Do you need ditchable weight?

[lairdb]

That's not how you calculate it. Assuming proper weighting, the only weight you are concerned about is the difference between the weight that results in neutral buoyancy at the safety stop with an empty wing and the point of maximum negative buoyancy (i.e. full tank at deepest point).

So 5lbs for the gas (Al80). Using your 24lbs surface buoyancy for the suit, we need to look at the difference in bouyancy between 5m and 35m. Assuming all the 24lbs is in trapped air bubbles, then it will provide 16lbs of lift at 5m and 5.33lbs at 35m (let's call it 5). So worst case is 5+(16-5) = 16lbs negative

Interesting. As I mentioned, I had excluded the compression to the safety stop, but you point out (correctly) that it is material. Using +16 at 15FSW due to compression, I think you still need to take tank buoyancy into account. The empty AL80 will be +3, for +19 you need to counteract with lead. That makes you zero at 15; you may want some extra to avoid corking...

@lowwall for a 7mm, one might choose to be neutral at a shallower depth to limit the buoyancy change between there and the surface. @rsingler's weight analysis tool (stickied in the Advanced Forum) indicates this could easily be more than 10 lbs from 15 ft. Rather than ascend every dive upside down kicking to limit ascent rate, a little more weight makes that expansion a little more controllable. This could add another 5 lb or more to the buoyancy difference you mentioned that has to be overcome. This only further underscores the wisdom of redundant lift for such cases.

Agreed. My calculations showed 8lbs difference in bouyancy between the surface and 5m. I like to take that last ascent from the safety stop to the surface really slow, fighting several pounds of lift would make that tough.

So yes on the need for ditchable weight or a source of redundant buoyancy in a 7mm. Personally, I'll stick with my drysuit.

...which I see you guys already covered.




ETA: I thought this was an interesting mental framing:

[...]Ditchable weights are a form of redundant buoyancy.[...]

Quibble-able, but thought-provoking.

 

[inquis]

I think you still need to take tank buoyancy into account

That's taken care of by your normal/initial weighting. (The tank itself doesn't change shape or mass.) The ONLY things the wing does is compensate for the CHANGE in buoyancy. That's wetsuit compression (as depth changes) and gas consumed (as time elapses), as long as you don't pick up or leave anything behind.

 

[FishWatcher747]

Wisdom of redundant lift... as in an smb? Do we really think a very distressed (normal recreational diver) who has had a catastrophic BC failure and now finds themselves quite heavy at depth (and we assume alone I guess, because we have never talked about having a buddy help in the scenario) is going to be able to deploy, inflate and manage the expanding air in the smb while holding it during a true emergency?

Possibly we might think that... I'm not sure it is such an obviously superior solution to simply dropping enough lead to allow themselves to swim up. Dropping 4 or 6 or 10 lbs SEEMS a lot less involved and afterwards requires the management of a lot less moving parts.. as opposed to an smb for example.

There are detailed and quantitative analyses in the previously mentioned thread that (I think) address these issues well - if someone cares to actually read the material.

So on the topic of using the SMB for redundant buoyancy this is something that I don't think was taught to me. For example I have a sealed 6 ft Mares XR SMB that can be orally inflated or with LP hose. For safety reasons I don't ever plan to inflate it with LP hose (reel or LP connection snags causing runaway ascent). So am I thinking about this properly that you would partially orally inflate the SMB, hold onto it, and use the toggle on the overpressure valve to vent air as you ascend?

I have also assumed from a description I heard of being caught in a down current that someone would inflate SMB at depth, let it go to surface on reel and pull down on it and hand over hand ascend the reel line. Is this correct?

 

[Wibble]

I have a couple of Halcyon SMBs and would never orally inflate. For a start my lips won't wrap around the spiggot when freezing cold, but secondly you need a few lungfuls to fill the SMB.

I use the Drysuit inflation hose. Simply pop off with one hand and it's ready.

Full process...

• Get SMB out of pocket with spool (can use a reel, will use a spool for this example)
• Undo SMB and shake out
• Unclip double-ender from the spool and clip off to chest D-ring
• Tidy up string and arrange the SMB's inflation spiggot on top of the spool such that it's in your left hand with your thumb and middle finger in the spool as a hub, and your forefinger holding the spiggot down onto the spool
• Disconnect the drysuit inflation hose.
  
• N.B. if not using a drysuit, I will lightly gaffatape the drysuit inflation hose to the SPG hose; it sticks out a couple of inches below the SPG
• Get the inflator on to the spiggot. STOP.
• Look around and up for anything that could be a snag or other divers!
• Press the inflator fully home on the spiggot and give it the beans. When it's really pulling simply lift off your forefinger and off it goes
• Spool unwinds in your left hand; reconnect the drysuit inflation hose (there's a knack!) or wait until the spool's stopped (or grip the spool to pull you up - advanced trick).
• Ascend to your first stop, winding as you go, dumping your suit and occasionally dumping from your wing
  
• Don't drop the bloody spool!
• Clip off the spool at your stop
• Relax.

 

[inquis]

@FishWatcher747 either approach works if your wing is busted. Whatever floats your boat, so to speak.

If you're stuck on the bottom with air (i.e., a bit of time), other options may be available. E.g., turning your gear upside down in front of you. It may now hold air if the damage was on the shoulder originally.

