Hi,

with ballast tanks there is no problem - they should be only in two states: completely empty or completely flooded.
Having them partially flooded (that means with air in the top part and water in the bottom part of the tank) was
not intended and adverse - as the depth changed, the external pressure also changed, influencing the amount of
water in the tanks (the deeper, the more water in the tank, and the air in the top part of the tank more compressed)
and in turn changing the U-Boat buoyancy.

However, sometimes were situations, when the partial blowing of the ballast tank was needed (when - due to the leaks
into the pressure hull - the boat became to heavy, one could lighten the boat by blowing the tank. Partially - because
no one wanted surface prematurely). This case is covered in paragraph 187 of the diving manual: [uboatarchive.net]

However, this trick could be used only on type VII U-Boats, and only with the main ballast tank no 3 (under the control
room). Only this tank could be vented inside the pressure hull, thus preventing the escape of the air up to the surface.
All other ballast tanks were vented directly to the sea, so the air could rise to the surface and betray the position of
the boat.

There are other tanks, which can be partially flooded.
The regulation tanks are fitted with tubular level gauges. See these pictures:
[www.ubootwaffe.pl]
(two vertical pipes behind the metal cover with holes, note the lineal with scales)
[uboatarchive.net]
(two vertical pipes visible near the left edge of the photo)

The fuel tanks are fitted with dedicated sounding installations.
The description you can find here:
[models.rokket.biz]
[models.rokket.biz]

The water level in trim tanks, lubricating oil tanks, torpedo compensating tanks and bilges were determined by means
of the manual sounding with something similar to the dipstick, which was inserted by openings in the tank tops.

--
Regards
Maciek


[www.tvre.org]