Pneumatic test system??

Post questions and info about pneumatic (compressed gas) powered launchers here. This includes discussion about valves, ratios, compressors, and anything else relevant to launchers powered by compressed gas.

Postby ssb73q » Mon Sep 19, 2005 10:22 am

I would like to build a test stand to test pneumatic launcher components. The first test setup I would like to consider is the comparison of an unmodified sprinkler, modified electric, and modified pneumatic 1â€Â
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Postby tim jones » Mon Sep 19, 2005 12:23 pm

1. For a 3" chamber, 24" is about average. We don't see many shorter than 12", or longer than 36". Personally, I'd like to see comparisons with short, medium and long chambers.
2. We usually size barrels by their volume relative to chamber volume, I'll suggest using 4:1, 2:1 and 1:1 ratios would cover the normal range.
3. Sticking with standard projectiles with well-characterized aerodynamics is an excellent idea. Tennis balls and golf balls are good choices, but you should probably go the next step and standardize on a make/model for each. 1-1/2" SDR 21 has become something of a standard barrel material for golf balls (long barrels should be sleeved in 2" Sch 80 PVC pipe), and 2-1/2" Sch 40 PVC is the obvious choice for tennis balls.
4. Several members have the Shooting Chrony Model F1, which is relatively inexpensive.
5. Following the prefered number sequence, try 100, 80, 64 and 51 psi. Most of us use fairly cheep pressure gauges, but a test fixture would be better with an accurate gauge. One that conforms to ANSI B40.1 Grade A or 1A would be nice.

I don't think you would be repeating other's work. Most of our testing, with a few notable exceptions, has been subjective.
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Postby BewareOfDog » Mon Sep 19, 2005 12:33 pm

I would use a golf ball for testing. A tennis ball fits too snugly in the barrel. I would guess that the friction would vary from shot to shot, making your testing inconsistent.
A golfball on the other hand, fits nicely isdie of 1.5"SDR26, which can then be sleeved inside of a length of 2" SCH80.

A 3 foot long barrel would be more than enough, but a 4-6 foot length will yield much higher velocities.

A 12" long piece of 3" SCH40 for the reservoir will easily support a 10 foot long barrel.
(GGDT- default settings for Valve Data / Bore: 1.734" / Length: 120" / Friction: 0.5 /
Mass: 58 gm / Diameter: 1.68" / Init Position: 0.84" <font color=red"><b>469f/s</b></font>

I just read an article, and someone stated that "Oehler Chronographs tend to be the most accurate. Their high-end models are accurate to about 1 fps."

He went on to state:
"The nice thing about the Oehlers is that it measures the same shot twice, so you can look at the deviation of the difference between the 2 shots and get an actual measurement of the precision of your measurements. Most of the time with a 4' screen distance, I have
seen a precision of 1 to 2 ft/sec. Sometimes it is worse depending on the light conditions. If you go to an 8' screen distance it is pretty easy to get a 1 fps std. deviation of error. The absolute accuracy may be off more than that of course".

Being that your are limited to 110psi, might I suggest starting at 60, and working your way up in 5psi increments until you hit 110?

It's bizarre that you started this topic today. I was thinking (yesterday) about modifying the exhaust orifice on a 1" Orbit so that it vents to the outside of the valve, instead of venting into the outlet of the valve. This would keep the valve from repressurizing the top of the diaphragm housing, which makes the valve "honk". It would be nice to know if there were any noticeable difference in velocity.
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Postby tim jones » Mon Sep 19, 2005 1:00 pm

DR makes a good point about lower shot-to-shot variability with golf balls. Still, tennis balls are much easier to find/recover over long range. Were you planning on using a bullet trap or similar device that would eliminate this issue?
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Postby clide » Mon Sep 19, 2005 7:29 pm

I was discussing test setups with akb awhile ago and he mentioned it would be a good idea to setup the test in such a way that exaggerates the characteristic you are looking to test.

