[b215826]

> You actually want to drop ballast as you descend, to make up for the added seawater.

The ballast is dropped during the ascension, not the descent. This makes the ascension-step impossible to fail and requires no electricity [1]. Incidentally, I first came to know about the bathyscaphe while reading Peter Watts's Rifters trilogy (which is an amazing hard SF series set in the deep sea).

[1] https://en.wikipedia.org/wiki/Bathyscaphe#Mode_of_operation

[Cerium]

Ballast is dropped during both descent and assent. The linked document explains how the gasoline used for buoyancy is significantly compressible and becomes denser as the bathyscaphe descends. If no ballast was dropped during descent the rate of descent would continue to increase due to the feedback mechanism of increased pressure, leading to increased compression, leading to increased density.

[a4isms]

FWIW you don’t need a bathyscape to experiment with this. When Scuba diving in temperate locations (like Tobermory, Ontario), people with 7mm wet suits and extra vests or jackets experience this as they descend: The neoprene compresses, and if you don’t add small amounts of buoyancy to your BCD, you’ll start to free-fall.

[elzbardico]

The document says otherwise. As gasoline is slightly compressible, seawater is admitted in the float during the dive, increasing negative buoyancy, and increasing the rate of descent. So, to control the rate of decent, some ballast is released during diving too.

[babypuncher]

Some steel shot ballast was jettisoned to slow the descent. They didn't want to crash into the floor and damage the pressure hull.