12 replies to this topic

[pwnchu]

So I'm using a homemade (30 inches of 1/2"PVC-about 9 cubic inches, using the pump and valve from a SS MaxD2000). For fear of bursting the goop (and a leak); i'm only pumping it to about 3 atm or 45 psi (estimated based on the size of the pump and the times i usually pump it).

Anyways, by accident i found that adding water into the tube gives more power to the dart. I'm only adding a bit of water, so it wont affect the volume much. Another clue is that right after the water is added, firing it doesnt release a spray of mist. You have to fire it once or twice before that.

Would anyone know chemistry-wise how this works? (and hopefully maybe some people will find this useful)

[Pineapple]

So I'm using a homemade (30 inches of 1/2"PVC-about 9 cubic inches, using the pump and valve from a SS MaxD2000). For fear of bursting the goop (and a leak); i'm only pumping it to about 3 atm or 45 psi (estimated based on the size of the pump and the times i usually pump it).

Anyways, by accident i found that adding water into the tube gives more power to the dart. I'm only adding a bit of water, so it wont affect the volume much. Another clue is that right after the water is added, firing it doesnt release a spray of mist. You have to fire it once or twice before that.

Would anyone know chemistry-wise how this works? (and hopefully maybe some people will find this useful)

I'm not certain if you've got it to 45psi...sounds like a lot...from what data we've obtained, most Nerf blasters (stock) generate less than 5 psi or thereabouts, so I'd figure more along the lines of 15-20 psi, maybe. I could be wrong there, so don't quote me on that.

One of our air-powered homemades was shooting 80 feet with the regulator set at "only" 10psi...it was an old regulator so it might have been off some.


Water doesn't compress, so the only thing I can think of at the moment is that the water added somewhat reduces the total volume of space in the airtank/pump line/valve, and hence you get a bit more output. Must be pretty messy comin' out, too!


I'm wonderin' if any of our members have pressure gauges attached to their slightly modified air blasters. Now you've got me thinking. CaptainSlug has some on his...I'll find out.



-Piney-

[pwnchu]

Well it's 30 inches of 1/2", while the pump (at least where i pull it up to) is about 3.25 inches of 1/2". So i pump it about 18 times meaning it probably gets to 3x normal, which is atm, converting to 15 psi per atm.
(Well actually since i recalculated with 14 pumps which is what i usually use it's more like 2.5 atm, or 36 psi [including the normal 14.7])


Isn't standard pressure like 15psi? (1 atm=15psi) Maybe i've got the conversions all wrong

Anyways, i mean like there is only a little water (which would be easily offset by a difference of one or two pumps, which i did). Anyhow, even after you let the water drip out, it still works better until the mist starts running out.

Anyways, i use elbows so the non aerosolated part stays away from the valve, so it wont come out unless you hold the thing upside down. The mist is actually kind of cool to watch, like a smoking gun. The annoying part is that the leak makes a whining noise when you have water.

[frost vectron]

If I am not mistaken, I always thought that pressure gauges measure how much above atmospheric pressure is there.

If you have a PVC tube that is open on both ends and you put a pressure gauge onto it, it will read zero PSi.

This does not mean that inside the PVC tube it is a vacuum.

Somebody correct me if I'm wrong.

[Prometheus]

If I am not mistaken, I always thought that pressure gauges measure how much above atmospheric pressure is there.

If you have a PVC tube that is open on both ends and you put a pressure gauge onto it, it will read zero PSi.

This does not mean that inside the PVC tube it is a vacuum.

Somebody correct me if I'm wrong.

You are correct.

[pwnchu]

Well of course the pressure gauges should be offset so that standard pressure is at "0".

But the main point is that if you can add pressure by measuring the volume of the plunger versus the volume of the pipe tank, then I'm operating at 1.5atm or 22psi, probably a little less, because of the slight leak that i can't fix. I probably shouldn't have said 3 atm/45 psi in the beginning, that's like saying i pumped earth's atmosphere to 1 atm. So that was a mistake on my part.

Anyhow, the description of my homemade was just a side detail in case it helped.

What I'm trying to figure out is why it works better with water. It is not because of the volume reduction based on non compressible water as by these reasons:
-I hardly add any water in the first place
-I accounted for the volume offset of the water by pumping it less by one or two times than with drier conditions. Indeed it works better with 10 pumps than a dry condition at 16.
-After draining the tank by holding the valve open and letting it all drip out, it still performs at the elevated state
-The elevated state is not attained immediately after water is added
-The elevated state seems to be tied to the amount of mist/droplet spray. i.e., after adding water it kicks in after pumping and releasing a few times, and this is also when the mist becomes much thicker. At the other end, the mist begins to dissipate after firing it many times (assuming the water is drained), and this is when the gun does not perform as well.

I am sure that you will agree with me that the volume reduction is not why the darts are more powerful.


Another note that is kind of related but may be distracting
If i fire it upside down (so that the pooled water is near the valve) then water will come out. It barely shoots the dart out.

