How much overpressure could the gas bags support?
I still try to find it out, did not find it yet. The USS Macon for example had a maximum pressure altitude of 900 meters. But I know that the German Zeppelins reached higher without problems. One military Zeppelin had a "static altitude" of 2000 meters, but had to vent 1% of its hydrogen for every 80 meters climbed.
Looks like German Zeppelins had rather been using a maximum volume of the gas cells as reference, rather than the pressure inside the gas cell. Especially the pre-129 gas cells had been really fragile, they had been made from one special skin of a cows stomach, because it is very gas tight and light, and multiple pieces can easily be glued together to one large bag by using salt water.
That was used in the Akron class. Helium was very expensive in the 1930s and venting was out of the question.
The Los Angeles also used it, since she was also filled with Helium after arriving in the USA.
---------- Post added at 19:32 ---------- Previous post was at 18:50 ----------
:facepalm:
Of course, they did not mix hydrogen and air inside the gas cells. 95% hydrogen meant that the gas cell was inflated with just 95% of its maximum capacity with pure hydrogen.
Thus, not accounting for tension by gravity on the gas cells, the pressure inside the gas cell is equal to the outside pressure until reaching maximum volume. Once the pressure starts to increase significant, the gas cell is getting overinflated. So, practically, it might be as little as just a few Pa over outside pressure to start venting.