Things to ponder with how much pressure you need.

[Redkey]

I would guess... without the plots to prove it...

it takes a certain number of ft-lbs of air to launch a ball.

Higher pressure guns use a smaller volume of high pressure air to fire the ball ie... short duration high pressure spike.

Low pressure guns use a larger volume of low pressure air to fire the ball ie... long duration low pressure spike

In theory, a gun running at low pressure should use the same amount of uncompressed* air as a gun running at high pressure to fire a ball at 300 fps. Although, I keep on seeing posts saying how inefficient low pressure guns are, I have never seen data to prove it.

*by uncompressed I mean the volume of air released into the atmosphere when the gun is fired.

So, I don't think all low pressure guns have the problem Tom is talking about... just the ones that don't use enough low pressure air.

This brings up another question... what affect does the barrel porting have on this phenomena?

Tom, have you actually measured negative pressures after the ball has passed a certian point in the barrel?

Interesting...

[CHK6]

Flow distrubance

Actually I didn't do so well in math in college. Mainly stems from to many red-eye nights playing ROTT over the net and never using the correct symbols that intructors wanted to see on papers and tests. Blahh, what do they know? The answer was correct.

I have been looking at other marker designs in how they deliver gas and possibly the flow behavior that comes from that design. There are equations that help explain how gas flows when the gas passes junctions. I found the angle of the junction makes a difference in how the gas flows and the pressure released. Porting in nothing more than miniture junctions in a large pipe.

I understand that porting is usually done at a 90 degree angle straight down to the center of the pipe. But can anyone explain why angled porting hasn't been done? Could it be that foward angles create small graders like a cheese grader and backward angled porting would only help with excess pressure?

I think these are interesting questions. Not in the sense of major break throughs, but just tweaking the small teaks and possibly creating unique barrel designs specific to a certain marker.

I'm also curious as to why o-ring seals in barrels haven't come about? I know that it's cooler to have a two-piece barrel or a sleeved barrel design, but a simple o-ring inside the barrel can do the same things and you won't need a ball dente.

That's my ten cents, inflated to thirteen cents, and Uncle Sam taxes it to 2 cents.

[steveg]

I suppose the reason that porting holes are perpendicular
to the barrel is simply because it costs less.
I'm guessing that most barrel manufactures drill
the holes and whatever other decoration on the same lathe
that that the barrel itself was turned on. As an alternative
the barrel, after turning would be mounted
on a rotary table (4th axis) on a machinig center. All
things considered, the first method would be the prefered
method because the part is handled only once if at all.
Angle heads for lathes are available but are very expensive
and very easy to break.
So the short answer Cost.

[AGD]

Guys,

Yes we have actually measured negative pressures in the barrel that go away once the ball exits. If this is an important factor or not we can't say without looking into it further.

AGD

[steveg]

Here is a link for the math for energy stored in compressed
air. https://www.media.mit.edu/people/ari...00000000000000
appropriately, the example uses a 68/3000 paintball tank

Math guys, get to work!

[BEEFYSAUSAGE]

ok

so to get a barrel with good efficencey and that shoots a tight group the gas pressure in the barrel should be as close to 0 as posible before before the porting or end of barrel(non ported).ok if that could be done would the variable size of paint be to much of a vairable to make it feasible(vairing friction)or would it not have that much affect on the speed of the pellet?
BEEFY.

[Ityl]

Now, with the gas being wasted by the ball exiting the barrel before the end of the barrel, couldn't this mean that the point of where the ball changes from acceleration to deceleration occurs later on. So wouldn't this create slightly more range?

[AGD]

Ityl,

This has been a long standing myth in paintball that a ball decellerates differently out of different guns. The fact remains that when the ball exits the barrel it hits a 170 mph headwind that starts decellerating it instantly. There is no chance for it to do something different.

AGD

[athomas]

The negative pressure thing had me stumped for a moment or two. I was trying to reason why there would be negative pressure behind the ball when the entire volume of air initially stored in the chamber would still be in the barrel and chamber. I think I have an understanding. Correct me if I am wrong.

The ball is accelerated to a velocity in the barrel determined by the initial pressure, the chamber size and the valve opening/flow rate. At a point in the barrel where it reaches its peak velocity, it also has its maximum kinetic energy. At some point farther down the barrel, the kinetic energy stored in the ball will allow it to maintain some/most of its velocity which is greater than the expansion rate of the compressed gas following behind it. This will create a vacuum effect and give the negative pressure reading.

The moment of zero pressure reading not maximum velocity would be the desired exit point from the barrel, even though there would still be wasted gas following behind the ball. This would give best shot grouping/less turbulence. The maximum velocity point on the other hand would give best air efficiency.

[bjjb99]

Negative pressures?

athomas,

I think you've got the basic explanation down, though the ball is not exceeding the expansion rate of the gas behind it. Instead, the pressure behind the ball has simply dropped below the normal atmospheric pressure. The ball is now feeling more pressure ahead of it than it is behind.

