Build log: New CAP water gun

Build a homemade water gun or water balloon launcher and tell us about it.
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SSCBen
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Build log: New CAP water gun

Post by SSCBen » Sun Jun 01, 2008 3:30 am

I've started working on a new constant air pressure water gun. In this water gun I want to address some problems with CAP that became evident after I built my first CAP water gun. This one will be significantly smaller, use a piston, not use a backpack, be pressurized with a bike pump, and have some more safety features (some needed for the piston). It'll likely still end up being pretty long for a water gun but it won't be hilariously huge like SuperCAP and some of the length will be tucked under your arm so it will be comparable to other water guns.

The advantages to a CAP system have always intrigued me and this design will go a long way towards making a very practical CAP water gun. I'm designing this water gun to have two full shots before needing to pump again at 40 PSI. Depending on how I make the gun each shot could be 4 to 6 liters big (I'm aiming for 5 liters but might go lower). I also can adjust the power by adjusting the pressure regulator. If I want to conserve water and air, I can use a smaller nozzle and lower pressure.

Another advantage is that I can attach a WBL to this and use the built in air reservoir or the excess air left after a shot.

The biggest disadvantage of a CAP system is that the design promotes excess but I'm really trying to avoid that here. The price also is a disadvantage, but for me this will be cheap because I'm using mostly unused parts.

Likely I won't finish this water gun for a few weeks. The main problem is that I'm going to specially order pressure rated fittings for this to prevent any potential problems. Before I used DWV fittings which hold less pressure than the pipe (2/3 as much from what I remember according to testing), but they're technically not rated so for a project that will use pressures over a long interval I'm not taking any chances. Once I get the pipe and fittings construction should take only a few hours.

To get an idea of how some of this stuff will look I've worked on most of the air regulator/transfer assembly: http://www.sscentral.org/images/dscn0031.jpg

The bottom is where the air chamber will attach. The top is where the air-piston-water chamber will attach. There is a pressure gauge for the air chamber and one for the regulated pressure. A schrader valve is attached at the moment but it can be swapped with air couplers. The two ball valves are necessary to refill the chamber but they also help with safety. The small valve on the middle left is a safety valve that will open when/if pressure goes over a certain value. I'm going to set the valve to open at 105 PSI to prevent pressure from building up too high if the gun is left in the heat.

A detailed guide to building this will be put online shortly after I finish the water gun. CAP water guns are rare because there isn't too much information about them out there so I made this project a priority to get the information out there.

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used man
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Re: Build log: New CAP water gun

Post by used man » Sun Jun 01, 2008 3:45 am

Awsome i really liked the cap concept and the look of the huge hose on the original.
"dont mess with used man!"-sloganizer.net
thanks to dx for link to website

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Silence
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Re: Build log: New CAP water gun

Post by Silence » Sun Jun 01, 2008 3:52 pm

I hope this goes well since it sounds more like what you had imagined CAP would be.

I'm guessing what you have in the picture takes up about a square foot. You might want to reconfigure it to lie alongside the main chambers so that there's less wasted space, but maybe I've just visualized a poor layout.

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SSCBen
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Re: Build log: New CAP water gun

Post by SSCBen » Sun Jun 01, 2008 4:43 pm

It's not quite a square foot because it's a little less than a foot long on the wide part and probably about 9 inches tall but that's pretty good for a guess.

The air transfer part shown won't be laid out exactly as shown. The 1/2 inch ball valve and the safety valve will be angled to fit pipe under them. The top will be where the shorter water chamber goes and the bottom will be where the longer air chamber goes. I guess I could reconfigure it to lie along side the chambers but the top chamber is going to be shorter anyway so I thought this would make them end at similar lengths.

Maybe I can reduce the overall length by putting more on the sides though. I'll have to think about it.

aEx155
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Re: Build log: New CAP water gun

Post by aEx155 » Sun Jun 01, 2008 4:45 pm

Hopefully this CAP gun doesn't end up like the last. I look forward to it's completion.

What do you plan on using for the PC? Is is going to be like the Supercannon II? If you want, I can share a design I just thought up.

Just for the record, I know a lot about the CAP concept and it is pretty advanced. If you want, I could help explain some things. (but I'm pretty sure you don't need any help with that Ben)

Technically, the Ubersoaker is a CAP gun, right?

