Don.
I guess a graphic could explain my ideas better that the poor wording, see below. I think we are in agreement with your first paragraph.
The weight of the submarine prior to the dive is the same at 50m or at 200m. I agree if you dont`t pump out any liquid or discard any objects from the submarine.
However the volume, displacement may be less because of hullcompression and some water may be pumped out of the regulating tanks to compensate. The water pumped out of the regulating tanks has no influence upon the volume/displacement but on the weight of the submarine, however if you blow out some water from the buoyancytanks it shall increase the submarines volume and thus the buoyancy of the submarine.
I could see the Uboat taking in 1.330 liters at a depth of 50 meters to compensate.
I assume you are referring to buoyancy tanks not vented. I guess you are missing a 0 here as at 50 m you have a total loss of buoyancy of 12,22m3- 1,33m3= 10,89m3 pluss the pressurehull compression 0,5m3 alltogether 11,39m3 water equal to 11.390 liters, a considerable amount of water to be pumped out against 50m.
At 200m the total regulating water which is required to be pumped out of the two regulating tanks would be some 15.200 liters to compensate for the aircompression in the buoyancy tanks if not vented. In case they are vented, nil. This is one of the main reason that the buoyancytanks should be vented prior to diving
The ballast tanks are normally not used for ballancing and trimcontrol. We use the regulatingtanks and the trimtanks as they are easier to control for such purposes. Again, a partly filled ballastank/ buoyancy tank create a fairly large free surface effect momentum. The regulating tanks are centrally placed and small length which minimize these unfavorable elements.
On the surface the buoyancy tanks have the vents shut, this is increasing the bow and stern buoyancy when the submarine is pitching, sometimes immersing the buoyancy tanks, hence together with the flared bow reducing the pitching which is the purpose for the tanks. At calm sea they have no function.
Re Graphic below.
If you don`t vent the buoyancy tanks submerged, the light blue area e.g. displacement becomes smaller as the seawater intrusion (dark blue) get larger( representing loss of buoyancy). A VIIC having a displacement variation between 6 and 11m3 not compensated would get trouble with the dynamic control of the submarine. Thus you have to compensate this decrease in buoyancy force by reducing the submarines weight, this is normally done by discharging regulatingwater by less extreme volumes..
On Kaura we kept normally the buoyancy tankvents shut at the surface and open as a routine in the divingpreparation.
In the event of a crash dive all the tankvents were opened including the Qs starting with the bow tanks.
Both Q tanks corresponds to a buoyancy of 4m3. If the buoyancy tankvents were shut they would represent a buoyancy force of 12,22 -4= 8,22m3 displacement, a hopeless figure for crashdiving and destroying the normal exellent short divingtime of 30 seconds for a VIIC.
the buoyancy tanks were not only vented in emergency cases, but always when diving .
Tore