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[Jumper]

I have been looking at RPMTech's floating piston ballast tank as shown on http://www.rpmtech1blog.com/

Could someone explain how this works please.  Would the pump pull water to move the piston or would the pump push air to move the piston?

Either way it seems like a nice solution for losing the space used by the motors and threaded rod as in the usual piston methods.

Thinking it may be a viable option for converting Robbe XXI to static dive.

Jumper

[Subculture]

It uses a positive displacement geared water pump to pump water in and out, the air behind the piston is compressed into the main dive cylinder. You will need to pinch up the hose to prevent air pressure from pushing water out through the pump and ruining your trim. You can also use a peristaltic pump, which would eliminate the need for a pinch valve, as they self seal. 

I don't think it will save you much space to be honest, but it does simplify construction slightly, and if you use a large syringe for the piston tank, you can put this together with off the shelf parts.

[Jumper]

Thanks for that Sub,

I like the look of this system - but there must be some drawbacks somewhere - eg friction on the pistons, one pump or two, synchronisation.  Would you, or others, have any thoughts on this please?

Jumper

[Subculture]

I'm not sure how well the system with two tanks and one pump would work out- that may cause problems with trim. A single tank will exhibit the same issue as any other single piston tank- a shifting centre of gravity.

In practice this doesn't seem to affect operation of subs that much. I've observerved boats with a single piston tank and ones with twin tanks, and they both work well.

No system has all the answers, you just pick one you like best and go with it.

[Davy1]

Not a lot of improvement over a bag, in my opinion.

Also quite difficult to make it proportional, unlike a standard piston tank.

David