A Great Challenge of Engineering

[Andrew]

It appears you could use a 'linear' arrestor, it does have a seperator AND uses air pressure. I would still try to find the other type as you can't vary the pre-charge pressure in one of these, so I couldn't even hazard a guess at the range it would give, nor can I provide a definite volume (it's diameter is not much more than 15mm pipe with a 13mm internal diameter).

It would be easier to hide up your sleeve though!!!

[Andrew]

Don't know about ACE but both Home Depot and Lowes, appear to sell the linear arrestor (pick above). Can't seem to find the other type though! I suppose being threaded, you can always swap pressure chambers later on if the linear ones are indeed pants! Combustion Depot sells similar types to the one in my earlier diagram, as does accentshopping, but I suppose there's always ebay and/or Amazon! I can get the white Varem arrestor (first diagram) for under £10 ($16 ish), which is LESS than the linear ones (though the opposite seems to be the case across in the US).

I think the linear type will work, but it's range and capacity may be less due to the lack of a schrader valve to change the pre-charge (you may find that you have almost no water in it when pressurised from mains supply, if the pre-charge pressure is too high, resulting in a ridiculously short shot time with poor range so check the pre-charge pressure BEFORE you buy a linear type and ensure that it is below mains pressure).

[atvan]

If you are any good at these things, it would probably not be that hard to drill a hole, add a shrader valve, and solder it.

[Andrew]

Atvan's suggestion may be the answer, just be careful with arrestors with a high pre-charge pressure. I personally don't like drilling into something containing air at high pressure, but if you can find a way of doing that safely then that might solve some of the issues (using a drill press with some form of guard perhaps???). Just make sure you're happy with the capacity first (more obviously equals longer shot time, but adds weight and makes it less concealable) and remember that a higher pre-charge means lower water capacity but a single, short, powerful shot may be exactly what you want! Also the additional price of the schrader valve and time required to modify the linear type, it may be cheaper/more worthwhile to pay a bit more for the other type (that is if you can't find a suitable linear arrestor). You could always take it back and claim it was 'unsuitable' for your purpose (just DONT mention that the purpose is a small homemade water gun, you may get some odd looks, and no refund! ). 45-50psi pre-charge should be just right for mains water (it is for UK mains water). You may want to try less (around 25-30psi-ish) as the arrestor is smaller than my large expansion tank, and the bladder takes up more of the internal space, so the air will be compressed much more. You should get a decent stream from 0-10psi, but would have MAJOR dropoff over the length of the shot.

[atvan]

Notice that he needed this 2 days ago. I love how when people new to the site ask a question, we get into a technical discussion, scare them off, and continue long after the purpose of the OP is obselete.

[Andrew]

Oh well, it's here if he ever needs it!
And hopefully he'll post the result if and when he's completed it

[sfchris]

I'm still alive, I've got two kills in four days using pen tubes filled with water. I got a chance to go to ACE on monday, and they had only the linear arrestors, but I figured that since they do the same job as the diaphragm type with less volume, they must be at higher pressure and I wouldn't be able to pump any water in. I've had midterms this week, so I haven't had time to get out much, but I should get a chance to go to home depot this weekend and hopefully they'll have the right kind.

[sfchris]

and even if I die, there'll be a next round

[atvan]

:d such a postitve atitude.

Would be funny if there was a guy wating for you at Home Depot.

[Andrew]

YAY we didn't scare him off!!!

I assume you're referring to death in the first assassins round!

I couldn't find any on the home depot website though. As long as the pressure of the linear arrestor is LESS than mains pressure it will fire SOME water when used in the design. Remember it is designed to absorb shock in water at mains pressure! Also McMaster sell both types (but not too cheaply, and you may need a free account to view them!).

You'll just have to ask the staff at home depot about the pre-charge (if the pre-charge pressure isn't listed on the side of the box).

As atvan said you could always fit your own schrader valve (you can get solder-able ones!)

Also, if you think it best not to try a linear type, try searching "watts water hammer" (including the quotation marks) in google or yahoo (even try images!!!). Most of these sites are for buying such parts. Here's one such site.

You can check out Combustion Depot

and accentshopping

aswell as bostonheatingsupply!

Annoyingly the linear types would, in theory, work slightly better (as the force is only applied in the direction you want the water to go!), if only we knew the pre-charge and capacity!!!

[sfchris]

okay, two bits of news, both sad:
1. I died today
2. I went to the home depot today, and they did not have any with diaphragms, which was annoying, and nobody knew the pre-charge on the ones that they did have
Also: I still want to make this, but the internet has become my source of parts so this could take even longer than it has already
and, neither I, nor my school's wood shop have appropriate facilities to drill into pressurized metal containers

[Andrew]

You could, possibly, make the first half of it (from the nozzle up to where the arrestor connects) first, and buy a linear type to test with. If it doesn't work well (as long as you don't damage the packaging, and you dry it out first) you should be able to get a refund saying it is unsuitable for what you need (once again DON'T tell them what you needed it for, they'll assume you'll use it for it's intended purpose).

If you think it will be too expensive, and because time is no longer of the essence, you could try and make you're own pressure chamber.

Perhaps along the lines of SEAl's SCH Here and Here just use 1/2" pipe istead of 3/4", and a smaller pressure chamber and it'll be a bit more compact!. if you put a 1/2" male thread on the bottom of the PVC pressure chamber, you can unscrew it later, and fit a smaller diaphragm arrestor!

Or even a mini version of Supercannon II (a piston separates the air and water, like the linear arrestor) but use narrower pipe for the pressure chamber. Again, a 1/2" male thread on the water side of the chamber, and you can swap it out later (although, if you can make this one work, there will be no need for the diaphragm arrestor).

Put a schrader valve in the top of the SCH or on the airside end of the mini Supercannon, and you can pre-charge them easily!

You can always convert it to the curent design when funding allows you to!