Lift Bags for Emergency Ascend

[beester]

I get you, and agree for heavy duty deep tech dives maybe.

I just consider the question of the Op, 20-50 Dives as experience and talking about emergency (?) ascent with a SMB or a lift bag.

Hi Freewillow: you are correct in assuming that a balanced system, and being correctly weighted will remove a lot of it. However there will in many cases still be an amount of air/gas in your bcd/wing to compensate for volume changes related to 2 things:

- Diving a wetsuit: Nowhere did the OP state that he's diving a drysuit (which would stay neutral). If he's diving a thick (7mm or more) wetsuit the compression of the suit at depth will signify a shift in volume that could be quite some kilo/pounds.
- GAS: The other thing changing as well is you use up your air/gas. 1 M3/1.2Kg so per 1000barliter (cubic m) used you are 1.2kg lighter. This needs to be compensated (and this is why you typically weigh yourself with a close to empty tank). I've done dives where the total weight of gas was so much (shallow cave dive) that we had to put helium in the nitrox 32 just to allow us to handle the stages. (14 to 15kg of gas)

 

[rongoodman]

@CPT Tightpants. I did not get it. Why would you need a lift bag on top of 1 - or even 2 - DSMB?

All the SMBs I've seen have the OPV at the bottom, as opposed to near the top as on a lift bag. Once you put air in them, it's not going to come out, since the bubble will be at the top. A lift bag can be vented under water if needed, since the OPV sits on top of the air in the bag.

 

[beester]

This is something I was taught to do in a recent wreck/ deco procedures course, and I believe it's a required skill in the TDI manual. We did two dives a day for 5 days, and the second dive each day ended with practicing deploying the SMB for a controlled ascent to the first deco stop. It seemed pretty easy and safe if performed as I tried to describe it. If you imagined we were being dragged up by the SMB , I did a poor job describing it.

Again, the skill was for open water ascents without a fixed line. We were not given the option of tying off to the wreck for ascents. Deploying an SMB from 140' directly to the surface doesn't seem much easier than what we learned to do. If nothing else reeling in 70' of line on the way to a stop at 70' seems like a lot of work. Also, if there is any current won't you have to play out (and reel in) a lot more line for the SMB to reach the surface?

By partially filling the smb with air from the bc (and holding on to it), you haven't changed your buoyancy at depth. The SMB floats a little above your head, with little or no line played out. Once we starting ascending it didn't seem very hard to maintain a proper rate, and the dive profiles from my computers seem to bear that out. Managing the bubble was little more than venting some gas every so often, and staying a bit heavy on the SMB. Letting it play out as we began to approach the stop didn't seem that much harder than launching it from a fixed depth. Did I hit every the first stop at exactly the right depth +/- 1 foot? No, not every time - hey, I'm new at this!

But I'm still learning, and am open to different ways to do things.
-Don

Hey Dberry. Thanks for giving more details. I didn't misread you that I think you did some uncontrolled ascends using your deployment method. Just that your method might be harder to do because you are inherently unstable when moving up the water column, ascending, and it's easier to do stuff (being taskloaded) while stopped at a certain depth. Like shooting an SMB.

What I would do depends on the circumstances:

SCENARIO 1: I'm doing a free drifting ascend, and this has been discussed with the charter captain. In this case I would not shoot the smb from the wreck. There is a risk that it gets entangled, and I'm going to free drift anyway so I want to get away from the bottom and shoot the SMB when I'm stable in the water column. The ideal spot to do so is when you slow your ascend (deepstops or first 1 minute stops), or when you are switching to another (deco) gas. Up to that point you are not wasting a lot of time. On a typical 140ft dive you leave the wreck, and ascend and you first start to slow down at 100ft or a bit less and you switch gas typically at the 70-60ft range. In this case you are staying at that depth for some time, you are stable in the water, you have nothing else to do so why not shoot the SMB. From that moment onwards your ascend is slow enough so you can slowly reel in the line on the reel/spool, and you have the benefit of a visual reference (the line of the SMB) in the deco portion of your dive.

SCENARIO 2: We planned an ascend via the anchor line and were instructed to ascend via this line. I couldn't find it for whatever reason, so this is like a minor emergency. I don't want to free drift in this case because the charter boat is still attached to the wreck and there might be other divers on the line. So what I want to do is make a new ascend line from the wreck to the surface. I would shoot an smb with a spool or reel (depending on depth), when the smb reaches the surface I would attach the line to the wreck (after attaching cut the line so you keep the reel) and now I have a permanent attached line from the wreck to the surface and can use this line/smb as ad hoc anchor line.

Hope this makes sense. In the end it's up to you, and practice makes perfect of course, but I do think that my scenario's are what is used most regularly by wreck divers.

 

[doctormike]

I don't understand the point of the TDI exercise either(why not just shoot the bag from the 70' stop?), but this was presented in a tech course, so between deco gas and required reserves of back gas the diver is never going to have an empty wing, unless there's a big problem.