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SSCBen
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Re: Build log: New CAP water gun

Post by SSCBen » Sun Jun 01, 2008 4:54 pm

The PC will be very similar to Supercannon II, yes. Post your idea too if you have another good one. I haven't built anything so I'm open to suggestions.

The Ubersoaker isn't CAP, it's CGP (constant gas pressure), which is the same thing but with a different gas. It uses CO2. CO2 isn't recommended for PVC water guns because CO2 is rather cold and would make the pipe more brittle, decreasing the pressure rating, but he uses mostly metal pipe so it's fine.

The most major change I've considered to this design is an HPA tank. HPA tanks are really high pressure air tanks. The disadvantages to using use is that it costs a lot of money, might not hold enough air for a few uses (I haven't checked), and couldn't be charged with a simple bike pump. I'll use a regular PVC air chamber for this water gun to use some parts I have leftover but in the future I might experiment with HPA tanks or maybe other higher pressure chambers.

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Silence
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Re: Build log: New CAP water gun

Post by Silence » Sun Jun 01, 2008 5:00 pm

Funny, 9 by 12 inches was exactly what I had in mind. I just rounded up.

Yes, the Ubersoaker is a CAP water gun. I'd forgotten about it until I googled it just now. The only real difference is that it uses compressed air. HPA (high pressure air), CO2 tanks, and propane tanks can all provide at least a thousand PSI when unregulated. For people who are willing to buy gas tanks, there is a really good option...

According to this page, propane's vapor pressure between 70 and 90 degrees Fahrenheit varies from 110 to 150 PSI. For those who don't know, that's basically the point at which liquid propane turns into gas. Propane is stored as a liquid and it vaporizes until the pressure in the tank reaches the vapor pressure. So if you hook up a propane tank to a pressure chamber, it'll automatically regulate to a suitable pressure (which depends on the temperature). This would be a very powerful, long-lasting, cheap and simple CAP system.

Maybe I should write an article about it.

EDIT: Didn't see Ben's post.

Thanks for the correction on CAP vs. CGP.

And yes, there are two things to watch out for here:
1) Cold tubing. If you've ever used a duster (with gas that shoots out a thin straw), you know it'll get cold after use. Vaporization is an endothermic process: substances need more energy as gases than as liquids, so they absorb heat. You'll need metal for a CGP water gun.

2) There's actual liquid inside. Aerosols and dusters have to be upright when used or some of the liquid might escape. The low temperatures or chemical reactions (depending on the substance) can make for some regrettable consequences. The tanks for the CGP system should be vertical.
Last edited by Silence on Sun Jun 01, 2008 5:05 pm, edited 1 time in total.

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Drenchenator
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Re: Build log: New CAP water gun

Post by Drenchenator » Sun Jun 01, 2008 5:50 pm

I'm actually unsure as to what Ben has in mind for this one, but I'm sure that it's going to be good. From what I'm seeing here at home, it looks to be a mix of SuperCAP and Supercannon II. I imagine it will perform on par with SuperCannon II, but then again I don't completely know what he has in store for it.
Just for the record, I know a lot about the CAP concept and it is pretty advanced. If you want, I could help explain some things. (but I'm pretty sure you don't need any help with that Ben)
Fortunately enough I already solved many of the basic equations for CAP--and believe me it took a while and a couple sheet of paper. That article should explain what's really going on with it, though this system may be more advanced than the "simple" CAP system I made those equations for. Maybe not too.
The Drenchenator, also known as Lt. Col. Drench.

aEx155
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Re: Build log: New CAP water gun

Post by aEx155 » Sun Jun 01, 2008 7:55 pm

Image

There you go, Ben. That was what I had in mind. You probably would have to try and find some form of 4" SCH 80 pipe for the air tank if it was going to be feasible; otherwise, HPA might work. After reading about CAP vs. CGP, you probably wouldn't be able to use CO2 for this, but if you get a large enough air supply, this would work pretty well.
Last edited by aEx155 on Mon Jun 02, 2008 3:48 am, edited 1 time in total.

