Portable air, NOT a compressor

[playinatwork]

Sorry dwalby but I didn't get your earlier link. I could rig a way to carry this thing I'm sure. So you believe the 125psi 20 oz tank will be sufficient and I don't need the high pressure like in the soda fountains and paintball applications?

 

[Blrfl]

Standing alongside the bike in the dark night rain waiting on that slow compressor...

I reviewed a compressor a couple of years ago that gets a ST front tire from 0 to 42 PSI in under three minutes. The Slime compressor and its ilk always impressed me as being a little on the wheezy side but adequate if you're willing to wait for it to do its job.

So you believe the 125psi 20 oz tank will be sufficient and I don't need the high pressure like in the soda fountains and paintball applications?

It's all about how much air you pack into how much space. Boyle's law says that P1 x V1 = P2 x V2, where P is pressure and V is volume. If you know the interior volume of the larger of your tires, its inflation pressure and the volume of any tank you propose to hold compressed air, that would be enough to figure out the minimum pressure the tank should hold. I'd actually double whatever number you come up with for that to account for leakage and so you have enough oomph to get the tire completely filled quickly.

I'm skeptical that a 20 oz tank filled to 120 PSI would be enough air to fill an empty rear tire to 42 PSI, but anybody who deals in this stuff for a living should be able to help you figure it out and recommend something.

--Mark

 

[dwalby]

Sorry dwalby but I didn't get your earlier link. I could rig a way to carry this thing I'm sure. So you believe the 125psi 20 oz tank will be sufficient and I don't need the high pressure like in the soda fountains and paintball applications?

its in post #8

same tank, different regulator setup, used by homebrewers as a portable CO2 source for small kegs of beer. Not as streamlined as the one you found though, yours looks better.

the 125psi spec refers to the max pressure setting for the regulator low pressure output side. All CO2 systems have a working pressure around 1000psi, that's the pressure CO2 requires to keep it a liquid at room temperature. It 'boils' in the tank until it creates that pressure, then stops 'boiling' until you release some of the gas and reduce the internal pressure. Then it boils some more until the required pressure is once again achieved.

edit (last of several): just noticed its a 20oz container, but I think that's the weight of the CO2 to fill it, not the volume. Did a web search and 1 pound of liquid CO2 will produce 247 liters of gas at room temperature and 1 atmosphere pressure. Since you need around 10 liters at 4 atmospheres to fill the tire, the 20oz cylinder should be good for several tire fills. 20oz will produce about 300 liters at 1 atm, so about 75 liters at 4 atm.

 

[Blrfl]

the 125psi spec refers to the max pressure setting for the regulator low pressure output side. All CO2 systems have a working pressure around 1000psi, that's the pressure CO2 requires to keep it a liquid at room temperature.

That makes a carload more sense. Thanks for the clarification.

--Mark

 

[BakerBoy]

It will get very cold as the CO2 vaporizes, as it may not be designed for fast discharge. That may slow down the fill, but an interesting prospect nonetheless. If someone gets one, I'd love to hear how it worked for you on a flat tire.

 

[dwalby]

It will get very cold as the CO2 vaporizes, as it may not be designed for fast discharge. That may slow down the fill, but an interesting prospect nonetheless. If someone gets one, I'd love to hear how it worked for you on a flat tire.

From what I can gather, his 20oz tank should hold close to 1 liter, and 20oz of liquid CO2 is about 0.5 liter. If that's the case, then 0.5 liter of gas at 1000psi should be enough to fill the tire with existing gas in the tank at ambient temperature. As that gas is expelled, then the expansion cooling will begin for the newly formed gas, so it may be a little cooler than ambient, but I doubt it would get seriously cold. Then subsequent fills would have proportionally more gas, less liquid, as more fills are performed, so it will be less affected by expansion cooling as it empties over multiple uses. Just to be safe, I'd ride a short time and as the tire warms up take another pressure reading, and bleed off any excess pressure that might have occurred as the cold gas warmed.

 

[BakerBoy]

No, it is not about spare volume Dwalby. Heat of vaporization (enthalpy) required to convert a liquid to gas is constant (freeboard volume doesn't matter). Pressure in the bottle remains unchanged as it vaporizes (until all liquid is vaporized). But we digress.


