bladder systems

Hi folks, 'm starting to think about the bladder now I can see the end of the WTC with its peristaltic pump so and thinking about the bladder.
A lot seem to use a cut down cuff from a blood pressure meter, but since I new a longer thiner bladder I think to fit I was thinking about a length of bicycle inner tube. In theory the pressure differential should be zero apart from possible over inflation at the surface as far as I can see.
I don't want it too long or I can see the same stability problems as with a long thin water ballast tank, but removing the valve core and with and adapter it should connect quite nicely with the air pump.
Originally I intended to have a seperate air reciever but for simplicity I might just use the WTC which as it is large vs the bladder the change in pressure should not be too great. I'm a bit bothered that the silicon tube type control rod seals might act as a valve venting air out or that excess pressure might pop out the shaft seal (though it will have a keeper fitted) but I've not seen that mentioned as problems anywhere.
In theory as the WTC pressure increases the external pressure should also increase as the boat dives, but I doubt they'll keep in step!
Has anyone tried an inner tube as a bladder?

It has been done. The pressure you build up is minimal, however, make sure your endocarps are secured somehow. Those can pop off.

Bike inner tubes have been used in the past (but not by me I have only used medical type bags.)

The problem was, I think, sealing the cut ends.

I happened to see that Norbert Bruggen offers a bag (which I think you can cut and glue). It is charmingly called a Gummisack in German! His product may give you some ideas.

I think I saw elsewhere ? that you were thinking about using a float and fishing line distress indicator. I am feeling my age because I remember thinking, several decades ago, that was not a very good  idea (float buoyancy, friction, water depth, angle of wreck) hence the various types of Pinger.

However I seem to be the only person using these and most people in model subs seem to only sail in clear, shallow water! Which seems a shame because it is so rare (in the muddy UK anyway.)

Lovely work on your "WW2 mini sub" by the way

David

being an owner of one of norbert's delta submersibles, i am quite familiar with his pump/bladder systems. the bladder is made out of a sheet vinyl that is cut and folded, with a plastic tube (think fish tank filter/pump tubing) glued together with the black rubbery CA glue. I have found functionally identical vinyl sheet on McMaster/Carr's website.