I received a fish box yesterday, shipped overnight by air, containing two large polyethylene bags. The sender filled the airspace in each bag from an oxygen tank and sealed the bags with rubber bands, but upon arrival both bags were nearly flat; NO air space above the water.

Is it possible that the O2 is diffusing out through the plastic and causing the air space to collapse? But it seems like if O2 can diffuse out, then N2 (a slightly lower molecular weight than O2) should be able to diffuse in, and so there should still be air space when the gases reach equilibrium inside & out. Could certain kinds of polyethylene be O2-permeable but not N2-permeable? I thought somebody here might know what's going on here, or at least know who to ask.

I realize that low external pressure during flight would make gases diffuse (or leak) out of the bags faster, but the total collapse of the entire airspace in the bags has me mystified. Any ideas ??

The only other possibility we've come up with is that maybe the bag tops aren't twisted tight enough before the rubber band goes on, and so the O2 is leaking out through the loose twist. But still, I wouldnt expect total collapse.

Thanks!!
Gerald

If the bag was shipped in a depressurized portion of the plane, then the oxygen would have readily diffused out of the bag due to the pressure difference. There's something like one seventh the pressure at that altitude than there is at sea level (if I'm remembering the 'explosive decompression' episode of Mythbusters® correctly). That kind of pressure difference can force gas to diffuse across the membrane. Returning to sea level wouldn't cause the bag to fill up, just crumple flat. A bag is similar to a rope that way; you can only pull a rope, not push.

Here is a plastic bottle that was closed at 30,000 feet. http://en.wikipedia....ssurization.jpg It is crushed by the pressure of the atmosphere at sea level. That's the same force that drove the oxygen to diffuse out of the polyethylene bag.

They should've used thicker plastic.

Edited by EricaWieser, 20 October 2011 - 06:58 PM.

I received a fish box yesterday, shipped overnight by air, containing two large polyethylene bags. The sender filled the airspace in each bag from an oxygen tank and sealed the bags with rubber bands, but upon arrival both bags were nearly flat; NO air space above the water.

Is it possible that the O2 is diffusing out through the plastic and causing the air space to collapse? But it seems like if O2 can diffuse out, then N2 (a slightly lower molecular weight than O2) should be able to diffuse in, and so there should still be air space when the gases reach equilibrium inside & out. Could certain kinds of polyethylene be O2-permeable but not N2-permeable? I thought somebody here might know what's going on here, or at least know who to ask.

I realize that low external pressure during flight would make gases diffuse (or leak) out of the bags faster, but the total collapse of the entire airspace in the bags has me mystified. Any ideas ??

The only other possibility we've come up with is that maybe the bag tops aren't twisted tight enough before the rubber band goes on, and so the O2 is leaking out through the loose twist. But still, I wouldnt expect total collapse.

Thanks!!
Gerald

I recently received a fish shipment that had very little air left in the bag and some water leaking. The fish arrived in good health and the sender is a nanfa pro. My thought is that the bags are quite heavy and therefore do not seal well. I had not considered air pressure. Hmmm... I have also had several shipments of fish in breather bags which seemed to leak, in some cases quite a bit- though well sealed and again by pros. I wonder if the pressure would cause water to leak from the pores of the breather bags?

I wonder if the pressure would cause water to leak from the pores of the breather bags?

I've had water leak from breather bags a few times while using them for moving fish. When other bags sit on top of breather bags, they have enough weight to squeeze drops of water from them. I used to take breather bags every month to the OCA meeting and I learned to wrap a towel around them so I could dry them off when I got there. They were always just a little bit damp.

Also, I rephrased my post above in an e-mail and think it make sense worded this way, so I'm going to post it here. Here's a rewording of what I said in the previous post:
The pressure difference between the bag and the high altitude atmosphere was enough to drive the O2 out of the bag, but there was no corresponding pressure gradient to drive N2 back in because the bag membrane collapsed, touching the water and doubling up on itself, which increased its thickness. It's like a rope. You can only pull items with a rope. You can never use a rope to push things. Gas can only diffuse out of a bag, not inwards because the bag lacks the structural strength and just collapses.

There. I think that explanation is a little bit clearer.

Thanks everybody. A chemist on another forum (Carolina FishTalk) told me that polyethylene really IS more permeable to O2 than to N2, so even without a plane flight a bag filled with O2 would slowly collapse as it loses more O2 than it gains in N2. However, thousands of O2-filled poly fish bags get flown around the world every day with barely any detectable volume loss, so the diffusion rate must be pretty slow. So, either these particular polyethylene bags have unusually high O2 permeability, OR (more likely) the O2 is simply leaking out from an insufficiently tight seal at the rubber band or from a leaky bag seam.

Re Kordon Breathing bags, several years ago I got a bad batch of their small bags with weak side seams and they DID leak, sometimes just a little, sometimes a LOT. It was deceptive bcuz they didnt leak right away, even if squeezed gently after tying the knot. The side seam weakened and split a few hours after they were filled. Other than that, I have never seen any water seep through them, and I have taken them on planes. Of course if the water in the bag is cooler than the surrounding air, water vapor can condense on the bag.

It's said that if you ask two people in different fields the same question, they'll answer it differently based on where their specialty lies. It's true here; the chemist's mind immediately thought of the behavior of the two compounds as molecules, while the chemical engineer's mind immediately thought of fugacity. I'm kind of proud of myself for answering it as I did And both answers are correct; the nature of the molecules and the driving pressure both contributed to the emptying of the bag.

Edited by EricaWieser, 21 October 2011 - 03:07 PM.

Cool, fugacity. Now I have a new word to try out in Bananagrams. Despite my best biological jargon, my wife always whoops me with her knowledge of Medieval English and old Celtic words.

If the bag was shipped in a depressurized portion of the plane, then the oxygen would have readily diffused out of the bag due to the pressure difference. There's something like one seventh the pressure at that altitude than there is at sea level (if I'm remembering the 'explosive decompression' episode of Mythbusters® correctly). That kind of pressure difference can force gas to diffuse across the membrane. Returning to sea level wouldn't cause the bag to fill up, just crumple flat. A bag is similar to a rope that way; you can only pull a rope, not push.

Here is a plastic bottle that was closed at 30,000 feet. http://en.wikipedia....ssurization.jpg It is crushed by the pressure of the atmosphere at sea level. That's the same force that drove the oxygen to diffuse out of the polyethylene bag.

They should've used thicker plastic.i am a retired aircrewmember, modern airliners have the entire cargo and pax area pressurized. so unless they lost cabin pressure that did,nt happen, but if they filled the bag at sealevel the cabin would get to 8000 ft ,expanding the bag until the closure faied causing deflation, the bags need to be left partially filled to allow for expansion,