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Kory. I have always thought trim tanks were to adjust trim, not compensate
for varying loads.
Water density changes with temperature and salinity, but not very much. It
is possible to run into a salty current or water layer and trim can be added or
reduced to compensate.
Sometimes subs have forward and aft trim tanks to change the angle of the
sub while still maintaining neutral buoyancy.
Adding lead ballast before a dive, depending on load, is easier than having
a separate, large system with associated piping and tanks which
requires maintenance, cost, and monitoring.
With a few dives, you know how much added weight your sub needs to
reach neutral, and then you adjust with weights from there for added
passengers, cargo, new equipment, or whatever.
Trim tanks need to be at one atmosphere, whether full or empty.
Ballast tanks are ambient.
If your trim tank is half full and open to the sea, ( like a ballast tank
) the air will compress when you get deeper, and you'll start sinking
fast.
Large, one atmosphere tanks for trim will require heavy walled tanks, fill
valves, drain valves, pressure monitoring, and take up room and add weight
because they're thick steel.
And you have to flush them out after every dive so they don't
corrode.
Better to have small tanks for trim and carry some lead.
Just my opinion....Frank D.
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