Avelo--I guess there's no need for me to recommend fundies anymore....

[lowwall]

There has to be at least a couple of liters of empty ballast space outside the bladder when it's at full rated pressure. Otherwise, not only would you need a pump that can force a couple of liters of fluid against a force increasing from 300 up to 360 bar (5200 psi) in a few seconds, but you would be subjecting the bladder to that pressure on nearly every dive.

None of the extremely high strength fibers that you could make the bladder out of stretch appreciably, so you could build a 12 liter bag in a 15 liter shell and use the extra space for holding either gas or air at ambient pressure.

Now that we've designed it, what do we think of the concept?

 

[NothingClever]

Now that we've designed it, what do we think of the concept?

Too fiddly.

This thing is designed for the kind of people who avoid Bonaire because the trucks have standard transmissions.

 

[BRT]

I'm trying to visualize this. We've got an outer tank wall inside of which is a big bag containing the breathing gas. Between the bag and the wall (or maybe just between the bag and the bottom of the tank) is a ballast space that is initially filled with gas at the same pressure as the bag?

I really don't see how that would work. What happens when you want to descend? You are going to have to replace what could be several liters of gas compressed to 200 to 300 bar in that ballast space with a fluid. If you dump the gas overboard, it will have the same effect as purging off a quarter of an AL80. You obviously can't boost it back into the breathing bag. And can you imagine forcing dirty lake water or seawater at that pressure through a pump? I guess they could pack a couple of liters of hydraulic fluid in the pump housing, but maintenance would not be trivial.

And what if you pick up a kilo of doubloons or your buddy's dropped weight pouch and try to ascend? Are you really going to have to be willing to give up 10% of your remaining breathing gas (1 liter of gas at tank pressure) to regain neutral buoyancy?

All this leads me to think the ballast area must be at ambient pressure. Or rather, ambient at the depth it last had gas added to it.

Anyway, however it works, I'm starting to warm up to the idea of this as a rental for tropical diving. I like a minimalist setup and this is pretty much just a backplate and harness and tank. No need to pack the BP/W and you can leave your inflator hose behind as well. Not sure how a dive shop could afford a bunch of these things, but I guess the company would lease them rather than force a shop to pay upfront.

I'm not so sure it would work for local diving. How many people own just one tank?

You don't add or remove gas. You just compress it to higher pressure by making it's volume smaller or take it to a lower pressure by making it's volume larger. This is just a well pump pressure tank running at a higher pressure. Essentially a hydraulic accumulator. The outside fiber tank has to be capable of higher pressures than the initial fill pressure. And the regulator has to be capable of higher pressures than the initial fill pressure.

Don't think that I think this is going to be a big seller. It won't be.

 

[Gone for diving]

Its only filled to 3000psi, the rubber bladder is against the inside of the tank, it's the most buoyant it will ever be,
Now you want to sink, to do that, start pumping in water into the tank at the bottom,
Doing that, it compresses the air to a higher pressure, like 4350 psi, and reduces the air bubble, which lowers buoyancy,
Because the tank is rigid the buoyancy remains unaffected, ( as you breath the air bladder will change slightly, and you will need to open a valve and let some water out, to increase bubble size,)
To increase buoyancy, let water out of the tank and the bladder will go back to its original size,,, (the inside of the tank,) that is assuming the is still air in the bladder.

Its actually a very simple system,,, but what kind of tank are they going to use so the tank will not rust???

Probably carbon fiber or glass lined tank????

 

[Etmutt]

So if I start at 300bars and reduce 2.5 kg boyancy by adding 2.5l of water, the pressure will be at 380 bars, I wonder how they tackle this issue

For the weight part I'm not too convinced though : you need an over built tank for more volumes and more pressure

 

[archman]

How much lighter is this thing?

 

[Angelo Farina]

I suspect the animation was trying to convey the idea of water entering the outer shell rather than being an accurate rendition of what is occurring within the system.

It's hard to imagine a membrane that can live in saltwater for years, be flexible enough to accommodate several inches of movement and be capable of withstanding 300 bar. That's roughly 100 tons of pressure over an area the size of the base of a scuba tank. A thick enough sheet of Dyneema cloth could theoretically handle the pressure, but how would you go about bonding it to the stub tank (or tank valve)?

There is no pressure difference on the two sides of the membrane if some water is inside the shell...

 

[vondo]

If I understand what people are saying about this, forget about the fact that it looks like one tank. imagine it as two separate "tanks".

Tank 1 is a high pressure, 300 bar carbon fiber tank which holds 106 c.f. of air. You can buy this today; people don't use them because they aren't very practical. But aside from the pressure, size, and material this is just a standard scuba tank.

"Tank" 2 is a ballast tank. A rigid volume which can be filled with water (neutral) or air (positive) buoyancy with just enough air in a bladder to fill it at a few atmospheres (pressure inside the ballast tank can be above or below ambient pressure).

Then you need enough weight to make the whole thing negative. Seems to me the ballast tank bit could be done with a membrane and PVC piping and a hydraulic pump if you needed.

 

[RobPNW]

I could break it...

 

[dlofting]

So why would I want something that costs money, introduces another failure point (or at least one) and requires that I change the way I dive when it took me over 200 dives to get my buoyancy in reasonable shape with the gear I use now.