In this case you are looking to compair opening speed right? So a large chamber, a short barrel bigger than the valve, and a low mass, low friction projectile should exaggerate the effects of opening speed.

Perhaps a short 1.5" sch40 barrel shooting milkcaps.

I was going to do some quick tests of the solenoid actuated valve with a golfball barrel for you because I already had all the stuff, and I already had modded valve data, but unfortunately the unmodded valve cap I had was damaged where the solenoid seals.
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Postby tim jones » Tue Sep 20, 2005 11:44 am

I like clide's idea about exagerating the measured characteristic. For an alternate ammo, I would suggest using the poly-foam practice golf balls that Target sells ($5.00 per dozen).
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Postby ssb73q » Tue Sep 20, 2005 1:48 pm

Hi Tim, wouldn't I have a problem with the exterior ballistics using a low-density projectile? I ordered a Chrony F-1 to measure projectile velocity and the "screens" need to be placed some distance from the muzzle to avoid muzzle blast from giving false readings. Wouldn't air resistance immediately begin to overwhelm any low density projectile if the chronograph must be some distance from the muzzle?

I thought of doing the experiments in my basement and I'm considering some sort of projectile trap. Would a carpet or 2 stop a golf ball? Box filled with packaging corns? Any ideas on an appropriate trap that won’t need to be replaced after every shot?

Since the intent is to "test" valves for practical application, I had though that a common high-density projectile would be more appropriate? Does exaggerating the valve speed attributes just muddy the water for practical application requirements?

I have been leaning to using fully optimized (GGDT) designs for testing. Straight pipes with no constrictions and no turns to minimize any conductance issues. Would that be a poor decision?

To all that replied so far, thank you for your input, your replies are the help I need to design appropriate tests. Keep the input coming.

Regards,
Richard
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Postby Mr.Plow » Tue Sep 20, 2005 2:03 pm

Aturner uses a trashcan filled with plastic sheeting (you can get large plastic drop cloths for nearly nothing at Home Depot) as a trap for potatoes, and we've shot golfballs into it with no problems. You could easily make a golfball trap from a frame conposed of 2x4s that have several layers of carpet hanging from them. If just the carpet is not sufficient, you could angle the carpets to direct the golfball downwards into a layer of sand. A while back we discussed traps, so see if you can find the topic using the search function at the top right of the screen.

I would use golfballs not only because I like the fit better than tennis balls but because they are more dense.

The least amount of dead space you have the better. I don't know how noticeable an effect there is by having a 180 degree turn between the chamber and the valve. People say that having the air change directions, even though its before the valve, affects power, although I can't say whether that rumor has any merit.
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Postby clide » Tue Sep 20, 2005 2:37 pm

Exaggerating the valve speed allows for more fine tuning of the GGDT attributes, which can then be applied to any gun. A little tweak here or there can change the velocity quite a bit on an exaggerated model whereas a little tweak on a standard gun may not result to much on your specific model, but if someone builds a gun with the same valve but different type of gun it may make a difference to them.

Light projectiles are also a lot easier to stop, which comes in handy when you are doing a lot of shooting.

I typically place my muzzle only a few inches from the chrony and I haven't ever noticed any strange effects that could be attributed to muzzle blast. (all pneumatic launchers though)

As for a golfball trap I use a big wad of cotton, wrapped in a golf practice net, in front of a couple doormats and a bunch of foam in a big box. Seems to work pretty well, but at higher velocities(~400fps) the net can tend to rip.
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Postby jimmy » Tue Sep 20, 2005 3:02 pm

It would be nice if you could measure the opening time for the valve. An electronic pressure gauge would be great but the sensors are pretty expensive. Instead, perhaps include a piezo element in the air resevoir and record the output with a tape recorder or pc? I think you might be able to get the valve opening time, and perhaps information on the projectile moving through the barrel.

You could also put the piezo in front of the valve but behind the projectile which might give a better readout.
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