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Achieving consistent muzzle velocities

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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Achieving consistent muzzle velocities

Unread postAuthor: btrettel » Wed Jul 08, 2009 7:48 am

Does anyone here have any specific tips for improving the consistency of muzzle velocity for a pneumatic? I did some tests a few days ago with the intention of finding the drag coefficient of my Nerf darts, but there is too much variability and this seems attributable in part at least to the inconsistency in the muzzle velocity of the test gun.

I'm willing to accept that I won't know the drag coefficient very precisely, in fact, I expect that.

My procedure involves shooting a test gun, measuring the muzzle velocity and range, and then recording that data along with other data like projectile mass. I can plug these numbers into a reasonably realistic but simple external ballistic simulation I wrote to find the drag coefficient.

Here's my current plan to reduce variability: The same dart will be used repeatedly to eliminate any variability from variability due to mass. A fixed starting height and flat angle will be used. Dual regulation (the air compressor's plus another one I have lying around that's much better than the one I used for testing earlier) will be used to keep pressure as consistent as possible. I'm also thinking that choking flow with the barrel (basically using a 3/4" QEV to power a 0.53" diameter Nerf dart) will eliminate variability associated with how fast the valve opens.

There was some light wind on occasion when testing a few days ago and this likely had an effect as well.

Any other suggestions?
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Re: Achieving consistent muzzle velocities

Unread postAuthor: spudtyrrant » Wed Jul 08, 2009 8:14 am

How far are you shooting away from the chrony wind can either have a very big or unnoticeable effect.

When shooting make sure you use a digital pressure gauge, because even the slightest change can make a noticeable difference.

Do you have your gun in a vice, or are you shooting from the shoulder i would suggest putting it in a vice while shooting.

Make sure you are shooting on a day with either very light wind or no wind at all to try and keep your results consistant.
Edit: there you happy lol
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Last edited by spudtyrrant on Wed Jul 08, 2009 8:29 am, edited 2 times in total.
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Unread postAuthor: btrettel » Wed Jul 08, 2009 8:24 am

Thanks for the reply.

Using periods when necessary is very helpful to people who want to read your posts.

I intend to mount the test gun on a something, likely a tripod, and use a level to ensure the barrel is level. A vise or clamp would be good for holding the gun.

My Chrony was (and will be in the new tests) less than a foot from the tip of the barrel.

I don't think a pressure gauge is even necessary aside for checking because the regulators should be consistent. An accurate and precise digital pressure gauge would be very nice, but I don't intend to burn a lot of cash on this and my 2% accurate gauge should be fine for any noticeable changes.
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Unread postAuthor: spudtyrrant » Wed Jul 08, 2009 8:35 am

Yeah, being less than a foot from the barrel can be a problem unless your using a wire break chrony.

The air is moving out of your cannons barrel faster than you projectile and sometimes this can give erratic readings on your chronograph.

I would move about 3' away when trying the test

b.t.w- You can get digital gauges at autozone for $5, but i didn't know you were using a reg i thought you were using a bike pump.
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Unread postAuthor: Technician1002 » Wed Jul 08, 2009 9:24 am

Just a quick question.. What type of valve are you using and how is it triggered.

Chamber sealer valves (including most sprinkler valves) are highly dependent on the speed and flow of the pilot valve. Variability in the triggering can make variability on the output.

Let me know your setup and I'll look for unstable parts. Part of the design I used was for highly predictable shots for a competition.
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Unread postAuthor: spudtyrrant » Wed Jul 08, 2009 9:35 am

Technician1002 wrote: Variability in the triggering can make variability on the output.

he's using a 3/4 qev to shoot a nerf dart as he said it should eliminate variability on how fast the valve opens and qev's being as touchy as they are could be triggered by a pressure drop of 1 psi so i don't think that is something to worry about
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Unread postAuthor: btrettel » Wed Jul 08, 2009 11:45 am

Yeah, being less than a foot from the barrel can be a problem unless your using a wire break chrony.

The air is moving out of your cannons barrel faster than you projectile and sometimes this can give erratic readings on your chronograph.


Interesting. Is this a well known effect? It goes contrary to my understanding of how the chronometer works.

The air doesn't cast a shadow unless discontinuities in it exist (i.e. shock waves and temperature changes) and this fact is used to make shadowgraphs. And as far as I know very bright lights must be used to make these discontinuities appear.

Of course, I could be (and often am) wrong, and as this isn't my area of expertise, I don't know if I am.

b.t.w- You can get digital gauges at autozone for $5, but i didn't know you were using a reg i thought you were using a bike pump.


Good to know. I wouldn't suppose a $5 gauge would be very precise but I think I'll buy one for where precision isn't necessary. Thanks.

I mentioned I was going to use dual regulation for consistency. The combination of the regulator on the air compressor and a second one should be consistent.

