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Pressure vs. Vacuums

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Pressure vs. Vacuums

Unread postAuthor: caspar97 » Thu Jun 16, 2011 8:21 pm

Imagine that one had a pneumatic potato cannon in space; the design and specifics of the cannon are irrelevant. The pressure is pumped up in the confounds of the International Space Station, but the projectile has yet to be put into the barrel. One then walks into space. Assuming that the cannon does not explode from the extreme pressure differences, one then puts the projectile into the barrel. The pressure in the pressure chamber is 100 psi, the barrel, on both sides of the projectile, is a vacuum, because one is in space. Would the psi firing the projectile seem greater because there is no air pressure on the other side, or would the vacuum take the pressure and lessen it, as if the vacuum was a number below zero and the psi a number above: add the two and a get a lesser number? :shock:
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Unread postAuthor: saefroch » Thu Jun 16, 2011 9:00 pm

Well for one, you seem to have a slight misunderstanding of atmospheric pressure... it's only about 14.7psi at sea level. Which is really pitiful, except when dealing with the forces on an object trying to contain 14.7psi, when the object has to be large enough for a human to live in.

Pressures are normally measured in gauge pressure, which does NOT include atmospheric pressure, since it is everywhere, it produces no net force. To model your spudgun in space, you'd just have to change all your pressures to absolute pressure, or add 14.7psi to whatever you measured the chamber pressure as inside the pressurized spacecraft.

Vacuum isn't a negative pressure, it's rather like absolute zero. It only looks like you can have a negative value because we use units that weren't conceived with the understanding of "vacuum" or "absolute zero."
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Unread postAuthor: caspar97 » Thu Jun 16, 2011 9:06 pm

Assuming that I did add 14.7 psi to the 100 already in there, it wouldn't really matter...I guess what I'm trying to say is would the force of the projectile be the same as it would on sea level or would it be greater due to the vacuum?
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Unread postAuthor: saefroch » Thu Jun 16, 2011 9:08 pm

Yes it would matter, because you now have another 14.7psi behind the projectile...

I do feel the need to :roll: at your question, to somebody like me who is both nit-picky and physics-oriented, it's poorly worded. I think you're asking about the net force on the projectile, and yes, that would be greater.
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Unread postAuthor: DYI » Thu Jun 16, 2011 9:50 pm

Well, in fairness to the OP, evacuating the barrel before the shot prevents creation of shock waves and very high pressure regions in the front of the projectile during the shot. It not only changes the pressure felt (albeit very slightly), but reduces the effective projectile mass by getting rid of the air slug in front of it. Now, unfortunately for the OP, these shock waves don't really mean much with 100psi propelling pressure... :roll:
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Unread postAuthor: Technician1002 » Thu Jun 16, 2011 10:59 pm

Where I work most of the time we measure pressure in absolute most of the time. We work with Torr and portions measured in millitorr.

When we open up process chambers the pressure inside goes from a low pressure to about 780 Torr or one atm. Depending on the weather, this pressure varies and is not constant.

Inches of Mercury mm (Meters) of Mercury, and millibar (Bar) are two other common units of pressure measured in absolute pressure.
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Unread postAuthor: caspar97 » Fri Jun 17, 2011 9:27 am

@ saefroch, you are right, that would affect the velocity of the projectile. Therefore, I am now modifying the question so the pressure in the chamber on Earth is greater to account for the 14.7psi in atmospheric pressure. That would solve the imbalance right?

Would the vacuum before the projectile act in a way that it would suck the air out of the chamber faster than normal, causing the velocity of the projectile to be greater or would the vacuum before the projectile absorb some of that pressure so to speak and therefore reduce the net force acting on the object?

@ DIY, I do not understand what you mean by saying evacuating the barrel.
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Unread postAuthor: inonickname » Fri Jun 17, 2011 9:38 am

caspar97 wrote:@ saefroch, you are right, that would affect the velocity of the projectile. Therefore, I am now modifying the question so the pressure in the chamber on Earth is greater to account for the 14.7psi in atmospheric pressure. That would solve the imbalance right?