I may ask a chemistry teacher soon (i tried a physics teacher but she didn't know because pressure is more of a chemistry thing...), but im thinking it either has to do with the water being better at transferring energy or diffusing faster than air.

Edited by pwnchu, 15 September 2007 - 03:59 AM.

[CaptainSlug]

Science time!

The standard pressure generated by any given Nerf gun varies a good deal. The Titan, Hornet, and many other semi-auto or single shot Nerf guns have over-pressure release valves set to 35psi.
Automatic Nerf guns (Magstrike, Powerclip, RF20, WF20) have a much higher operating pressure between 40 and 60 psi, but I'm unsure of the overpressure valve setting because I never measured it.

The pressure level needed to fire a dart is extremely dependent upon the amount of volume being used. The lowest effective volume you can use to fire a dart at 40psi seems to be 2.5 cubic inches. Most of the better performing air guns have atleast twice that available. To fire at lower pressure you need much more volume (a minimum of 6ci). Firing a dart at 10psi requires that you have a good deal of volume.

In the next few weeks I would like to run some practical experiments on volume so that I can finally get a formula and baseline worked out for what makes an effective configuration.

The above assumes that the passageways from the air supply blast chamber and the barrel are nonrestrictive, in the sense that the area of any given passage doesn't not go below that of a 1/4" diameter passageway.


As for the water, I'm assuming you are using a high volume chamber. The water could be doing two things at once.
1. sealing and lubricating
2. affecting the dimensions of the foam

Edited by CaptainSlug, 15 September 2007 - 09:20 AM.

[Pineapple]

^^^^^^^

My hero.


40 - 60 psi for a Titan? Whoa.


Yeah, the homemade in question had a 36" long 3" SCH40 PVC chamber. I think it's still in the Homemades forum somewhere.

Thanks for the info, Sluggy. Only the best dwell here.



-Piney-

[CaptainSlug]

The Titan overpressure is set somewhere between 35 and 40psi. So if you plug the pump you're going far beyond that. The fittings and tubing that Nerf guns use are rated up to (but not beyond) 80 psi.

Edited by CaptainSlug, 15 September 2007 - 11:25 AM.

[pwnchu]

Science time!


The pressure level needed to fire a dart is extremely dependent upon the amount of volume being used. The lowest effective volume you can use to fire a dart at 40psi seems to be 2.5 cubic inches. Most of the better performing air guns have atleast twice that available. To fire at lower pressure you need much more volume (a minimum of 6ci). Firing a dart at 10psi requires that you have a good deal of volume.

The above assumes that the passageways from the air supply blast chamber and the barrel are nonrestrictive, in the sense that the area of any given passage doesn't not go below that of a 1/4" diameter passageway.

I think i am using about a 9ci tube at around 22 psi. The valve opens very rapidly because of a spring on the trigger. The diameter is around 1/4".

As for the water, I'm assuming you are using a high volume chamber. The water could be doing two things at once.
1. sealing and lubricating
2. affecting the dimensions of the foam

So for #1 does that mean that the performance is due to the presence of water in the barrel?
I dont think the foam needs to absorb water for it to do better
Is water truly something useful for pump guns, or does it just counteract a flaw in my homemade?

Edited by pwnchu, 15 September 2007 - 03:11 PM.

[CaptainSlug]

1. So for #1 does that mean that the performance is due to the presence of water in the barrel?
2. I dont think the foam needs to absorb water for it to do better
3. Is water truly something useful for pump guns, or does it just counteract a flaw in my homemade?

1. It may be aiding the sealing the surface of the dart to the barrel.
2. Polyethylene foam isn't going to absorb much water because it's a closed cell foam.
3. Useful? Maybe as a kind of barrel lubricant or sabot mechanism.

[pwnchu]

1. I believe you are correct on this. After reading your post, i decided to try it out. I have two 15" barrels, so
-I ran water in the inside of the first
-I put tissue in, then plugged it to my nitefinder and fired until it popped out the other side for the second.

The first one had the powerful characteristic, but not the second. Of course putting water in the pipe tank will give the effect of constantly supplying water to the barrel.

2. So if it doesnt absorb water, then how would its dimensions change?


Thanks for helping me with this, it really had me confused for a while.

[CaptainSlug]

Your experience attests to what I have discovered with practical tests. Air guns work best when there is very little dart and barrel friction, up to the point that this ease of movement does not negatively affect the ability of the dart to keep an effective airtight seal. The best barrel material I've ever used with an air gun is .527" ID aluminum, mainly because it's extremely smooth on the inside. But I haven't used it as much because it requires added barrel length. I may make more barrels with it in the future.

The opposite is true for spring powered guns however. The barrel needs some tackiness to hold the dart in place while the plunger action can build up enough pressure behind the dart. This makes PETG and CPVC more than ideal.

2. So if it doesnt absorb water, then how would its dimensions change?

Experienced nerfers will attest to both relative humidity and elevation changes being able to affect the performance of their darts for better or worse. How this dimensionally or characteristically affects the foam has never really been measured imperically. Nor could it be with any great deal of accuracy without a laboratory setup.

Edited by CaptainSlug, 15 September 2007 - 11:45 PM.