 

[johndiver999]

So on the topic of using the SMB for redundant buoyancy this is something that I don't think was taught to me. For example I have a sealed 6 ft Mares XR SMB that can be orally inflated or with LP hose. For safety reasons I don't ever plan to inflate it with LP hose (reel or LP connection snags causing runaway ascent). So am I thinking about this properly that you would partially orally inflate the SMB, hold onto it, and use the toggle on the overpressure valve to vent air as you ascend?

I have also assumed from a description I heard of being caught in a down current that someone would inflate SMB at depth, let it go to surface on reel and pull down on it and hand over hand ascend the reel line. Is this correct?

So you are asking me? The guy who expressed doubt that an smb is a viable solution for a recreational diver on a deep dive with a failed BC and a lot of unditchable lead and a thick suit and who has no help from a buddy?

My perspective, if I have not already made that clear, is to simply find out how much lead YOU need to drop and still be able to swim up with a failed BC. I kinda doubt that is something you can do with your current configuration, but I don't really know - do you?

If you are going to go the route of thick wetsuit and deep water and very little ditchable lead, then you can practice with an SMB. Either fill it a little at depth (however you want) and then allow expansion to take over as it goes to the surface and then climb/swim up a a string OR inflate it a good bit and try to hold it, or sit on it, and vent it as needed and watch your ascent rate. You can easily practice these, just dump 100% of air from BC at depth and then try to ascend with all your lead. See how it goes. I have to admit, I have NOT personally practiced this. I usually wear a weightbelt that I am willing to drop on a recreational dive.

I generally don't wear a thick wetsuit however, so my personal situation is different than yours.

 

[johndiver999]

@FishWatcher747 either approach works if your wing is busted. Whatever floats your boat, so to speak.

If you're stuck on the bottom with air (i.e., a bit of time), other options may be available. E.g., turning your gear upside down in front of you. It may now hold air if the damage was on the shoulder originally.

Really,,, in an emergency you would suggest that a viable option for a recreational diver is to remove BC and scuba unit, diagnose where the leak/failure is and then invert the unit, inflate and then hold it in hands and try to swim up with it. ?? If you think that ranks anywhere high on the decision tree of "good options" your perspective is far, far different than mine.

What if they simply DROP the scuba unit for a second? With a thick suit and all their lead on the scuba unit... now who is going to be taking a quick ride - this time with nothing to breathe? Seriously?

 

[-JD-]

I keep a Dive Rite 50lb, open-bottom lift bag on the bottom of each of my plates.

It is in a mesh sleeve (shown poorly in the picture), attached with SS split (key) rings to the plate, and the d-ring is clipped to the right split ring with a double-ender. (Shown in my picture up-thread: Do you need ditchable weight?)

There is a pull-dump located 1/4 of the way down from the top, actuated by a pull string anchored through the center tab at the bottom. This allows most (approx 2/3-3/4) of the air to be released with the string. Since this is a true open-bottom, not a one-way duck-bill, emergency or final dump is to simply grab the top and pull it down. The wide-open-bottom is easy to fill with a purged reg or simply tilting the head and exhausting.

It is, unsurprisingly. handy for lift bag duty.

For use as an emergency buoyancy device (this is relegated to backup #2 since I now dive with redundant bladders in cold water) the strap is easy to grab, or it can be clipped to a shoulder d-ring with the ever-useful double-ender. The shoulder d-ring is convenient for exhaust filling and leaves that hand free for easy dumping with the pull-cord or reaching up to grab the bag and rotate down in a pinch.

Which reminds me this is on the lets refresh this skill list ...

 

[inquis]

@johndiver999, I had hoped the "on the bottom" (not sinking) and "with air/time" would be sufficient to suggest something less urgent than you seem to have in mind. One person's emergency may be another's minor irritation.

I'm not sure what I've done to earn your ire, but all I said was that it is an option. The reader should assess their own skill/knowledge when selecting an approach, preferably practiced/simulated in advance. I believe they still teach gear removal in OW classes, don't they?

It sounds like you're saying one would be ripped away the instant you doff it. Remember, your suit has very little lift due to compression... You know, the root cause of the unbalanced rig and inability to swim up.

invert the unit, inflate and then hold it in hands and try to swim up with it. ??

Hardly, the scenario I had in mind is separating the elbow or inflator hose. Invert it and it's a functional BC (orally inflated), easily offsetting the lead and trivial to ascend at a leisurely pace. Of course hook your arms through the straps, but my kid's school bag weighs more than that rig under water. Again, options (maybe you forgot your DSMB that day), and yes *I* would invert the BC in that situation before ditching weight. I'll just end with YMMV.

 

[BRT]

Part of what was being addressed with the "balanced rig) approach, is that with proper weighting in a recreational environment you shouldn't be more than about 8 lb negative even at the beginning of the dive at depth. And you can swim up 8 lbs (which will get less as the suit decompresses and you use up air) and then ditch weight at the surface to attain positive buoyancy. If you ditch at depth, you are committed..... no getting it back and you run the risk of a rapid uncontrolled ascent.

Respectfully,

James

If you are 8 lb negative when you get in and you hit 100' with a 7 mil, you are going to be more than 8 lb negative.