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Powerful Ammo Can Seals


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I pulled one of my caches last summer because it had been plundered. I recovered the ammo can and put it in my car. The temps were close to 90.

 

Advance to last weekend. I hosted a breakfast event and needed a container to hold the 26 travel bugs that were showing up. Hey, why not use the ammo can that's been sitting in my car all this time? The overnight temps were about -5.

 

The suction from the cold air in the ammo can required two people to break the seal on the can! :D

 

I'm keeping the can inside the house now. I'm afraid of what cold air sealed in an ammo can would do once the temperature goes back up...

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:) Use to work at NAPA Autoparts. They have threaded tire valves. You simply drill a hole in ammo can and insert vale and secure it with the nut on the inside of the container. The valve comes with gaskets so there would be no need for any sealants.

 

:blink: Wonder if the seal were tight enough, could you pump the cache full of Helium.. The cache hint might say, lighter than air..

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i'd sooner think it was an expansion/contraction problem rather than a vacuum/pressure thing. Regular ammo cans don't hold that much air, but the lids can fit pretty snug. IMHO

 

Nah, that sucking sound you hear when you open it tells me it's air pressure. Now frozen ones are a different story. Took me 10 minutes to get one open a few weeks ago.

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:blink: Served 8 years in the Army and never heard of an ammo can exploding. Think about it. If the can is full of ammo, there is not much room for air. Now I would avoid putting in chemicals that react with water, like the heat packs that you sometimes got with MRE's. We use to have a little fun blowing up water bottles with them. Thought about giving out the recipe for this home made firecracker but it might pose too much trouble... Edited by Hogarth
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Sure it was a vacuum problem and not just a sticky rubber seal? Some ammo cans seem to get a bit tough to open when they've been sitting for a long time, even when they are full of ammo in my closet. No major temp changes. Might try a thin coat of Vaseline on the gasket to keep it from happening again, at least for a while.

Edited by Mopar
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Sure it was a vacuum problem and not just a sticky rubber seal? Some ammo cans seem to get a bit tough to open when they've been sitting for a long time, even when they are full of ammo in my closet. No major temp changes. Might try a thin coat of Vaseline on the gasket to keep it from happening again, at least for a while.

Might have been, but so sticky that it took two people to pull it open?

 

Let's break out the physics... (Hopefully I remember this right...)

 

Remember the PVT formula?

P1 * T1     P2 * T2
-------  =  -------
   V1          V2

The box is about 30 cm x 15 cm x 25 cm or 11.25 litres. This essentially stays constant, so we can plug this in for both V1 and V2.

 

The starting temperature was about 90°F which equals 303°K That's T1.

The ending temperature was about -5°F or 252°K. That's T2.

 

The can was initially sealed at atmospheric pressure. Since I'm at about 700 feet above sea level, air pressure was probably less that 1 Atm, but the conversion factor would cancel out on both sides of the equation, so we'll leave P1 at 1 Atm.

 

Solving for P2 then gives you an internal pressure of the ammo can of 0.83 Atm. With 0 Atm being a perfect vacuum and 1 Atm being the pressure at sea level, this would be the equivilent of almost a fifth of the air getting sucked out of the ammo can. You'd probably need to do some other experiments to test this, but this seems to be a significant amount of suction, enough that with a hard-to-break seal, could be the cause for the can being hard to open.

 

OK, my brain is tired now... :blink:

Edited by The Cheeseheads
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Might have been, but so sticky that it took two people to pull it open?

 

Let's break out the physics... (Hopefully I remember this right...)

....

....

snip

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Solving for P2 then gives you an internal pressure of the ammo can of 0.83 Atm. With 0 Atm being a perfect vacuum and 1 Atm being the pressure at sea level, this would be the equivilent of almost a fifth of the air getting sucked out of the ammo can. You'd probably need to do some other experiments to test this, but this seems to be a significant amount of suction, enough that with a hard-to-break seal, could be the cause for the can being hard to open.

 

OK, my brain is tired now... :ph34r:

I think the formula is right. But, you need to finish it. The top of the can is about 60 sq. inches, with a pressure difference of about 2.5 PSI. That gives you a force of 150 pounds, if you had to lift it straight up. Figure yeah, it would have been tough to open.

 

Things you do when bored on a business trip.....It is too cold, dark and sleeting right now to go hunting. But tomorrow.....

 

Allan

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Let's break out the physics... (Hopefully I remember this right...)

I think you are looking for PV=nRT Where P is pressure, V is volume, n in the number of mols of gas, R is the universal gas constant and T is temperature in Kelvin.

 

n, R, and V are constant, so pressure is directly proportional to the change in temperature.

 

So The pressure is 1-(303/252) times lower, or 20% lower.

 

F=Delta(P)*area

 

F = (0.20*15)*72 = 216 lbs of force! :ph34r: That’s assuming you are pulling strait up. If you are prying it off you have about a 2:1 advantage.

 

That's what the brain does when you go back to school. :ph34r:

 

1 ATM = 15 psi

 

30x15cm lid = 12 x 6 inches = 72 square inches.

 

The starting temperature was about 90°F which equals 303°K That's T1.

The ending temperature was about -5°F or 252°K. That's T2.

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