Maximum PSI for layered LRT

Threads about how water guns work and other miscellaneous water gun technology threads.
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Leonard
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Maximum PSI for layered LRT

Post by Leonard » Thu Mar 13, 2008 3:58 pm

Hey guys!
I'v just ordered the LRT i will use for my compressed air powered CPS. I'll use 3/8 to 3/4 to 1'', so three layers, McMaster Carr shows max PSI of about 30 PSI, is it the same for layered LRT? If my total wall is 9/16 '', how will be the max PSI the CPS chamber can take?

Thanks a bunch for your answer.

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SSCBen
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Re: Maximum PSI for layered LRT

Post by SSCBen » Thu Mar 13, 2008 4:20 pm

Welcome to SSC!

You seem confused about how latex tubing works, so I'll explain. The pressure figures reported are what the tube will pressurize at. You can not put more pressure than that in--the air will expand out until it meets the pressure figure. The fluid can put in at a pressure lower than the figures reported. In CPS water guns (rubber CPS, that is), there is no air pressure. So if you're thinking you have to fill these tubes with air to make constant pressure, you're mistaken. You just pump water in and it's pressurized to the pressure reported on the tube.

We explained a little bit about how this works here: http://www.sscentral.org/physics/works.html . More of an explanation obviously would be helpful. I might be able to write/make something over spring break for that.

The pressure the chamber makes depends both on wall and the inner diameter. Could you post the part numbers for the tubes you used? Then some reasonable estimate could be made.

With that being said, we don't have any equation for pressure from wall thickness and inner diameter. I've been trying to figure out one, but it's more difficult than it sounds. You also have to factor in the space between the tubes which seems to make a big difference in pressure, as well as if you will make the outer tubes expand lengthwise, which also will make a big difference. The only sure way to know is to buy the tubes and test it out.

Some sort of article on this would be helpful. Post your results and it'll help. ;)

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Silence
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Re: Maximum PSI for layered LRT

Post by Silence » Thu Mar 13, 2008 9:14 pm

Welcome to the forums, Leonard! :)

Ben's right - the rubber itself stretches and exerts force, creating pressure. The pressure increases only slightly as the rubber fills to accommodate the water you pump in - the marked PSI is the maximum PSI, reached when the bladder is full. As a rule of thumb, the max PSI of layered LRT is equal to the sum of the pressures from each layer.

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SSCBen
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Re: Maximum PSI for layered LRT

Post by SSCBen » Thu Mar 13, 2008 9:40 pm

The pressure rating listed is for full expansion only (like Hooke's law). If you layer tubes, the outer tubes are not expanding fully, so you do not get the full benefit. I'd say it's hard to estimate the pressure because of that. ;)

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Silence
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Re: Maximum PSI for layered LRT

Post by Silence » Thu Mar 13, 2008 11:02 pm

Well, it's obviously not precise. But if you assume the pressure is constant as the bladder expands (and it isn't), you could also assume that the summed pressure is constant and easily calculated (and it isn't).

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SSCBen
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Re: Maximum PSI for layered LRT

Post by SSCBen » Fri Mar 14, 2008 12:09 am

It might be okay to assume that pressure is constant as the bladder expands, but I don't know much about that so I can't really say. The force applied does increase as the area increases, so it should be relatively constant nonetheless. I was mainly saying that the pressure will be marginally lower. In my experience the pressures from layered tubes don't add up as you might think.

After some thought, you have to consider that the wall is increasing, but one layer is expanded more than the other. The internal diameter is the same for both tubes, so the larger tube's pressure figure is low if you think about it like this. Smaller diameter, same wall as a larger tube will have higher pressures.

It's obviously not quite as simple as adding the two pressures, but I can't say for sure that adding the two pressures is completely inaccurate.

Perhaps I should find someone who could be able to solve a problem like this... I don't know if it's a simple as using the tensile strength in a cylindrical vector (somehow--I don't know precisely how to do this because the cross sectional area is staying the same as both diameters increase and I don't know how the tensile strength fits into that) on a surface integral divided by the area of the cylinder's outside. That might seem complicated to some, but it's as simple as I could think of it. I suppose that's how this problem would be solved.
Last edited by SSCBen on Fri Mar 14, 2008 2:09 am, edited 1 time in total.

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Silence
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Re: Maximum PSI for layered LRT

Post by Silence » Fri Mar 14, 2008 12:32 am

First, we need to find an equation for the pressure of a single chamber. The pressure clearly does increase as you fill the bladder: range drops a bit as you shoot. With the right equation there, you could measure the rate at which the inner bladder increases pressure compared to the rate of the outer bladder.