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SSCBen
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Re: Build log: New CAP water gun

Post by SSCBen » Sun Jun 01, 2008 8:32 pm

Nice design. That's similar to the CAP water gun I wanted to build last year. The main difference was that I had two air chambers and one water chamber but other than that they were pretty close.

If that's meant to be a backpack you shouldn't use pistons to reduce the cost. They wouldn't help because the angle would be correct unless you were lying on your stomach.

I'll probably eventually make something more similar to that but I hope someone beats me to it. Backpacks are where CAP designs make most sense.

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Silence
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Re: Build log: New CAP water gun

Post by Silence » Sun Jun 01, 2008 10:41 pm

Nice diagram, and that is pretty neat. My only suggestion would be to rethink the proportions of volume for each tank. For example, the unregulated air, depending on its pressure, could be enough for 2 shots or 500 shots, which you can balance by changing that tank's volume.

Also, the fill port for water should probably be on top. That way, you won't have to flip over a heavy water gun.

And finally, you don't need pistons in a vertical tank, as Ben said, unless the water is in the top. Additionally, you probably don't want the piston in the top of the regulated air tank anyway, as it stops the air from actually getting to the other tank where it pushes on the water.

Ben's using pistons in his new CAP water gun so that the water and air can be separated even in a horizontal tank. That should allow for vertically compact designs that can be held entirely in one's hands.

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SSCBen
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Re: Build log: New CAP water gun

Post by SSCBen » Sun Jun 01, 2008 11:20 pm

After some thought, I'm going to first build this design with a large air chamber chargeable by a bike pump and then modify it to use an HPA/nitrogen tank. After doing the math, the basic system CaptainSlug described here hold 2.7 times as much air as the largest system I've thought about trying. I'll just need to figure out a cheap way to recharge these tanks at a remote location and HPA/nitrogen could be the way to go. HPA would make it much more compact and much safer, but much more expensive.

Though, I figure this water gun is already rather expensive so that might not matter. If this turns into the $300 "professional" water gun, so be it. I figure whatever works, works. And I know I'll have fun building it.

aEx155
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Re: Build log: New CAP water gun

Post by aEx155 » Sun Jun 01, 2008 11:42 pm

Okay. Thanks for the suggestions and compliments on the design.

Technically, having pistons means that my whole design would work upside down, right? Plus, pistons reduce the amount of charging you have to do because it prevents air from leaving when you're done firing.

If it's needed, you probably could add a larger tank to the back and still keep it within reason. You could also probably go into metal for the tanks.

Does anyone have a link to pressure ratings for SCH 80 pipe? What about the gas temperatures for HPA tanks? Those would help.

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SSCBen
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Re: Build log: New CAP water gun

Post by SSCBen » Sun Jun 01, 2008 11:49 pm

Technically, having pistons means that my whole design would work upside down, right?
That's correct.

I've noticed two small problems with your CAP design now that I've examined it a little closer. Air regulators have a direction indicated on them. I don't think they're designed to operate backwards, so to vent the pressure on the air side of the pistons you'll should move the air vent to the regulated pressure part. You'll also need a ball valve before the regulator to prevent air from the air chamber from venting when you just want to vent the regulated chamber.
Does anyone have a link to pressure ratings for SCH 80 pipe?
PVC reference is helpful for a lot of things like that.
What about the gas temperatures for HPA tanks?
HPA is just highly compressed air, so it doesn't have a vapor pressure because it doesn't change phase. It'll be at whatever temperature the inside of the tank is.

aEx155
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Re: Build log: New CAP water gun

Post by aEx155 » Mon Jun 02, 2008 12:32 am

Image

Corrected design.

I figured that 24" of 4" pipe at SCH 80 would hold 301.71428571428544 (cubic inches) = 1.30612245 US gallons of air at a max pressure of 190 PSI. Not too good.

If I were to use this design, I would probably go with A) metal air tanks or B)HPA/Nitrogen like Ben said.

Then again, this is just an idea. I probably will never make this unless I'm paid to do so...

I think I'll just stick to LRT and CPS; I'll be here if anyone needs me though.

EDIT: My 100th post!!!! (I think...) YAY!
Last edited by aEx155 on Mon Jun 02, 2008 4:00 am, edited 1 time in total.

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