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Can reservoirs be additive?

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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Can reservoirs be additive?

Unread postAuthor: psisquared » Mon Feb 18, 2013 10:41 pm

As analogous to series and parallel in circuitry:

Can two 2 liter bottles at 70 psi be treated as one bottle at 140 psi?

Thanks!
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Unread postAuthor: Zeus » Mon Feb 18, 2013 11:02 pm

No, the amount of stored energy is doubled(amperes), but the pressure stays the same (voltage). You also won't get the same power, as there's only x pressure on the projectile. Double the pressure, and you get 2x. That's why we like HPA.
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Unread postAuthor: psisquared » Tue Feb 19, 2013 12:11 am

I got confused on the 'won't get the same power' part but that's just my own doing I guess. It makes sense overall.

Dah well, I was hoping for something like this (two 2 liters | looks weird a on mobile phone):
    __________
__||__       __||__
|        |       |         |
|        |       |         | -more air
|  Air  |       |         |
|        |       |.·.··....|
|        |       |·.·.·.·.·| -water
 \      /         \ ·.·.·./
   \_/             \.../
                      ||
                      | ¯¯¯¯¯¯¯:::::-> Super Soaker²

Thanks for the help!
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Last edited by psisquared on Tue Feb 19, 2013 5:48 am, edited 3 times in total.

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Unread postAuthor: jackssmirkingrevenge » Tue Feb 19, 2013 12:38 am

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Unread postAuthor: psisquared » Tue Feb 19, 2013 1:06 am

Thanks for the link. I've either got some learning to do or I'm just going to wing it.

I'm probably going to wing it, lol. I'm dealing with water over a long time, so I'm guessing I need a higher volume to keep the high pressure constant.

Thanks guys! You seemed to know your stuff, so I figured I'd ask here (even though I'm not propelling spuds). Thanks for not shooing me out!

Edit: If I could redirect my own thread here, does 'expected psi' change with an object in the chamber? If 70 psi is average for a two liter, would that fluctuate depending on if water is in it and/or how much water there is?

Like if it were full of water minus an inch...could I still pump a bicycle pump to 70, would it just reach 70 psi faster?

I could wing that, but I don't really want to make a loud boom for my neighbors, heh.
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Unread postAuthor: Zeus » Tue Feb 19, 2013 1:42 am

If the chamber is half full of water, then charged to 70psi, you'll have 35psi when it's empty. It's just ratios.

Two other things, I've had good luck with a near laminar flow nozzle and a chamber at 150psi, filled to 90% with water. Also, there's a water gun forum around, quite active, but I can't recall the name. Super Soaker Central rings a bell.
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Unread postAuthor: psisquared » Tue Feb 19, 2013 1:57 am

Thanks for the answer and the tips!
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Unread postAuthor: jackssmirkingrevenge » Tue Feb 19, 2013 2:30 am

You might want to consider other pressure sources that are more compact.

Take a 12 gram CO<sub>2</sub> cartridge, for which holders and adapters are widely available. This holes about 6.5 litres of carbon dioxide at atmospheric pressure. A 2 litre bottle filled with air to 70 psi (with no water) would hold around 9.3 litres of air at atmospheric pressure, that's more than the cartridge but not a huge difference, plus it would allow you to have both 2 litre bottles full of water for more "ammunition" capacity.
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Unread postAuthor: psisquared » Tue Feb 19, 2013 4:14 am

So quick question on that, if you don't mind:

I'm guessing that more pressure but less volume would mean I'd have a stronger jet for less time and less pressure but more volume would mean a lower pressure but for a longer duration.

Specific to my use, I guess it's about finding the right balance of pressure and time, but I was just wondering if that's right.

Sounds like the co2 would give me a stronger jet but for a shorter time and the 2 liter would be a weaker jet (but might still be okay for my use) for a longer time.

Just trying to making sure I have the logic of volume vs pressure right.

Edit (trying to think of an example):
A 70 psi 2 liter can push water out of another 2 liter longer and weaker than a 1 liter would at 140 psi (pretend land), but the 1 liter would propel the water much stronger but for a shorter time and, consequently, less water would be used (or would it all be used still, just faster?), right?