The regulator I'll be using in the gun this data is for is NOT consistent as the outlet pressure depends greatly on the inlet pressure. I figure this is not too important for a Nerf game as you can adjust the pressure as needed during the game. The size of the regulator (3" long and about 1/2" in diameter) is why I use it.

Just a quick question.. What type of valve are you using and how is it triggered.


The original test setup used a 3/8" Deltrol QEV with a Clippard MJVO-3 valve as the pilot valve. Clippard's catalog says the valve has a flow rate of 14.0 scfm at 50 psig and 25 scfm at 100 psig. This is good for Nerf but is inadequate for this testing.

My new planned test setup is designed to make flow saturate quickly. I have a 3/4" Deltrol QEV with a 3-way 1/4" NPT brass ball valve as the pilot valve. The ball valve has reduced ports, but it should be very adequate. Ball valves can open slowly, but there essentially is no pilot volume, and the idea is to let the barrel be the flow restriction, so I don't think this will be a problem.
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Unread postAuthor: Technician1002 » Wed Jul 08, 2009 12:35 pm

btrettel wrote:
Yeah, being less than a foot from the barrel can be a problem unless your using a wire break chrony.

The air is moving out of your cannons barrel faster than you projectile and sometimes this can give erratic readings on your chronograph.


Interesting. Is this a well known effect? It goes contrary to my understanding of how the chronometer works.

The air doesn't cast a shadow unless discontinuities in it exist (i.e. shock waves and temperature changes) and this fact is used to make shadowgraphs. And as far as I know very bright lights must be used to make these discontinuities appear.

Of course, I could be (and often am) wrong, and as this isn't my area of expertise, I don't know if I am.

b.t.w- You can get digital gauges at autozone for $5, but i didn't know you were using a reg i thought you were using a bike pump.


Good to know. I wouldn't suppose a $5 gauge would be very precise but I think I'll buy one for where precision isn't necessary. Thanks.

Just a quick question.. What type of valve are you using and how is it triggered.



My new planned test setup is designed to make flow saturate quickly. I have a 3/4" Deltrol QEV with a 3-way 1/4" NPT brass ball valve as the pilot valve. The ball valve has reduced ports, but it should be very adequate. Ball valves can open slowly, but there essentially is no pilot volume, and the idea is to let the barrel be the flow restriction, so I don't think this will be a problem.


In air cannons, the sudden drop in temperature often causes an intense fog. Here are a few taken with high speed photography. I hope these photos help.

Next on the QEV trigger..
Cool Hwhip stated
he's using a 3/4 qev to shoot a nerf dart as he said it should eliminate variability on how fast the valve opens and qev's being as touchy as they are could be triggered by a pressure drop of 1 psi so i don't think that is something to worry about


If the QEV triggers with a 1 psi drop, send it back it's broken. Now on to the operation of QEV's and their intended use. They are intended to be used on a long control line.. READ large pilot volume.. and suddenly release the vent on an air cylinder, air brake, etc.

How it works. It remains closed while the pilot volume pressure bleeds off slowly. Then the balance point is reached.. umm way more than a 1 psi drop., and then as it unseats the pressure hitting the now exposed face of the piston snaps it open.. Note, for this to work, the pressure in the pilot should not suddenly rise as the piston moves in to the pilot space. A super small pilot volume on a QEV can make it twitch, open and then move right back as the piston movement compresses and causes a pressure rise in the pilot.

The same effect also happens if the inlet plumbing into the pilot valve is not right next to the "cylinder" chamber it is venting. Either a sudden rise in pressure in the pilot area, or a sudden drop in pressure in the "cylinder" chamber port has the same effect and tries to re close the valve.

I hope this makes sense. Add at least 6 inches of tubing to the pilot line to the ball valve and try again.

A comment on your new setup. How fast can you open a ball valve? The time line on top is in seconds. The entire event in the picture took a total of about 0.004 seconds.
Image
This is the typical time of a small air cannon. If you take 0.1 seconds to open the valve, your entire shot is done while you are still opening the valve. The sound is from a 1 inch cannon with a barrel about 30 inches long.
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Unread postAuthor: btrettel » Wed Jul 08, 2009 2:28 pm

More interesting points.

Condensation wasn't something I had considered. Thanks for bringing it up. Makes me want to get some sort of high speed photography or video setup!

I'm already well aware of the durations of shots, etc., and the full dynamics of solidly subsonic pneumatics (apparently not condensation though!). I've been working on filling in the details for transonic projectiles but only indirectly recently.

The "twitch" you've mentioned makes sense, though, I'm confident my pilot volume is far larger than the displacement volume (assuming the pilot valve flow rate is slow enough such that this is a problem). I'd be more worried about the bounce of the diaphragm from the impact.