I'm a little confused about the thread of this conversation too... when I did my TDI tech training, we used an SMB for redundant buoyancy in case of wing failure when diving in a wetsuit and doubles. When doing that, we didn't deploy the bag on a line in the standard manner, we rode the bag up with the valve close at hand so that we could dump gas to control our ascent and stops.

I'm not an instructor, but I think that there was an earlier thread here about the way that different agencies handle this. PADI TecRec mentions a double bladder wing if a dry suit is not worn, but says that an SMB is not reliable. TDI allows the use of the SMB, and GUE seems to stress the balanced rig, and does not recommend the double bladder wing.

I personally use a double bladder wing when diving doubles in a thin wetsuit (in Truk, I was in a neutrally buoyant dive skin). I know that people have issues with them, but I never saw the problem. Certainly easier than trying to ride an SMB to the surface, and with a balanced rig even that shouldn't be necessary unless you need to bring something (or someone!) else up with you.

 

[Freewillow]

with a balanced rig even that shouldn't be necessary unless you need to bring something (or someone!) else up with you.

 

[dberry]

If this question is too off-topic I can post it as a separate thread, but how do folks define a balanced rig in the context of diving doubles?

In my prior experience single tank recreational diving, my goal for a balanced rig (loosely defined) was to have only enough weight (lead + backplate, etc) so that with only small amounts of air in the wing I could control my buoyancy and depth mainly with my breathing, and only rarely needing to add/vent air from the wing. Smaller bubble in wing = smaller changes in buoyancy with depth. Being able to hold a safety stop after using XX pounds of air required a bit of extra lead.

With double LP85s I carried no lead and needed air in the wing to be neutral at any depth. I was diving in a 3mm wetsuit during the course, so the wing was the only bubble I needed to manage (other than my lungs.) I could adjust the air in the wing to be fairly "balanced" within a certain depth range on the wreck (i.e. minimal wing adjustments), but it was always "overweighted" in terms of what I was used to for single tank diving. I thought this is an inherent aspect of "tech diving", and also the reason why we practiced with the SMB as redundant lift in event of a wing failure.

Is there a unstated assumption that diving doubles always requires a drysuit? My course was taught in South Florida and I saw lots of doubles on the boats but not a single drysuit.

One thing I didn't try, but wish I had, was to experience how hard it would be to swim up off the deck after venting all the air out of the wing.

So, how should I define a balanced rig if I am inherently overweighted in doubles?
Thanks for your advice.
-Don

 

[DevonDiver]

There isn't a singular "tec answer" about DSMB. That advice of an individual instructor for a specific course reflects local needs and/or practices.

A DSMB has to be visible in the foreseeable conditions. Height/volume is dictated by issues like surface conditions (waves) and current (distance traveled). A 3' bag is no use in 4' waves.... or if you might be swept 400m from the boat.... etc etc

Where I live (Subic Bay, Philippines) there really isn't any current or wave issues. It's quite placid. I never use larger than a 3' DSMB. In other places; like the English Channel...where the tide can get strong and there's heavy boat traffic.... I'd want at least a 6' bag. Maybe even a 9' bag on a few sites.

In short... take the bag/s you need for the actual dive you're doing. Deciding your need is a part of the planning and preparation process.

I've never heard of a requirement that you have to be able to 'hang negative' under the DSMB. It shouldn't be a crutch for weak buoyancy control. Technical divers should have the capacity to float neutral on stops. How's it gonna work hanging negative from a DSMB in a 1-2m swell.....?? That's some pretty atrocious up - down - up - down antics right there.....

You need to launch a bag where it'll be seen by your surface support. Yes... in general, shallower is better.... but you don't always have that luxury.

Oral inflate DSMBs are the easiest and safest to operate. If set-up properly and a good technique, you can pop them single-handed and no risk of the line getting fouled up on equipment. They're available in sizes 3 to 9'. With oral inflate bags... you simply need to understand the depth from which a single breath is sufficient to fill the bag volume letting air expansion do that hard work for you. At, or below, that min depth and you've got a full bag on the surface, but you don't get positive transferring a breath from your lungs to the bag.

Spools are the easiest for DSMB deployment. They aren't prone to jamming.... and even if you have to let go of one... it'll unspool and come back down to you. K.I.S.S. Spools hold less line though... so sometimes you'll have to use a reel if deploying from deeper depths. Side-handle reels are infinitely better than top-handle reels IMHO. There's a few top-handle reels I've virtually banned from my technical and wreck courses because they persistently foul.

I recommend that reels/spools aren't deployed deeper than 2/3 to 1/2 the distance of the line capacity. This accounts for any drift etc.... lines rarely go straight up. Again, this would depend on the current/drift. Sometimes there can be different currents at different depths.

 

[DevonDiver]

There isn't a singular "tec answer" about DSMB. That advice of an individual instructor for a specific course reflects local needs and/or practices.

