Psi?

[RonnyMcDYO]

To make a long story short I have a paintball gun that im trying to design and ive been trying to wrap my head around how to figure out the air pressure required for it. Problem is that I cant figure out exactly what PSI is measuring. Cause it cant actually mean that there is a pound of air per squar inch...otherwise our air tanks would a few hunder tons when full. Can someone explain to me what exactly PSI is measuring?

[El Pirata]

Pounds of pressure per square inch. So a tank that holds 3000psi can hold 3000 pounds of pressure. A 4500 tank is higher capacity and you guessed, 4500 pounds of pressure..

[tgaffner]

It means that it is putting ,# of PSI, pounds of force against a given object.

The atmosphere is 14.7 PSI. That means that there is 14.7 pounds of force pressing on everything.

[RonnyMcDYO]

So at normal pressure air is 14.7 PSI? And if thats the case then in a 45/45 tank for every cui there is 4500 lbs of air. So how do we measure the air inside at this point? I calculated it to 202,500 but im not sure of the unit? There is 202,500 cui of air at normal pressure?

[El Pirata]

Dude KISS, Keep It Super Simple. You have 45 cubic inches at 4500 pounts per square inch, you don't need to complicate it more than that.

[RonnyMcDYO]

Dude im trying to design a paintball gun I kinda have to make it complicated.

[Padlock6]

To make a long story short I have a paintball gun that im trying to design and ive been trying to wrap my head around how to figure out the air pressure required for it. Problem is that I cant figure out exactly what PSI is measuring. Cause it cant actually mean that there is a pound of air per squar inch...otherwise our air tanks would a few hunder tons when full. Can someone explain to me what exactly PSI is measuring?

Air Pressure is measured in a force (pounds) divided by an area (square inches). So as we approach earth from space (a vacuum) the air starts to exert force on our bodies. This totals ~14.7 pounds on each measured square inch of our body that is exposed to the atmosphere. If we continue our descent into the ocean, the pressure continues to build in the same way. Consider what happens when you suck through a straw: our mouth forms a lower pressure than the atmosphere, which forces the slushy up through the straw.
So, the 45CI air tank will hold 45 cubic inches (a cube 3"x3"x5") of whatever you want to stick in it. If you start to pack air into it, it will compress and allow more air to enter. Thus, when full, the tank would hold a room full of air, pushing against the tank (and valve) at 4500 pounds for every square inch of tank.
I could ramble on some more.... If you are still awake, email me and I can drone on some more.

dc

[ITO]

The Pound is a unit of measurement for MASS or FORCE.

When used to describe pressure the word pound is used as a measurement of FORCE per square inch. Just as a liquid would exert pressure on the sides and bottom of a container, a gas will exert pressure on all sides of a container. The force of the this pressure is expressed in pounds per square inch or PSI, which is the force felt on each square inch of the inside of the container.

1 pound of force feels like gravity pulling one pound of mass towards it, but this pressure is equal on all sides of the container (Nitro Tank) included the top, sides, and bottom. So the actual tank only has the weight of the actual volume of gas inside and not the weight of the pressure measured inside.

[RonnyMcDYO]

ohh ok that makes more sense...so the psi is only measuring the force the air is exerting on its container. So now I need to figure out the amount of energy air has as its expanding and the rate of expansion...fun stuff

[ITO]

http://airsoldier.com/miscellany/index.shtml

This site sells extruded body parts and barrel thread taps if you are serious about making your own marker.

[RonnyMcDYO]

Thanx man appreciate it =)...I actually had a gun design that would really take me way to much money to produce so I almost gave up on it...but I had an epiphany one day and simplified the design so much that I may actually be able to pull it off. Anywho thanx again for all the help guys

[LayinRocker]

would you have to take into consideration that a high pressure tank usually outputs around 800 psi and a low pressure tank puts out around 450? this might be stupid but why are you tryin to figure out what is in the tank instead of figurin out from the output of the tank. balls per tank maybe??

[RonnyMcDYO]

Yea pretty much...I mean the biggest thing all players want in a gun is air efficiency...how many shots can I get per tank. As well as lower pressure which is also part of the design. And trying to figure out how much air is in the tank itself will help me to fully understand how it all works...im one of those people that needs to see the big picture I cant just settle for knowing a little bit of something lol

[Bigal1]

I know this is an old thread but if you're still intersted, this equation is helpful (and it's in Trigga's signature if case you ever forget it...)

PV=nRT
Pressure times Volume equals the number of 'moles' of gas present times constant associated with the gas you're using (air in this case) times absolute temperature.

http://scienceworld.wolfram.com/physics/IdealGasLaw.html (helpful link)

Now the left hand side of the equation above is pretty easy and straightforward. The right hand side can be intimidating but not to worry. Becuase if I take a gas and change it's temperature, pressure or volume as long as I don't change the amount of gas I'm working with, I can write this equation instead:

Pressure number 1 times volume number 1 = pressure number 2 times volume number 2 (assuming temperature 1 equals temperature 2)

This second equation is useful to you because if you look at the units associated with pressure times volume, you'll see it has the same units as energy or work. (ft-lb)

you stated above you want to know how much energy you need to shoot a ball down the barrel and now you know how much energy you have stored in your air tank.

Note that air is not an ideal gas so technically it doesn't work in the equation above but most engineers will tell you that it's close enough.

[RonnyMcDYO]

Thanx Al thats a huge help....and im slowly understanding all this PSI stuff now...I just need to conquer the math aboot it so I can start building this gun of mine...if I ever had the time to sit down and study it.

[Bigal1]

Just for giggles, I figured out how much energy is in a 4500psi/45cubic inch tank:

16,875 ft-lb

You can appreciate the risks/concerns associated with dropping your tank or puncturing it as all that energy is going to be released in about a 1/2 second.

[Eclipse363]

PV=nRT is for an ideal gas, air is hardly ideal so any calculations will be wrong (air is also a mix of gasses so the exact composition will throw your calc. off too)

[Bigal1]

PV=nRT is for an ideal gas, air is hardly ideal so any calculations will be wrong (air is also a mix of gasses so the exact composition will throw your calc. off too)


I said that above.

it actually works pretty well.

[Eclipse363]

oh, my bad, missed that you said that, but pretty well is always relative.

[Trigga Nometry]

oh, my bad, missed that you said that, but pretty well is always relative.Yeah, the ideal gas Law is really good for understanding how gas responds to temperature, pressure, and changes in volume. Very cool equation, if you like that stuff like I do. Of course, I'm a nerd, so....

[Bigal1]

oh, my bad, missed that you said that, but pretty well is always relative.


I've actually seen calculations using the ideal gas law for steam. Now steam is in no way an ideal gas but the calculations I saw were within 30%. Sometimes, you just want to get a ball park idea and it works quite well.