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C:B ratio?

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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C:B ratio?

Unread postAuthor: Mitchza89 » Mon Sep 17, 2007 8:08 pm

G'day guys,

This is a bit of a silly question coming from someone with 250+ posts but I've never figured out how to calculate the C:B ratio. I need to figure out what would be a real good ratio for my bb gun.

the barrel is a meter of 9/32 brass tube and the chamber is 3/4in copper tube.

Anyone got a clue on a good size for the chamber?
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Unread postAuthor: TwitchTheAussie » Mon Sep 17, 2007 8:22 pm

I found an easy way. Take ya chamber and block it at one end and pour water into it. Block ya barrel at one end also and pour it in with no spills. Cut it where the water level is. Perfect 1:1 :P
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Unread postAuthor: spudgunnerwryyyyy » Mon Sep 17, 2007 8:36 pm

Well yes the water method is the most accurate but the ideal ratio isn't always 1:1. For Combustions .8:1- 1.5: 1 is generally accepted as ideal. For pneumatics 1:1 - 2:1.
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Unread postAuthor: Mitchza89 » Mon Sep 17, 2007 8:51 pm

Cheers guys, Thanks alot.
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Unread postAuthor: DYI » Mon Sep 17, 2007 8:54 pm

The "ideal" ratio for a pneumatic operating at normal pressures is around 0.2:1, and you can use larger barrels at higher pressures. The often quoted 2:1 ratio is only for convenience, since some people don't like having massive barrels. A 2:1 ratio on a pneumatic is a massive waste of air in my opinion. In a normal combustion, anywhere from .6:1 to 1:1 is fine.

Now, to answer the actual question: It really depends on the pressure you plan on using. Your barrel is about 2.4ci. If you are aiming for the best performance possible, you will need a chamber at least the same volume or larger. However, with such a small barrel, the pressure used is of greater importance than the size of the chamber, due to the short space for acceleration.
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Unread postAuthor: Mitchza89 » Mon Sep 17, 2007 8:58 pm

Well I'm going to be using 300+ psi so hmmmm...I'll have to try and figure out 2.4ci for a 3/4in copper tube
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Unread postAuthor: acoop101 » Tue Sep 18, 2007 3:05 pm

so if the ideal ratio is 2:1 i am assuming it is 2 chamber 1 barrel then y not fill the barrel 2 times and dump it into the chamber pipe. this would get you the right ratio. if it is reverse then reverse the system :roll:
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Unread postAuthor: markieclarkie » Tue Sep 18, 2007 3:55 pm

The water method is great. I wish I'd thought of that! Thank you Archemedes!! The mathmatical way to figure the volume of a cylinder is. Pi X radius X Radius X length. I.E 3'Dia.18"long Chamber Would be.. 1.5 X 1.5 X 3.145 X18 =127.3725 cu.inches I have found that when I use a 1.5:1 ratio, In a normally aspirated combustion gun, the burn ends exactly at the point where the potato leaves the barrel. If the chamber ratio is any larger, you will notice a big flame shooting out the end,when you shoot it at night. The gun will also be noticeably louder. Witch looks and sounds cool, (I even build em on purpose that way sometimes)but will result in the loss of some range and muzzle velocity. If the chamber ratio is too small, the burn will end before the potato leaves the barrel resulting in a loss of perfomance due to friction. Thanks for the info on pneumatic ratios guys. Tell me More!!
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Unread postAuthor: DYI » Tue Sep 18, 2007 7:43 pm

The water method is great. I wish I'd thought of that! Thank you Archemedes!! The mathmatical way to figure the volume of a cylinder is. Pi X radius X Radius X length. I.E 3'Dia.18"long Chamber Would be.. 1.5 X 1.5 X 3.145 X18 =127.3725 cu.inches

Or you could just enter the chamber specs on GGDT... :lol:

