Solving Pressure Shooter Kink: Find Volume & Air Needed

[mmartin]

I've made this pressure shooter for school, but I can't quite figure out this kink. If the cylinder has a volume of 4,994.57 cubic centimeters, and I plan on filling it with 100 psi of air (I want to maximize the pressure) how much air am I going to need? I could just flow it in, but I need to know how much to buy to shove it in (without blowing the container). Any help would really be apprecaited, I want to get his ready in a month or so and the schedule is looking down right now.

 

[Sirus]

You will need temperature and the molar mass of air. Use the equation $PV=nRT$, where P is pressure in kilopascals, V is volume in L, n is moles of gas, R is the universal gas constant (8.314), and T is the Kelvin temperature. Make the appropriate unit conversions. I say you need molar mass of air because I'm assuming you want to know the mass of air you need to bring the pressure to 100 psi.

 

[poolwin2001]

How do you find Molar mass of air?
Is it corect to say 70/100*28 + 30/100*32 where 32,28 are molar masses of oxygen,nitrogen and 70,30 their % in air?

 

[Sirus]

If you ignore the other gases present, yes, you can do it that way. You could look it up somewhere as well. Mmartin, is it important that you have the air at exactly 100 psi, or just something close to that?

 

[Gokul43201]

More importantly, what mechanism do you have for pumping ("shoving") air into your cylinder ? Do you have a compressed air tank with a p > 100 psi pressure regulator ? Do you know for sure that your container won't fracture at 100 psi ?

 

[mmartin]

I am fairly sure (which is a great assurance) that the container will not fracture at 100 psi, it's pretty strong. I will be using a compressed air container, I'll have to check later if it reads x>100 psi. No, the air does not have to be exactly at 100 psi, but close, plus or minus 5 psi. I originally tried the PV=nRT equation, but I couldn't figure out the Pressure to Temp ratio. That said, still trying to find out how many gallons. This was one man's suggestion, how does it check out, I think it just about answers it.

"Assume 100psi absolute. 100/14.7 is about 6 to 7, so you should need 6 to
7 times 1.2 gallons of uncompressed air to be compressed to get your
cylinder up to 100psi-absolute. For 100psi gauge, you need about 1
additional gallon of uncompressed air."

I would say that that would be the logical approach? Any scientific qualms?

Thanks.

 

[Gokul43201]

No...none. As far as the calculation is concerned, that's fine. However, if you buy a 100 psi compressed air tank with the equivalent of 1.2*7 gallons of uncompressed air, you will not be able to get your container up to 100 psi...you'll only get to about 50 psi.

Either the compressed air should be at 200 psi, or you'll need more air. I think they specify the pressure and the volume at that pressure. So, you can use the PV relation, as before, to determine what will work. But keep in mind that if you let the tank equilibrate with your container (which will be required if the tank pressure is not much more than 100psi), the final volume is the sum of the two volumes.