My ball valves open just as slow as anyone else's (probably slower). As far as I know, the point of a ball valve as a pilot valve is that they are relatively high flow valves and thus don't need to be fully open to match the flow of a faster-to-full-open pilot valve (if that makes sense). Phrased as a question, if valve A can reach 10 L/s in 0.1 seconds but valve B can reach 1 L/s in 0.015 seconds, which is "faster" (assuming a linear opening profile)? What matters here how quickly the pressure can drop, which depends on how quickly the volume of gas is displaced (this is a time, not a flow rate, which is a little confusing).

In addition, the barrel will be the flow restriction here so the QEV does not need to be fully open for full flow. Perhaps I had not made this clear enough; in a system with a 3/4" valve driving an ~0.53" projectile, the barrel obviously is the limiting factor. This I believe will improve the consistency by reducing the "opening time" (i.e. time to full flow). I recall some discussion about this (perhaps at Spudtech way back) for high performance cannons.

Also, for everyone's edification, according to the literature about with my valve a pressure differential of 3 psi minimum is necessary to move the diaphragm. In my experience provided numbers have a tendency to be very optimistic.
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Unread postAuthor: jimmy101 » Wed Jul 08, 2009 3:00 pm

One more thing to consider is how long you wait between pressurizing the chamber and firing the gun.

If you are pressurizing from a resevoir, then the gas in the gun is cold to start with (the gas cools as it expands from the resevoir into the gun's chamber) and the pressure will rise as it warms up to ambient temperature. The longer you wait between pressurization and firing the higher the pressure in the chamber will be.

If you are pressurizing directly from a pump (no storage tank) then you are pressurizing with hot air. The longer the chamber sits after pressurizing the lower the pressure will be because of the gases cooling off.

Not sure exactly how big an affect those two are but I know that on my compressor the air line between the pump head and the tank gets too hot to touch when the tank is pressurized from 1 ATM all the way up to the compressors shutoff pressure of ~120 PSIG.
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Unread postAuthor: Technician1002 » Wed Jul 08, 2009 3:05 pm

Thanks for the link to the literature. The control to operate it is vented to the atmosphere.. the trip point is not specified. The shift referred to in the literature is in reference to Figure A on the right. Manufactures want to know how much pressure loss there is operating the valve.. what is the pressure in the cylinder.. Depending on model.. it 3-8 PSI below supply pressure to move the valve and start filling the cylinder. Once the valve has moved, the loss is less.

It is often not clear in literature what the 3-8 psi shift is all about. In the case of QEV's, it is not a 3-8 psi drop in supply pressure to fire it but a 3-8 PSI pressure above the cylinder to change it to fill mode.

It is just spec'ed at it will fire when vented to atmospheric pressure. No trip point is specified.
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Unread postAuthor: btrettel » Wed Jul 08, 2009 3:23 pm

@jimmy: Good observation. I'll make sure to charge up the compressor and wait a while for the air to reach the steady state temperature. And wait too for the other pressure chamber.

@Technician: Ah, makes sense, especially given that I considered the number to be optimistic for the tripping point of the valve.
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Last edited by btrettel on Wed Jul 08, 2009 4:17 pm, edited 1 time in total.
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Unread postAuthor: Ragnarok » Wed Jul 08, 2009 3:39 pm

Given I'm working towards an MOA capable spudgun, this is a question I've had to ask a few times.

To extend on jimmy101's point, having studied the results of Apocalypse, I'm sure that gas temperature, although not as major an element as gas pressure, can be a notable part of the problem.

So, you might want to see if you can increase the consistency in the gas temperature. There are quite a few ways of doing this - the simple way being what jimmy suggests, but you can obviously get more complex.
My own solution to the problem is the oddly (but explainably) named Daemon's Kettle - a souped up version of an automotive intercooler which uses boiling water to keep things cool.
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Unread postAuthor: btrettel » Wed Jul 08, 2009 4:23 pm

Waiting for the gas to cool in the compressor and gas in the gun to heat up is the easiest approach. I know a thing or two about heat transfer so I might run the numbers to see how long this'll be. I'm not going for anything fancy at the moment (but I won't discount anything in the future).

If the time is fairly long I might try dipping the pressure chamber in ice water, obviously with a thermometer to maintain a certain temperature.
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Unread postAuthor: Technician1002 » Wed Jul 08, 2009 6:42 pm

btrettel wrote:Waiting for the gas to cool in the compressor and gas in the gun to heat up is the easiest approach. I know a thing or two about heat transfer so I might run the numbers to see how long this'll be. I'm not going for anything fancy at the moment (but I won't discount anything in the future).

If the time is fairly long I might try dipping the pressure chamber in ice water, obviously with a thermometer to maintain a certain temperature.


Gas in a small container will rapidly adjust to the container temperature. Take for example a hair blow dryer.. The gas spends very little time warming up. Take a car radiator. Again very little time is spent going through. A couple seconds and it's mostly adjusted. Look at the length of time the fog remains in a barrel after a air cannon shot. It's gone after a few seconds.
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