300+psi sounds like fun, and I'm glad that at least some people know that low volume high pressure chambers are usually better than the reverse for small calibres. Obviously, the bigger you can make the chamber the better for a given barrel length, up to about a 4:1 ratio. After that, gains are negligible.
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Unread postAuthor: markieclarkie » Tue Sep 18, 2007 9:31 pm

`Enligten me. 'WAS GGDT? I rilly don't know.
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Unread postAuthor: Mitchza89 » Tue Sep 18, 2007 9:58 pm

GGDT is a program that calculates muzzle velocity. It's a good little program to get a rough idea but most of the time, I find it to be pretty dodgey.
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Unread postAuthor: markieclarkie » Tue Sep 18, 2007 10:07 pm

Ever heard of Archemedes?
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Unread postAuthor: markieclarkie » Tue Sep 18, 2007 10:56 pm

archemedes happenend about 300 B.C.
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Unread postAuthor: jimmy101 » Wed Sep 19, 2007 11:00 am

Another way to look at the relative dimensions of the chamber to barrel is to just base it on the two diameters.

Since volume of a cylinder scales as the square of the cylinder's diameter (or the square of the radius) then the relative volumes of equal lengths of the two pipes, expressed as it is normally for C:B ratios is;

CB ratio = (volume chamber)/(volume barrel) = (area chamber)/(area barrel) = (diameter chamber)<sup>2</sup>/(diameter barrel)<sup>2</sup>

Your barrel is 9/32" and the chamber is 3/4", we'll assume that those are the IDs of the pipes. Plugging in your numbers;
CB ratio = (3/4")<sup>2</sup>/(9/32")<sup>2</sup> = 7
for a gun with the chamber the same length as the barrel.

The C:B of 7:1 is pretty high so you might consider just making the chamber half the length of the barrel, that'll give a perfectly reasonable C:B of 3.5:1. The 3.5:1 ratio won't perform as well as the 7:1 ratio but the difference in performance will be negligible and the shorter chamber will be much easier to fill and handle.

The "optimal" CB ratio of a compressed air gun really has no meaning. Efficiency and performance do not optimize to about the same ratios as they do in a combustion gun. For a compressed air gun a bigger chamber will perform better in terms of muzzle velocity and worse in terms of efficiency. So, what do you want to maximize? If you have a shop compressor then the "cost" of filling a big chamber is essentially zero. So use a big chamber. If you are pumping up the chamber with a bicycle pump then the "cost" of pumping up the chamber is large. Use a small chamber to maximize the efficiency of your manual pumping effort.

A "reasonable" C:B for a compressed air gun is greater than 2:1, that is the chamber is at least twice the barrel volume. Lower CB's will work, and might even work very well, but you are gaining performance by lengthening an already long barrel instead of the much easier, and more length efficient, lengthening of the chamber (or increasing the chamber's ID).

Water weight is a great way to measure the volume of a container but in the case of a compressed air gun it is really a waste of time. You still need to do the math to convert weight to volume. So, you might as well just calculate the volume based on the dimensions. For most spud guns the actually volumes do not have to be calculated all that accurately. Heck, if you can get the chamber and barrel volume calculated to plus or minus 20% that is close enough. (You should be able to get the numbers accurate to +/- 10% with very little effort.)

In spud gunning, the only time when you need a really accurate measurement of volume is when you are building a combustion gun. It is very helpful to have an accurate measurement of the chamber volume so you can calculate the proper fuel load. It is also helpful to know the volume of a meter pipe with pretty good accuracy. The volume of the barrel doesn’t really need to be known to a high degree of accuracy. Heck, in a combustion gun the barrel can usually be changed by a factor of almost two in length without a measurable change in the performance of the gun (for example, a 0.6 CB performs nearly exactly the same as an 0.8 or 1.0 CB.)

In a pneumatic gun the volumes of the barrel and the chamber really don't matter all that much, so there is really no reason to try to measure or calculate them to a high degree of accuracy. Simple geometry will give an answer that is accurate enough. Whether your final C:B is 1.9:1 or 2.1:1 really doesn't matter a whole lot. So why bother measuring thing to an accuracy of 1%?
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