Leonard
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Re: Maximum PSI for layered LRT

Post by Leonard » Fri Mar 14, 2008 3:01 am

Oh. I realy tought you would remember the post i made a few days ago :
http://forums.sscentral.org/t4813/

If you take a look at the diagram i uploaded in my previous post, i will be a little more clear. Sorry for being vague ^-^

Basicly, the lower container is filled with water and then pressurized with compressed air (source of the air being : Compressor or small manual air pump attached on lower container). The compressed air pushs the water down into the green tube, water travels in the tube to go into the CPS LRT chamber! So i get water pressure without building a water pump.

My question was : I am pressurizing water in the LRT tubing to a certain ammount of PSI, am i right? Or does water pressure is named something else? Aniway, i still have to choose an ammount of AIR pressure to pressurize the lower container, wich will equals the force of pressure of the pressuized water sent in the LRT tubing. So that's the number im looking for : How many psi can i shoot into the lower container without bursting the LRT by over-pressurizing water in it?

By the way, you guys are real proffesionals, im glad i found this site.

Thanks a bunch.

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SSCBen
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Re: Maximum PSI for layered LRT

Post by SSCBen » Fri Mar 14, 2008 3:37 am

I thought you might have decided to go with something else. ;)

The LRT does not just accept things pressurized and then keeps them at that pressure. The extra pressure beyond the LRT's rating is expanded out so the pressure drops. You can't "overpressurize" LRT. The pressure will expand out and it will pop because there is too much volume. You can "overload" LRT.

For example, if I pump air in to the LRT at 100 PSI and the tube is rated at 35 PSI, the air will expand in volume to fill out the chamber. This follow's Boyle's Law that basically says the total amount of a gas is the the pressure times the volume. If I have 1 liter of air at 100 PSI that is pumped into LRT rated at 35 PSI, the air will take up 2.3 liters in that tube (given that all of the air was put into the tube, just to give you an idea of the scale). Again, the extra pressure is reduced by expanding the volume until it matches the LRT's pressure.

If you pump water in at higher pressure, it will expand out too, but the expansion is unnoticeable (which is why Boyle's law doesn't apply to fluids). The pressure differential will cause a flow. If you use a hand pump then you won't have a problem because water won't keep pumping in. But, I'll explain how the flow might be a problem in your design.

That design will be delicate now that I think about it. The air pressure will need to be higher than the LRT pressure to get flow into the LRT chamber. If the pressure drops, you might not have a problem if it drops below the LRT's pressure. But if you want more than one shot, the pressure shouldn't drop that low. So the LRT chamber will burst unless you add a valve to stop the flow manually. I'm assuming that's what #7 is. You will have to be very careful to close the valve before the chamber pops.

I'd like to reiterate what some others said in that thread. That design is too complicated. If you want air powered CPS, use a regulator. A simple air regulator will be more constant than rubber tubing, cheaper, simpler to build, require no replacement down the road (LRT thins with use and needs to be replaced), and be more reliable. :)

Let us know if my explanations aren't adequate enough. I reread them and I didn't think they were perfectly clear, so I edited them.
Last edited by SSCBen on Fri Mar 14, 2008 3:50 am, edited 1 time in total.

Leonard
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Re: Maximum PSI for layered LRT

Post by Leonard » Fri Mar 14, 2008 3:55 am

Thanks for those explanations. I got your point, and it makes perfect sense. But i doesn't realy matter now, because Mc Master does not ship in Canada... (Yup, im a french canadian, wich explains my poor english)
I'll probably go with the pressure regulator. But the point of my design was to repressurize the LRT chamber during a fight without having to pump. I would have only needed to open up the ball valve until the LRT chamber is pressurized up. With the lower container pressurized to 100 psi, i could have filled up the LRT chamber a few times.

Oh well.

Question for you Ben, what was the PSI used in the supercannon II? The point of the cannon was to get the most linear flow, is it?

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SSCBen
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Re: Maximum PSI for layered LRT

Post by SSCBen » Fri Mar 14, 2008 4:41 am

Your English is better than some "native" speakers. Don't worry. I would have never known. ;)

The pressure I reported on the article was 100 PSI. But, I could get about 65 feet of range (which is about 90% of the full range) at 60 PSI, so don't think that 100 PSI is necessary for very good performance. ;)

The point of the water gun was to make a very efficient design, yes, so the linear flow was necessary. I've found linear flow is not completely important given that you have high flow, but it obviously does help to a certain extent.

If you're set on your design, give it a try. There are probably more local LRT suppliers or ones that ship worldwide. Try some Google searches. LRT is fun stuff to mess around with.

Oh, and by the way, if you want to buy a regulator and can't get one from McMaster-Carr, look at home improvement stores like Home Depot or Lowes. They should have regulators very similar to the one I suggested in your other thread.

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