RE: New ballast system
I don't mean to criticise your work just pointing out things to consider..
You will find that after relativly short use the gear pump will let more and more water threw especialy since it gets dry in some part of the cycle ,these pumps don't like to run dry at all and the valve efect will be lost,I found that out and had to install a solinoid valve for larger subs and cintered brass filter for smaller ones as even just 2 inches bellow the surface the pump would leak water past.The paristolic pump is the best for bag and tank systems.
I also found that the pump gets warm and even hot so I made my tank hold the watertight pump which was sealed for that use,the one you use can also be watertight with just GOOP and that way it stays cool ,run it in the wet or in the tank if a bag is not used.
Howe ever there is the amount of ballasted water used to achieve the desired water line,as you said your sub would be almost nuetraly ballasted,that is great for some subs but most in general require a higher free board area for more scale like operation,that is the problem with most bag systems.
In tanked systems that is not the case as the intire tank is used as is in the full scale subs.Fully adjustable to a full normal range of boyancy that is also why bag systems are not used on full scale subs ,as far as I know.Also at depth the bag will get fuller from external water pressure and preciese depth control will be dificult and cost more in power as constant pumping will be needed.
At depth out at sea I would not want that bag to fail no mater what its maid from ,too risky.
In my opinion its saffer to make a tank and to any size needed by the way than a few choices of bags.
I had experimented with variuse bag systemsyears ago and only had bad results from internal leaks to CG shifting something I did metion before,with a tank and baffles the water sloshing is kept undercontrol in larger subs,though both do have a CG shift at least it can be morecontroled with a tank system.
The tank and pump system has ben around for a very long time and has been tried and true to prove that it works as it does to this day on the full scale machines.
It is great to see otherCaptains building and experimenting with there subs as it should be but I thoaught itwould be benificial to render some experiance in this area as I have alot of and have made my own very true to life ballast systems even to this day and still always looking to improve them.