If you think of it not as "negative pressure" but rather "pressure lower than the ambient atmosphere," it becomes easier to visualize. The paintball is initially accelerated by a much greater pressure behind it than in front of it. As it travels down the barrel, the volume behind the ball increases, and the behind-ball pressure decreases accordingly.

If the barrel is long enough, at some point the pressure behind the ball will drop below the ambient pressure present ahead of the ball, and the ball will begin to decelerate. This is the point where the behind-ball pressure "goes negative" relative to the ambient atmospheric pressure. If we include frictional forces from rubbing against the barrel walls, then the point at which the paintball begins to decelerate will not be coincident with the pressure "going negative".

Keep your fingers crossed, 'cause I'm gonna try to attach a picture showing the forces/pressures involved.

Many pressure sensors output what is known as gage pressure (psig), which is pressure relative to ambient conditions. A gage pressure sensor behind the ball will measure negative pressures when the absolute behind-ball pressure drops below ambient.

BJJB

[athomas]

In order to have a negative pressure (compared to ambient atmosphere), we must have a vacuum effect. Eg. If the inside of a five inch long barrel is suddenly lengthened to 10 inches (ball moving down a barrel) without allowing the addition of more air to allow equalization of atmospheric conditions, then the 14.7 psi (0 reading on the gauge) would be 7.35psi (-7.35psi on the gauge).

The pressure inside the barrel of a paintball gun firing a paintball never theoretically reaches 0 psi (relative to atmospheric pressure). As long as the ball is in the barrel it is holding the chamber pressure from exiting the front of the barrel. The only way to create negative pressure in the barrel is to create a larger volume faster than the available air can fill it up. This is where the paintball guns with the lower chamber pressures have problems. The available air in the chamber is forced through holes(nozzle) in the front of the bolt. The rate of discharge is related to the pressure in the holding chamber. As the gun is fired and the ball accelerates down the barrel this low pressure gets even lower. Eventually the kinetic energy of the accelerated ball allows it to maintain a speed causing the barrel volume to increase faster than the rate of air discharge from the chamber. Here is where we have the pressure differential we are talking about. The pressure sensors read a negative pressure (vacuum). Higher pressure guns like the mag don't experience this phenomenon to the same extent if at all because the rate of air discharge from the chamber is greater due to the higher chamber pressure.

[Ityl]

So you're saying that it's impossible for a ball to still accelerate after it leaves the barrel? Or that it's just that the amount is trivial?

It must accelerate some when it's out of the barrel, and the faster the ball is moving the longer it would accelerate out of the barrel. I'm not sure of the numbers on this, they may not even be measureable easily.

[athomas]

In order for a ball to accelerate, there must be a force behind it greater than the force in front of it. The moment the ball leaves the barrel, the forces behind it disappear. Any residual gases dissipate in all directions of least resistance. The frictional forces of air plus the pressure in front of the ball cause it to immediately start to decelerate.

In conclusion there is no acceleration beyond the barrel.

Any gun that causes acceleration beyond the barrel would have to waste a hughe amount of air to keep the ball engulfed in the pressure blast.

[bjjb99]

athomas,

The pressurized gas behind the paintball is capable of expanding at far greater velocity than that of the paintball itself, even when it is only slightly above ambient pressure. Reference Tom K.'s statement regarding transonic flows in the thread titled "FEM model of a paintgun?". The restriction in flow caused by the bolt/valve tube/etc. should not be enough to limit this expansion to the point where it falls below the paintball velocity.

The pressure inside the barrel of a paintball gun can reach zero relative to atmospheric pressure (0 psig), Tom Kaye mentions this in his February 13 post to this thread; a "popular low pressure gun" which he won't name actually ends up with a behind-ball pressure below atmospheric pressure prior to the ball exiting the barrel.

BJJB

[athomas]

BJJB:

Looking back at our previous posts, I think we are saying the same thing.

The real question is why does this happen sooner in some guns than others. The test using the unnamed gun was conducted using a vented barrel. This could/probably did contribute to the pressure reaching relative 0 within the short time the ball was in the barrel. Further movement of the ball down the barrel due to built up kinetic energy would definately create the vacuum effect which would result in negative pressure.

In a mag with a nonvented barrel, the the negative pressure phenominon would theoretically occur somewhere around 35" down the barrel, unless the ball can travel down the barrel faster than the air can follow it.

Lets try an experiment. Use a nonvented barrel. Tape up one end leaving an opening the size of the automag valve(use a straw). Insert a squeegy from the open end of the barrel. Place it all the way at the closed end of the barrel. Now tighten the squeegy so that it forms a seal. Pull it from the barrel at some high velocity. If there is no vacuum created in the barrel then the rate of expansion/compression of air through the valve can keep up to the ball traveling down the barrel. If there is a vacuum created, then a full speed ball can create a vacuum partly because the air in the barrel and chamber cannot keep up at low pressures. Note-this only occurs when the pressures are low, usually at the end of the expansion cycle.