The amount of CO2 in the tank (at any given air temperature) has nothing to do with the inernal pressure. It'll remain the same regardless of the amount of CO2 remaining, as long as there is a single drop of CO2 in the tank.
Remember that the CO2 in the tank is a *liquid*....





Fred

 

By your logic there would be no need to have a safety/blow off valve. Liquids expand and contract. And lower volumes will cause lower changes in pressures than higher volumes within a finite space. And the internal pressure of the tank drops gradually as it empties which is contrary to your single drop theory. If you have actual experience with CO2, and have paid attention to the internal tank pressure, you would see that pressure changes dramatically, as I said from zero to 2500 psi or more depending upon volume of CO2 remaining and ambient air temperature.

Here is a test for your "theory."
Tank a normal CO2 tank and have it filled to specs. Say a 10 pound tank is filled with 9.75 pounds of CO2. Place the tank in a freezer for an hour. Check the internal pressure. Now place the tank on a propane burner for a few minutes. You will see that temperature has dramatic affects on internal pressures. Most wheelers see similar extremes. I know I have been in 40*F below zero(Gunnison, CO) and have parked my rig at the mall in Phoenix on a day it reached 115*. So inside I figure the cab reached 170* easy. That is something people need to be aware of. CO2 is the safest gas there is, but there are some minimal hazards that you are dismissing. I had a tank blow a safety valve while I was in the garage with the rig. Only the front half was parked in the sun and the cargo area and tanks were in the shade-- it was about 95*F when it blew. I had to leave the garage to prevent being asphyxiated.

 

I've used CO2 for over 7 years to air up tires when wheeling and I use it sometimes when I weld.
At any given air temperature that pressure inside the CO2 tank will remain the same regardless of the amount of CO2 remaining in the tank. At normal temperatures the pressure in the CO2 tank will remain in the 800 ~ 900 psi range until there is no more liquid CO2 in the tank, then the tank pressure will drop very quicky as the last of the gas is exhausted.
CO2 is in a liquid state at 800 or so psi, yes, depending some on the outside air temperature.
Yes, as you mentioned, the pressure can and will dramatically raise if the out side tank temperature is increased, however a gauge showing tank pressure still isn't of much, if any use.
It won't tell you how much CO2 is left in the tank, you have to weigh the tank.
We've had a couple of tanks blow the safety disk but they were all overfilled and left in the vehicle, directly exposed to the sun out here in Tucson.

I've never know of one to blow the safety disk that wasn't overfilled.

The place that fill mine use to put 6 lbs in the 5 lb CO2 tank and that's one of the ones that popped the saifety disk.




Fred

 

Yeah.

I'm just going to throw this out there.

I've been taught, by this crazy physics thing people are raving about, that increasing the temperature of a piece of matter is effectively increasing the average kinetic energy of the molecules of said piece of matter. One way to increase the kinetic energy of said molecues, is to apply heat.

Now onto pressure. This, i have been taught, is a measure of the force per unit area. Force can be applied, by, say, some molecules banging into the side of a container. If i increase the velocity, which increases the kinetic energy, say, by adding some heat, then the pressure on the sides of this container is going to raise, i would think.

So, to put it all together, if i apply heat to my CO2 canister, i will increase the average kinetic energy of the CO2 molecules inside the container, which will increase the pressure on the sides of the container. One way to apply heat would be to put my container in direct sunlight in a hot car, i would imagine.

However, i will admit that physics is a bit shaky, and that i could be totally off base, but i would think that there is at least a small argument to be made that leaving a CO2 tank in a hot car, could possibly have unexpected, and unwanted consequences. Just gonna throw that out there.

 

PV = nRT

If you increase the temp, hold the volume constant, then the volume must change.

I am foolish and I get that.

Buy a Power Tank.

 

I won't claim to be any expert but I can kinda see both sides of this argument.
Here are my thoughts.
CO2 as a liquid doesn't change volumes....liquids don't change volumes with changes in temperature. pv=nrt is for gases not liquids.
As the CO2 liquid decreases from 10lbs to 0lbs, the liquid in the bottle goes down (obviously). There's a gas on top of that liquid that follows gas laws and increases in pressure as the temperature goes up.
Therefore, when the bottle is filled the pressure should stay relatively constant. When the bottle only has a drop of CO2 liquid in it then the pressure can fluctuate quite a bit depending on the temperature.


Like I said, I'm not expert so correct me if I'm wrong.



Oh and for the actual topic at hand. I put together my CO2 setup using stuff from www.beveragefactory.com. I can't find the regulator I have but it goes up to 160psi. The whole setup was about $160. I think the power tanks are great but a little too pricey for me. My regulator has never froze and I have even used it in cold weather after wheeling in the snow. I usually have the pressure set to 140psi and that day I filled my 35s and a friends 31s so I don't think I will have any issues with it freezing in the future.

 

there is always a little gas in the bottle, up near the top.