Your point about putting cold CO2 in the tire up to pressure, then the CO2 continuing to warm up is an important point. The tire can be overinflated. Another subtle point is that CO2 is a bit inferior to air for use in tires due to thermal conductivity, specific heat, and is a small molecule (CO2 leaks out through places that air can't). So if CO2 is used to pump up the tire, it would be a good practice to later let the CO2 out of the tire and refill with air (or better yet, nitrogen, if one is so inclined).

 

[jonz]

Interesting and informative. I don't know if it's the same with tires but when I use CO2 for inner tubes in my bicycle, the inner tube rubber seems to be permeable to CO2 and the next day, my tire and inner tube are almost flat. It's not a leak because when I pump up the tire, it holds air. Just something to check on should you use CO2 to repair a tire.

 

[CruSTy]

Quickest way to seat a bead out on the road. The key is quick volume not pressure.

https://www.youtube.com/watch?v=abH9__WKMhc

 

[dwalby]

No, it is not about spare volume Dwalby. Heat of vaporization (enthalpy) required to convert a liquid to gas is constant (freeboard volume doesn't matter). Pressure in the bottle remains unchanged as it vaporizes (until all liquid is vaporized). But we digress.

Oh yeah, I forgot about the fact that the expanding gas from 1000psi to 50 psi will also cool, just like the expanding liquid to gas will cool, duh. But it is true that the intermediate gas will acclimate to the ambient temperature around the cylinder, so there is some temperature equalization in the process (assuming the cylinder is sitting around long enough to temperature equalize before expelling the gas, which is reasonable). This is a topic I'm not too familiar with, how does the temperature drop from liquid to gas compare to the temperature drop going from 1000psi to 50?

 

[BakerBoy]

Vaporization requires the most energy, expansion cooling relatively little by comparison. The bottle and regulator will become very cold while discharging. The bottle will generally equilibrate to ambient temperature day in, day out while it is not discharging.

[thermodynamics]
A quick search of vapor pressure, enthalpy of vaporization, and other boring Thermodynamic stuff shows that if the CO2 liquid is above about -40*C and the bottle pressure drops below say 125 psi, the CO2 liquid will vaporize (boil) to produce CO2 gas, and in doing so heat is extracted from the surrounding gas, fluid, bottle at the rate of 247 BTU per lb CO2. A 20oz CO2 container would require about 308 BTU of energy transferred into the fluid as it vaporizes to convert the entire contents from liquid to gaseous CO2. In other words, 308 BTU of heat needs to be extracted from the surroundings and there's not that much heat available instantly, so the vaporization slows down to the rate that heat can be 'found'. For example, if the bottle were submerged in warm water, the vaporization of all the CO2 would pull enough heat out of the water to freeze up to 2.1 lb of water onto the outside of the bottle as energy is extracted from the water. In absence of water, the vaporization process needs to extract that heat from the atmosphere, and there just isn't enough heat in the vicinity of the bottle, heat is extracted from the bottle and liquid so that the liquid approaches -40*C, and the vaporization is slowed down to a trickle rate as ambient energy is slowly transferred into the liquid (where it is transferred into the gaseous vapor as it is vaporized).
[/thermodynamics]

 

[Ozzie]

Get a NOS tank, rig up an adapter, go to town.....?

 

[bdalameda]

Lowes home and hardware stores used to sell a small CO2 tire inflation system that used a CO2 bottle similar to a paint ball gun. I have one I keep in my Jeep. I think it is a Kobalt brand name. I have used it to fill my jeep tires and it handles that without a problem. It also has a built in regulator to control pressure in the tire. Many 4X4 off-roaders use portable CO2 systems for inflating tires.

 

[Al st1100]

its in post #8

same tank, different regulator setup, used by homebrewers as a portable CO2 source for small kegs of beer. Not as streamlined as the one you found though, yours looks better.

the 125psi spec refers to the max pressure setting for the regulator low pressure output side. All CO2 systems have a working pressure around 1000psi, that's the pressure CO2 requires to keep it a liquid at room temperature. It 'boils' in the tank until it creates that pressure, then stops 'boiling' until you release some of the gas and reduce the internal pressure. Then it boils some more until the required pressure is once again achieved.

edit (last of several): just noticed its a 20oz container, but I think that's the weight of the CO2 to fill it, not the volume. Did a web search and 1 pound of liquid CO2 will produce 247 liters of gas at room temperature and 1 atmosphere pressure. Since you need around 10 liters at 4 atmospheres to fill the tire, the 20oz cylinder should be good for several tire fills. 20oz will produce about 300 liters at 1 atm, so about 75 liters at 4 atm.

