Hi all,
First post here and I have to say I'm hugely impressed with the level of knowledge on the forum.

I've recently been reading Herbert A. Werner's autobiography of his time on u-boats, Iron Coffins. And there's one thing he says, but doesn't really explain, so I wondered if anyone here could. At one point, they were being depth-charged by several destroyers and couldn't surface, but were also running out of oxygen. They were running out of air when:

"In the afternoon, we faced the fact that we had absolutely no more leeway, no more air to breathe. We now had to choose between suicide and surrender. In a last effort to steal another hour from death or imprisonment, Friedrich released some compressed air into the midship buoyancy tank to raise the boat. The hissing sound attracted our assailants' attention. A ferocious blast pulled the boat upward. As the air in her tanks expanded, she rose with increasing speed. But then a battery of canisters detonated, violently slamming into her starboard side, sending her down again for the final crush."

My question is, does anyone know what the compressed air release was supposed to achieve? I assume the buoyancy tank was, by its nature, a sealed entity. Releasing air into it wouldn't provide more air to breathe, would it?

Many thanks