Review Diving the Avelo System

[lexvil]

Yes. The displaced weight of water, from the volume, vs. the actual weight of the object. If the object displaces more weight of water than it weighs, it floats. If it displaces less weight of water than it weighs, it sinks.

Thus a boat that weighs 10,000 pounds can float but a golf ball that weighs an oz sinks. A balloon full of water will sink due to the weight of the balloon itself.

 

[-JD-]

Thus a boat that weighs 10,000 pounds can float but a golf ball that weighs an oz sinks. A balloon full of water will sink due to the weight of the balloon itself.

Maybe. The baloon "skin", while thin, does have its own volume and buoyancy depends on the relative density vs. the liquid. I'm not into latex, so have limited first hand knowlwedge, but latex is 0.92 g\cm³ so a baloon would float maybe?

 

[Wibble]

Sorry; think something's very wrong here...

With the gas savings of not having to worry about inflating and deflating a BCD, along with my 270 Bar starting pressure to allow the first pump to make you neutrally bouyant (which raised the tank to around 290bar) Iv noticed my bottom times going from 45mins, and gaining almost another 30 mins.

The 10 liter Hydrotank is rated to 300 bar/4350 psi. This includes the pressure change from pumping. The example given above by @jakeluther is a starting pressure of 270 bar and an increase to 290 bar after the first pump. An initial fill pressure of 270 bar/3915 psi would ...

How can the pressure in the gas cylinder increase when you're pumping water into the low pressure (i.e. atmospheric pressure) solid outer container?

You start with 270 bar then the pressure will just go down from there. Unless it's really cold topsides and the water warms up the gas in the tank -- of course that won't be the case.

AIUI there's the internal 10 litre 300 bar steel gas cylinder wrapped with a larger 14 litre (or thereabouts) outer solid container into which water is pumped in and out. This leaves (I don't know the exact numbers) about 4 litre of variable air space. As the outer casing is sold and gas-tight, it is a constant volume BCD where the weight varies with the amount of water pumped into it.

Comparing; a normal BCD has variable displacement volume because it is subject to ambient pressure, thus volume varies according to depth.

 

[scubadada]

Sorry; think something's very wrong here...



How can the pressure in the gas cylinder increase when you're pumping water into the low pressure (i.e. atmospheric pressure) solid outer container?..

You may misunderstand how it works. Water is pumped into the rigid outer container. That reduces the volume of the internal bladder that contains the gas, thus raising the gas pressure (same amount of gas in a smaller volume).

 

[lexvil]

Maybe. The baloon "skin", while thin, does have its own volume and buoyancy depends on the relative density vs. the liquid. I'm not into latex, so have limited first hand knowlwedge, but latex is 0.92 g\cm³ so a baloon would float maybe?

I was trying to remember if bits of balloon would be floating in the pool after grand kids antics, I feel like I had to dive down to get some of them but don’t remember fosho

 

[Wibble]

You may misunderstand how it works. Water is pumped into the rigid outer container. That reduces the volume of the internal bladder that contains the gas, thus raising the gas pressure (same amount of gas in a smaller volume).

View attachment 842923 View attachment 842924

Thank you for pointing that out. The increase in pressure then makes sense.

I didn't realise that the outer cylinder is the pressure vessel and there's an internal bladder to separate the gas from the water.

This does mean that cylinder testing is way more complex.


Am surprised that they didn't opt for a lighter outer cylinder and run the balast off the gas; getting rid of electrical systems (weight, reliability, complexity, slow pumping).

 

[RobPNW]

Admittedly I haven’t read all 23 pages but can someone explain what happens when this thing fails, because everything fails. Does the diver swim up. I hope it doesn’t fail buoyant. Either way, I feel comfortable with my simple gear and affordable replacement options for now.

From my limited experience, a lot of learning has come down to “feel”. How my body works in the water, how weight and breathing impacts buoyancy and trim. What gear and how I use it changes things. I have enjoyed all that along the way.

I think there’s a lot of value to that so even if this kind of system gets more mainstream, I hope new divers learn as we currently do because I bet it will give them kinesthetic experience that may help when dealing with adversity under water.

 

[EdMcNeill09]

Admittedly I haven’t read all 23 pages but can someone explain what happens when this thing fails, because everything fails. Does the diver swim up. I hope it doesn’t fail buoyant. Either way, I feel comfortable with my simple gear and affordable replacement options for now.

During the Avelo Certification Dives we train for 2 possible failures: a continuously running pump and a failed dump valve. In the first case we have the diver simulate a continuously running pump by running the pump twice thus adding an additional 4lbs of water to the tank. In the second we have the diver vent all of the water from the tank. Both scenarios result in about a 4lb change in buoyancy. Not a lot. This requires the diver to swim in a slightly head down or up position as they work their way back to their exit point and end the dive. During our certification dives we swim for 30 seconds under these conditions before ending the skill and adjusting our systems to get back to neutral.

 

[inquis]

what happens when this thing fails, because everything fails

From what I can tell, failures would pretty benign.

You use the pump when you're getting light (approx +1 lb) and don't want to breath a little shallow any longer. If the pump fails, you'd swim up (while being a little light). Thicker wetsuit could have an issue as it expands (*), but shallow breathing with a bit of finning down would make for a safe ascent. Honestly, I suspect even without finning it will be slower than many normal OW diver ascents (where the BC expansion outpaces their reactions).

The purge (ballast removal) is non-electrical, with water being pushed out by the air bladder. One cannot take on much ballast in the first place, so if the purge failed open at the worst time (end of the dive at reserve pressure, losing all your ballast), you would be at most +9 lb buoyant. Finning downward can overcome that.

If the purge could not be opened for whatever reason, you might be around -1 lb negatively buoyant (you'd put in around 2 lb when getting light). Easily swimmable and you have to climb the ladder with that ballast (similar to a normal rig where you climb with your lead).

What is not obvious to me is if the pump continues to pump: I would hope there is an overpressure facility of some sort for the water release valve (similar to an OPV on a BC). We know there would be one on the air valve, but it would suck if the continued pumping would take up more and more of the tank volume, compressing the breathing gas beyond the capability of the burst disk. However, I suspect the pump would not be capable of pumping to such pressure. So you might be a few lbs negative, again swimmable.

(*) Avelo claims you don't have to worry about wetsuit expansion/compression. I don't believe this, as there is objective evidence of a 30% surface buoyancy loss at 33 ft (10 m). I've measured an XXL 7mm Bare Reactive suit at 16 lb of surface buoyancy, so that's at least 5 lbs that will be regained (obviously more from deeper). [Bardy, Erik; Mollendorf, Joseph; Pendergast, David (October 21, 2005). "Thermal conductivity and compressive strain of foam neoprene insulation under hydrostatic pressure". Journal of Physics D: Applied Physics. 38 (20): 3832–3840.]

 

[lowwall]

I am well aware many of my posts on this topic seem to be critical of avelo, but that's mainly to give balance against the manufacture's view through (understandably) rose-colored glasses. In truth, I would totally dive this system in some situations, but I feel understanding both pros and cons is important.

Me too.

All equipment has tradeoffs. When you have a mature technology like open circuit scuba, an improvement in one aspect is very likely to come with a deficit elsewhere; in cost if nothing else. This thread has been about trying to understand these tradeoffs.