4 ms·

So, why not set the intermediate pressure (fixed) at the average of the external and internal (typical) pressures? The load on each pane is lower so I would th

So, why not set the intermediate pressure (fixed) at the average of the external and internal (typical) pressures? The load on each pane is lower so I would think they'd be less likely to fail. A few hypothesis: - If the main failure mode is a defect that doesn't depend too much on pressure, then we have increased likelihood of failure; - The manufacuring cost of pressurizing the midpane is high; - Having a varying load direction on the external pane (inward on ground and outward in sky) is not good. Any opinions?

oh_sigh 11y ago
It seems like you are asking why a more complex solution with no benefits and some drawbacks isn't being used. Is that fair?
alok-g 11y ago
In fact, one could package the three panes in a normal (or humidity-free) air without the hole. Assuming the cabin pressure is close to ground air pressure, the pressure difference will still be on the outermost pane as in the OP, and if the outermost pane fails, it will come automatically on the middle pane as in the OP. In other words, this reason cited by the OP does not explain at all why a hole is there. Am I missing something here? :-) The other reason cited by OP, fog and frost, would still be valid if it is not desirable to package the panes in a humidity-free environment.
- lukeschlather 11y ago
  Without the hole, the space between the two panes will be pressurized. I'm not sure what the exact figures are (I'm not a phycisist) but I would guess that without the hole the outer pane will be subjected to some force F due to the presure, and the inner pane will be subjected to some force F/2, or even equal. Without the hole the system would probably be better modeled as a single pane with an airgap between its inner and outer edges. It's likely that both panes would fail at the same time.
  - darkmighty 11y ago
    You don't need to be a physicist: if the panel is sealed there will be a constant pressure inside. The pressure on each face is then P_ext-P_int. It's just as likely that both panels will fail at the same time in that case as in the case with a hole; but the bonus in this case is that you can set up the internal pressure so both panels have lower nominal pressure (but the internal at least should be able to still handle the full pressure difference). I know it's routine to manufacture even household glass panels with different gases sealed in their interior (I think they put Argon inside? some low conductivity gas), but afaik it's at atmospheric pressure.

oh_sigh 11y ago

It seems like you are asking why a more complex solution with no benefits and some drawbacks isn't being used. Is that fair?

alok-g 11y ago

In fact, one could package the three panes in a normal (or humidity-free) air without the hole. Assuming the cabin pressure is close to ground air pressure, the pressure difference will still be on the outermost pane as in the OP, and if the outermost pane fails, it will come automatically on the middle pane as in the OP. In other words, this reason cited by the OP does not explain at all why a hole is there. Am I missing something here? :-) The other reason cited by OP, fog and frost, would still be valid if it is not desirable to package the panes in a humidity-free environment.

lukeschlather 11y ago
Without the hole, the space between the two panes will be pressurized. I'm not sure what the exact figures are (I'm not a phycisist) but I would guess that without the hole the outer pane will be subjected to some force F due to the presure, and the inner pane will be subjected to some force F/2, or even equal. Without the hole the system would probably be better modeled as a single pane with an airgap between its inner and outer edges. It's likely that both panes would fail at the same time.
- darkmighty 11y ago
  You don't need to be a physicist: if the panel is sealed there will be a constant pressure inside. The pressure on each face is then P_ext-P_int. It's just as likely that both panels will fail at the same time in that case as in the case with a hole; but the bonus in this case is that you can set up the internal pressure so both panels have lower nominal pressure (but the internal at least should be able to still handle the full pressure difference). I know it's routine to manufacture even household glass panels with different gases sealed in their interior (I think they put Argon inside? some low conductivity gas), but afaik it's at atmospheric pressure.