Pressure compensator, fluid filled solutions


Hi guys!

I found this forum just now :slight_smile:

I’m a student of Chalmers university of technology in Gothenburg, Sweden, and right now I’m writing om my master thesis. The project I’m working on is a device that will operate in the sea, maybe 10 - 30 down, and right now the focus is pressure compensation. In my pre study I want to cover the basics of pressure compensation and describe different solutions.

I found a handbook that I have been using but it is a bit old. The title of the book is “Handbook of Fluid-Filled, Depth/Pressure-Compensating Systems for Deep Ocean Applications” written by Thomas H. Mehnert for the Deep Ocean Technology Program in Naval Ship Research and Development Center Annapolis, Laboratory and published in 1972. And it can be found here: and then press “PDF Url”.

From there I can find basic information regarding:

  • Hard shell concept
  • Encapsulation
  • Fluid compensation

I believe the fluid compensation concept is the best suited for the device and I’ve been digging into what type of solutions there is, this is what I have so far:

  • Flexible bladder type
  • Bellows or convoluted tube
  • Diaphragms
  • Spring-loaded piston

But the book is from 1972…so am I missing something here? Some new sort of smart compensation device or system? :slight_smile: Any information would be helpful! Sorry for my poor English.

Have a great day


Pressure compensation is a cool concept, it helps to save weight and cost of specialized connectors. Be careful, all of the items you mention have some kind of polymer in contact with a metal. You need to think about cathodic delamination. Usually happens over long term submersion, but can happen very quickly if you set things up wrong.

For the depths you are planning to work at, encapsulation in a silicon compound should work well.
drawbacks are if you need to service parts that are encapsulated it gets rather messy. Delamination is also a concern.

You can put a balloon or condom on and suck out the air (or mold release compound) before hand if you plan on servicing the item later. makes it easier to clean the potting compound off.

I have tested many microprocessors (operational) and other integrated circuits to 20,000 psi just thrown in a bottle filled with mineral oil. The main failures you will see are electrolytic caps. For those I punched a hole in the cap, submerged it in silicon oil then pulled a vacuum to suck in the oil. I then put a little finger tip from a glove over it with silicon oil inside to act as a bladder then threw the whole assembly in the mineral oil filled bottle. Worked well.

Pressure makes most caps shift so if they are controlling frequency and it is an issue then you need to adjust.

There are some patents for this that may give you some insight. One I worked on at my last company.

Have fun, hope this helps!



The system I’m looking at is a only a part of a larger device and holds a turbine shaft, generator, bearings, and seals. It needs to be fluid filled and pressure compensated. As of now the system is compensated with a “top hat” rolling diaphragm with a loaded spring, it works well but I’m looking for a smaller device to use or to use it in a more efficient way to save space.

Since it’s a marine application and the sea water can be pretty tough we’ll have to think about the corrosion as well so thank you for your advice on cathodic delamination!