co2 will expand when it gets hot, used to paintball and if you left you tank in the sun it would eventually blow the safety valve. How safe would that be in your tires ??? an expanding gas sound like a recipe for disaster!! Just keep a can of fix a flat for emergency. Paintball compressed air tanks are over $100 more like 150 , need to be tested ($30) 3-5 years and a safety cover($25). but you can get a small bottle 45cu in @4500 psi... fix a flat $5.99

 

[dwalby]

co2 will expand when it gets hot, used to paintball and if you left you tank in the sun it would eventually blow the safety valve. How safe would that be in your tires ??? an expanding gas sound like a recipe for disaster!!

all gases expand at basically the same rate, so CO2 is no different than plain air in that regard. Also, CO2 contains no water vapor, like pure Nitrogen (which some racers prefer), that would be the only difference. As has been previously mentioned, the initial temperature of the CO2 inside the tire might be a bit cold from the expansion, so you'd get an initial pressure rise as it warmed back to ambient temp. After that its no different than air.

The reason your paintball tank would blow the safety valve is because the pressure rise over temperature change is proportional to the initial pressure. If you start with 1000psi and raise it 20%, you've added 200psi. With a tire you've added about 8psi.

 

[Al st1100]

all gases expand at basically the same rate, so CO2 is no different than plain air in that regard. Also, CO2 contains no water vapor, like pure Nitrogen (which some racers prefer), that would be the only difference. As has been previously mentioned, the initial temperature of the CO2 inside the tire might be a bit cold from the expansion, so you'd get an initial pressure rise as it warmed back to ambient temp. After that its no different than air.

The reason your paintball tank would blow the safety valve is because the pressure rise over temperature change is proportional to the initial pressure. If you start with 1000psi and raise it 20%, you've added 200psi. With a tire you've added about 8psi.

if all gasses expand equally why not use 134a ?? ///

 

[dwalby]

if all gasses expand equally why not use 134a ??

Interesting idea, do they still sell those can piercing devices with a hose and a schrader valve like back in the R-12 era? I see the cans of r134a are available online, so maybe that's another option.

The properties of r134a are such that you get 1/3 as much gas per unit weight of liquid when compared to CO2, but I think there's still probably enough in a 12oz can (weight) to fill a rear tire. Looks like about 15 liters at 4 atm from one can.

Just to clarify on the expanding gas statement, it didn't mean that all compressed liquid gases expand to the same volume of gas, they're all different in that regard. Once they are in the gaseous state, then they all follow the ideal gas law more or less, within the application of pressures used for tires. The ideal gas law starts to break down at higher pressures, so it doesn't apply to everything, but for the sake of this discussion they all behave similarly as far as the pV = nRT equation goes.

 

[Al st1100]

Trick question , 134, r12 have a temp to pressure property, for example an auto a/c system that holds 2lbs of Freon standing still, pressure will pretty much equal the outside temp. the same system with 1/2 lbs standing still will have the same pressure, therefore 134a in a tire would not be good , 90 degrees outside would equal 90 psi

 

[dwalby]

Trick question , 134, r12 have a temp to pressure property, for example an auto a/c system that holds 2lbs of Freon standing still, pressure will pretty much equal the outside temp. the same system with 1/2 lbs standing still will have the same pressure, therefore 134a in a tire would not be good , 90 degrees outside would equal 90 psi

show me the mathematical reasoning. 90F = 90 psi doesn't make any sense unless you put it in a mathematical context. In a gaseous state, all gases will have the same temp/pressure relationship based on pV = nRT, where the T (temperature) is in absolute temperature units, either Kelvin or Rankine, not Fahrenheit, so the fact that you're saying psi and degrees F are proportional is suspect. To raise the pressure by 10%, requires a temperature increase of 10%, in degrees Rankine, which is about a 50 degree F increase at ambient temperature levels. Got a link to explain the calculations behind your claim? Also, the pressure inside an A/C system with an active compressor/evaporator isn't necessarily comparable to the same gas being used inside a tire. Please explain your figures.

edit: here's a graph of r134a vs. the ideal gas law, which is what plain air will generally follow for the sake of this discussion.

If you find 4Bar and ~30C on the graph, you'll see that at most r134a is about 10% different than an ideal gas, maybe a little less, but not significantly different than plain air.

 

[playinatwork]

All this discussion is nice but I have a flat tire this morning. Welcome to Monday Tim!