Would the vacuum before the projectile act in a way that it would suck the air out of the chamber faster than normal, causing the velocity of the projectile to be greater or would the vacuum before the projectile absorb some of that pressure so to speak and therefore reduce the net force acting on the object?

@ DIY, I do not understand what you mean by saying evacuating the barrel.


Removing the majority of air pressure in front of the projectile (inside the barrel). I.e. in front (and initially, probably behind) your projectile is a vacuum*

*incomplete
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Unread postAuthor: Technician1002 » Fri Jun 17, 2011 9:45 am

The term evacuating refers to pumping out the atmospheric pressure. So yes it is a vacuum in the barrel.

Placing 15 PSI more in the chamber is not quite the same as evacuating the barrel. With the barrel evacuated, the projectile does have ~15 more PSI behind it in the differential pressure when it is not moving. However it has no air mass in front of the projectile to add to the projectile mass when it is moving down the barrel.

A common physics experiment is to launch a ping pong ball with 15 PSI of air and find the speed. Then put the ping pong ball in an evacuated barrel with a burst disk and use atmospheric air to launch it. It does much better in the evacuated barrel.

Not bad for only 15 PSI.
http://stokes.byu.edu/pingpong.htm
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Unread postAuthor: saefroch » Fri Jun 17, 2011 9:47 am

I think you may be misunderstanding pressure, or at least it seems that way from the way you word your questions. Vacuum does not "suck" on anything. A pressure in a fluid exerts equally on all surfaces, according to Pascal's Principle. The vacuum wouldn't "suck" at all, the chamber pressure would just be relatively larger. A cannon whose chamber pressure is 100psi relative to conditions in the barrel is the same on earth or in space. Flow is related to pressure differential, there is no magical "sucking power" of vacuums.

Imbalance? I am confused... A chamber whose pressure reads 114.7psi gauge pressure on earth's surface would operate pretty much the same as one that is pressurized to 100psi in a pressurized spacecraft then taken outside of the craft into space... because now the pressure in both chambers relative to the outside conditions, or the conditions in the barrel more importantly, are the same.

My posts are wordy. :?
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Unread postAuthor: caspar97 » Fri Jun 17, 2011 10:33 am

From all the knowledgeable answers in sounds like the projectile would have a greater velocity in space even if the pressure in the chambers were changed so they would appear to compensate for the loss of atmospheric pressure. However, as Tech1002 showed, it doesn't really compensate...Is that the jist of it, or am I still wrong?

@ Tech1002. The video on you-tube accompanying your link doesn't work.
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Unread postAuthor: saefroch » Fri Jun 17, 2011 10:41 am

The effect of the mass of the air in the barrel is very nearly zero, especially when compared to the mass of any projectile we'd launch. A ping-pong ball has incredibly low density and mass, so the effect shows up well. The effect of the mass of the air in the barrel would be nigh on immeasurable with a potato.

The projectile would travel slightly faster in space (though how much faster is highly debatable), even if you compensated for the external pressure change, yes. The effect of the mass of the air in the barrel has nothing to do with pressure, so I'm not sure you can say "it doesn't really compensate."

Is there any particular reason you're so intent on asking about this?
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Unread postAuthor: caspar97 » Fri Jun 17, 2011 11:06 am

No particular reason. The question just came to me and I figured you guys would be the people to ask, and clearly I was right. I am just extremely inquisitive.
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Unread postAuthor: saefroch » Fri Jun 17, 2011 12:34 pm

Inquisitive is good. We like inquisitive.
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Unread postAuthor: Tobin » Fri Jun 17, 2011 1:39 pm

Technician1002 wrote:Where I work most of the time we measure pressure in absolute most of the time. We work with Torr and portions measured in millitorr.

When we open up process chambers the pressure inside goes from a low pressure to about 780 Torr or one atm. Depending on the weather, this pressure varies and is not constant.

Inches of Mercury mm (Meters) of Mercury, and millibar (Bar) are two other common units of pressure measured in absolute pressure.


Atm? Atmosfärer? Then i asume youre from sweden?
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