Update 19.03.2016: So far 8 contributions generating 4839 visits. Thanks to all previous and coming participants. Feel free to join the development.
In terms of marketing the portholes provide probably the most important component of the system. They are fundamental reason for potential visitors to reserve a short or long stay in the system, tool for observing the outside world (during stays for educational purposes) and attraction to a look from the outside into the inside (tourism marketing).
At the same time they are one of the weak points of the system and small irregularities between the seals could lead to the collapse of the structure. Therefore, the thickness of the glass should be designed according to highest possible load.
BOOT fair 2007
On the BOOT fair 2007, we also had the pleasure to talk to Arnd Schöttler, then director of the diving sports department of BOOT Dusseldorf. We were especially interested in the large windows of the diving tower, a pool, in which for example underwater rugby was demonstrated, while visitors of the fair watch from outside. He told us the glass panels were 8 cm thick and would each weigh 300kg. At first it might seem very heavy, on the other hand, these panels were not curved but flat, which increases the thickness and therefore the weight. The panels in the diving tower were apparently made by the company Röhm in Darmstadt several years ago.
At an ambient pressure system the porthole would be pushed outwards by the emerging air. In this case the window holding sealing ring should be fastened from the inside with the station, so that the curvature of the disk protrudes through the opening to the outside (see sketch for porthole change).
Actually it would be staticly ideal if the buckle would extend in this case inwards. But then the outlook would be that much limited that it would be preferable not to have portholes at all.
In case of a surface pressure system (the outside pressure is then substantially higher than in the interior kept at 1 bar) the portholes are pressed from the outside inwards. The holding sealing ring should ideally be screwed from outside the station.
Replacement of a porthole
For the possible replacement of a porthole this proposal was made: “The porthole might be inserted from the inside into the slot and bolted to an outer ring. Should there be a problem with the seals or the porthole itself a back-up dome (repair attachment) would be placed from the outside. The water between the porthole and fuse dome would be pumped off (valve under the window-bay). After the porthole is repaired it would be replaced. The gap is flooded again and the backup dome removed.” In principle, this approach is likely to be able to operate. In a 1-bar system the attachment would have to be only placed from the inside (since the porthole is likely screwed from the outside), so that the porthole is replaced from the outside.
Another contribution suggested the idea of a safety dome in a 1bar system: “A security dome of metal could be permanently installed above the window on the outside. In case of a porthole burst an alarm system could trigger the closing of the dome over the window slot. It would then be pressed onto the slot by the inflowing water. The lower interior pressure would then block the inflow and seal the hole thus. This assumes of course that the respective seals at the edge of the dome are constantly cleaned. “