Ditching is for the surface. Doing it underwater is a last resort. Most setups only allow you to ditch half or all of their weight. If they were as overweighted as most divers tend to be, you now have a highly buoyant diver and no way to counter what is likely to be a fast ascent.
Review Diving the Avelo System
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I have reviewed this system several times and find two scenarios that bother me. First is the descent in cold or cool water where wetsuit compression is present. If the Avelo has limited lift with full tank or you have a pump failure slowing or stopping the descent becomes a major issue. This is somewhat remedied with ditching weight, but that then brings us to the second issue. If at depth, you flood the battery or pump dies for any reason how will the diver slow the ascent with a wetsuit that is becoming more buoyant as the diver ascends? In very warm water this may not be an issue, but wetsuit compression and decompression with a pump failure has not been addressed as far as I can see. Please tell me I am wrong and this is part of the training!
The pump is only used to add ballast water. To dump ballast, you open a mechanical valve and the gas pressure inside the tank bladder causes it to expand which expels the water.
I do agree with you that this system is problematic if you are going to use 7mm or more of neoprene. Avelo either ignores wetsuit compression in their marketing info or dismisses it as being much less than people think.
If the pump fails, decent is stopped by purging. The buoyant ascent due to wetsuit expansion with a failed pump still seems like an issue. Avelo's claim that wetsuit compression is negligible is disputed by actual testing (link somewhere up thread). Assuming you were not light when you started your ascent, I'd expect buoyancy to be positive 5-8 lbs in a single layer 7 mm suit -- corkage.
Or if you weight for holding a safety stop at the end of the dive, you are going to be around 5-8 pounds negative on the bottom.
Thinking of this from a safety perspective, how will you deal with a system that only has 2-4 pounds of buoyancy compensation at the start of a dive? This, BTW, is why Avelo falls outside the WRSTC standards and can't be used for discovery dives or OW training.
Of course experienced divers can use their lungs to compensate for another couple of pounds and watermanship/swimming skills for even more, although at the cost of increased gas consumption. But I'd personally be looking for neutral layers to add if a 3mm wasn't enough.
If you want to dive cold water with this, use a drysuit and bring an extra buoyancy source in case of a suit flood.
I guess. Of course, you'll need to do a 6 minute descent if you want to stay neutral while the pump is working. OTOH, if you are using an older reg, especially one with a yoke connection, you may end us dumping weight in the form of breathable gas very quickly well before you reach 4350psi
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I've only dived a 3mm full suit, started 2 lb positive, singleI guess. Of course, you'll need to do a 6 minute descent if you want to stay neutral while the pump is working. OTOH, if you are using an older reg, especially one with a yoke connection, you may end us dumping weight in the form of breathable gas very quickly well before you reach 4350psi
Many commenting in this thread have not dived the Avelo System. Misinformation has crept in, such as the yoke/DIN connection. Dry diving a system may be useful sometimes, not always.
Edit: pump time corrected
Thanks for the info. And that's not the only thing wrong with my post. After 25 years of diving, it can be hard to hard to wrap my head around adding ballast instead of buoyancy and I went the wrong way. Let me try again.
If you are wearing a 7mm and are starting at neutral at the surface, you'll need to increase your buoyancy at depth at depth by several pounds to offset the shrinking of the bubbles in your neoprene. Estimates of the amount of buoyancy lost at 30m compared to the surface for a 7mm single layer suit without hood and gloves range from 5 to 15 pounds.
Let's think about what we can do with Avelo assuming we will start the dive neutral. The only answer I can think of is that you need to start with a fill to 200 bar, wait on the surface for 14.6 minutes while the pump adds 7.3 pounds of water, adjust your weight so you are neutral and then begin your descent. During your descent you will purge water from the tank more or less continuously as you try to stay close to neutral. At the bottom, you'll be back to around 200 bar and hopefully neutral. As the dive progresses, you'll add ballast to compensate for the gas breathed as usual. You will likely have to do an extra addition of ballast if you make a major ascent.
At the end of the dive you now have the problem @inquis first suggested. Assuming you are doing a square profile and used 2/3rds of your gas and have added 4 pounds of water to offset it, you will be able to compensate for about 6 pounds of lift from your wetsuit decompressing by running the pump for 12 minutes as you ascend.
This might be enough to prevent corking if you have the time, but it will be a struggle to keep control if for any reason you require a normal or accelerated ascent rate.
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