Another edit:
One last question that I can think of. In that link you posted:
10 inch chamber at 100 psi - 537 feet per second
5 inch chamber at 200 psi - 684 feet per second

What if the 10 inch chamber was at 200 psi as well? Would the feet per second remain at 684 feet and would there technically be 'more air' to 'last longer' (if you were streaming the air instead of releasing all of it at once)?

Thanks!
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Unread postAuthor: jackssmirkingrevenge » Tue Feb 19, 2013 4:50 am

psisquared wrote:A 70 psi 2 liter can push water out of another 2 liter longer and weaker than a 1 liter would at 140 psi (pretend land), but the 1 liter would propel the water much stronger but for a shorter time and, consequently, less water would be used (or would it all be used still, just faster?), right?


Both would push out all the water, because their expanded volume at atmospheric pressure is greater than the combined volume of air chamber and water chamber.

If for example you had a 5 litre water tank, once all the water had emptied, you would still have 20 psi in the system for the low pressure high volume tanks, and 23 psi for the high pressure low volume tank.

In short, for a given weight of air, having it at higher pressure and lower volume will always give you greater power, and assuming that its expanded volume is greater than the total volume of air and water tanks, it will always be enough to empty out all the water.

One last question that I can think of. In that link you posted:
10 inch chamber at 100 psi - 537 feet per second
5 inch chamber at 200 psi - 684 feet per second

What if the 10 inch chamber was at 200 psi as well? Would the feet per second remain the same and would there technically be 'more air' to 'last longer' if you were streaming the air instead of releasing all of it at once?


Note that this is referring to a projectile launcher, while what you have in mind is more of a hydraulic system. In the case of a launcher, more chamber volume means better sustained barrel pressures, so the projectile will be under a higher impulse and thus leave the barrel faster.
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Unread postAuthor: psisquared » Tue Feb 19, 2013 5:18 am

So I think I've got it. That was a pretty nice explanation and it got me thinking about what's really happening with the air (how it wants to neutralize itself, it seems).

Just to be sure, though:

This means the co2 canister would have to have enough air to fill two 2 liters in it and even more in order to increase the pressure (or rate at which it pushes the water out). It's surprising to me to realize that a co2 cartridge contains such a large volume of co2 in such a small cartridge.

Is there any way to use a bicycle pump to fill a co2 cartridge (as the canister seems sturdy) to a psi that is close (as I'm guessing oxygen is too dense to achieve the same psi in that canister)? I know you said there were adapters but I'm guessing you didn't mean of that sort, heh.

Or, rather, do you have a suggestion of a similar sized container with a similar ability to hold a high psi for this type of use?

I'm interested in making it as compact as possible, but I really want to stick with a bicycle pump (as opposed to co2) for a few reasons.

Edit:
Looks like I can use an HPA tank, maybe? I didn't know what HPA meant when I read it earlier from Zeus' post (tried to look it up but encountered the wrong thing apparently, heh).

I did a quick search on "bicycle pump hpa tank" and it looks like it might be possible. Would a smaller HPA tank work well with a bicycle pump? I'm looking at an 8 ci one but can't really tell.
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Unread postAuthor: Zeus » Tue Feb 19, 2013 5:27 am

JSR in 5 minutes wrote:
Why not have both :wink:

Image


Edit: That ended up a bit bigger than I expected
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Unread postAuthor: psisquared » Tue Feb 19, 2013 5:45 am

Well with a pump if I run out of compression I can just top it off without having to find a supplier to keep me going. Self sustaining I guess? lol.

But yeah, both wouldn't be so bad, heh.
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Unread postAuthor: jackssmirkingrevenge » Tue Feb 19, 2013 6:09 am

HPA with a hand pump is not really practical, unless you're willing to spend a couple hundred dollars on an airgun pump - or you feel you can build your own: http://www.spudfiles.com/forums/easy-st ... 13277.html

A small fire extinguisher would be a decent and relatively cheap air tank, and 50 bucks will get you a stirrup pump that can go to 260 psi: http://www.ebay.com/itm/Bicycle-Bike-Al ... 416746073d

Remember that upping the pressure means that all your parts will need to be stronger, 260 psi with water in a pop bottle just won't cut it - unless you have the firing valve between HPA bottle and water supply, as opposed to at the nozzle.
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Unread postAuthor: psisquared » Tue Feb 19, 2013 4:23 pm

Sweet. Thanks for the help guys, I hope to have a lot of fun and I hope you guys do too!
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