambient pressure, going up to that 11,000 foot pass road, will increase the volume of the tire.  The tank is not affected by atmospheric pressure changes.

 

I know we are way in the weeds on "what is lowest pressure you should run in XXXXX tire"; but the information and comment of folks has been very interesting and appreciated.

 

Phil

 

Ps...Just contacted a friend who by profession is a welder.  He does believe that the CO2 has both a liquid and gas phase inside the cylinder [siphon system] and most probably should be used standing up.  Other welding gases like acetylene is liquid in a styrene matrix in the bottle and for that gas an explosion can occur if you use it after laying the cylinder on it's side.  CO2 is non-combustible but it sounds like you need the cylinder upright to draw off the gas and not the liquid.

Edited by Wallowa, 30 November 2015 - 05:38 PM.

[Shadyapex]

Yup, that's how the guys at the welding shop explained it to me. The cylinder contains both liquid and gas but the gas stays at the top of the tank. When I inflated my tires with the (full) tank on its side I was getting liquid and when I stopped (and didn't empty the hose) the liquid went to gas and exceeded the hoses strength. Made a helluva bang, too. I sure was glad that I had at least closed the valve.

[PJorgen]

Not to rain on anyone's parade but I'm having a hard time following the logic of using CO2 over other gasses...

The best gas to run in tires is actually Nitrogen which is about 70% of what generic air is made up of.
People who race cars/trucks for a living from the Baja 1000 to Formula 1 all fill their tires with Nitrogen because it is inert and not affected (as much) by temperature fluctuations.

The reason the OP can fill more of his tires with CO2 is probably due to the cylinder more than anything else.

 

As an amateur sports car racer I see this discussion/argument all the time at the track.  All gases obey the ideal gas law (remember PV=nRT?) and they all expand or contract with temperature the same amount.  The advantage to using nitrogen or any other bottled gas is that it's dry.  Water vapor in the gas does not obey the ideal gas law and will expand much more - and increase tire pressure - with increasing temperature.  

 

The average shop air compressor tank (mine included) is full of water from condensation.  This causes the air from the compressor to be saturated with water vapor and therefore a poor choice for inflating tires where minimal change in pressure is desired.  A good tire shop will have an in-line dryer on the air lines, or just use a dry bottled gas, nitrogen being the cheapest.

 

Not sure about the issue of CO2 becoming soluble in rubber, but I remember back around 2004-2005 in Formula 1 racing, Ferrari was experimenting with CO2 to inflate tires.  Not much came of it as I recall.

[Wallowa]

PJ,

 

You are correct, actually, Charle's Law, that all gases expand at the same rate if the pressure is constant.  And the moisture in some, not all, compressed air does result in more expansion in a flexible container such as a tire. But unless you are racing and need that 1/2psi accuracy/consistence, we off roaders are just fine with compressed air.  Incidentally compressed breathing air is very dry to prevent icing of delivery equipment; unless, moisture is deliberately introduced for respiration therapy. Our SCUBA cylinder compressors had very elaborate dehydrators as well as filtration.

 

For us garden variety folks, the use of a liquid phase gas to vaporize and fill our tires is an easy way to carry a lot of gas volume in a small package.  But I will just do it the old fashion way and use a compressor; of course backed up with a high volume mechanical "tire pump".

 

Phil

 

 

Edited by Wallowa, 04 December 2015 - 02:22 AM.