A DSMB has to be visible in the foreseeable conditions. Height/volume is dictated by issues like surface conditions (waves) and current (distance traveled). A 3' bag is no use in 4' waves.... or if you might be swept 400m from the boat.... etc etc

Where I live (Subic Bay, Philippines) there really isn't any current or wave issues. It's quite placid. I never use larger than a 3' DSMB. In other places; like the English Channel...where the tide can get strong and there's heavy boat traffic.... I'd want at least a 6' bag. Maybe even a 9' bag on a few sites.

In short... take the bag/s you need for the actual dive you're doing. Deciding your need is a part of the planning and preparation process.

I've never heard of a requirement that you have to be able to 'hang negative' under the DSMB. It shouldn't be a crutch for weak buoyancy control. Technical divers should have the capacity to float neutral on stops. How's it gonna work hanging negative from a DSMB in a 1-2m swell.....?? That's some pretty atrocious up - down - up - down antics right there.....

You need to launch a bag where it'll be seen by your surface support. Yes... in general, shallower is better.... but you don't always have that luxury.

Oral inflate DSMBs are the easiest and safest to operate. If set-up properly and a good technique, you can pop them single-handed and no risk of the line getting fouled up on equipment. They're available in sizes 3 to 9'. With oral inflate bags... you simply need to understand the depth from which a single breath is sufficient to fill the bag volume letting air expansion do that hard work for you. At, or below, that min depth and you've got a full bag on the surface, but you don't get positive transferring a breath from your lungs to the bag.

Spools are the easiest for DSMB deployment. They aren't prone to jamming.... and even if you have to let go of one... it'll unspool and come back down to you. K.I.S.S. Spools hold less line though... so sometimes you'll have to use a reel if deploying from deeper depths. Side-handle reels are infinitely better than top-handle reels IMHO. There's a few top-handle reels I've virtually banned from my technical and wreck courses because they persistently foul.

I recommend that reels/spools aren't deployed deeper than 2/3 to 1/2 the distance of the line capacity. This accounts for any drift etc.... lines rarely go straight up. Again, this would depend on the current/drift. Sometimes there can be different currents at different depths.

 

[dberry]

I didn't mean to say the requirement was to hang heavy on the SMB during stops... this has all been about using an SMB as emergency, redundant lift in the event of a wing failure. That was what we were practicing in the course, and is also related to the topic of this thread.

 

[beester]

If this question is too off-topic I can post it as a separate thread, but how do folks define a balanced rig in the context of diving doubles?

A balanced rig is a rig that has been setup as a balance between 2 weighing extremes, these are:
- being so heavy that you cannot ascend anymore from depth.
- being so light that you cannot stay down, keep a specific stop depth.

This weighing is based on 2 factors, one of which can be calculated, the other not and needs to be checked in water.
- The weight of the gas you are breathing, taking with you underwater.
- The weight/buoancy shift of material you take down and can compress (become less buoyant the higher the pressure is and the more it compresses).

GAS weight: This is easy to calculate. First of all my appologies for using metrics and metric tank volumes, Imperial is a bit hard for me and I would probably make mistakes. So air and air related gasses (nitrox) weighs in at about 1.2kg per 1000 barliter. So a full double 12L or 80cuft set will be filled with about 5 to 6kg of air (quite a bit less when it's filled with trimix but I digress). You will breath this set down so you become lighter during the dive. This you'll need to compensate with weight.

MATERIAL that compresses: Typically we are talking about neoprene (a wetsuit or non compressed neoprene drysuit). Since the material loses volume the deeper you go, you will get heavier the deeper you go. How much heavier depends of course on the amount of neoprene. With a 3mm shortie the difference will be negligible, with a double 7mm neoprene wetsuit it could be quite a lot of weight.

If you would combine a big set of tanks (so heavy because of the amount of gas in it) and a double 7mm wetsuit (which loses buoancy when it compresses), you'll end up with an "unbalanced" rig meaning, it will be so heavy at depth that in case of a failure to your bcd/wing chances are you won't be able to swim back up. This is where a compressed neoprene/trilam drysuit comes in the picture (there are other solutions like a double wing, or using an smb but I won't go there now). Since the material doesn't compress it stays neutral (doesn't change weight) during the course of the dive at whatever depth. Next it can act as a 2nd floatation device (if you keep horizontal) in case of problem with your bcd/wing.

Now what is a balanced rig to summarize:
- A rig (not just your leadweight, everything on your rig has weight or is buoyant and will affect your weighing) that will be heavy enough so you can keep a shallow stop at 3m/10ft with almost no gas in it, but at the same time is light enough so in case of an emergency at max depth you can still swim up. If the latter is not the case you'll need to think of some weight that can be dropped (like a weight belt or a big battery of your light).

So typically you want to be as light as possible while able to maintain a stop depth, How do you check this. get in the water with all the equipment you'll use. Empty your set and try to stay at a 3m stop depth with the min amount of weight necessary. Let a buddy check your bcd/wing to make sure there is no gas in it any more, if there is still gas in it , dump it and check if you can lose additional weight.

Next you'll want to check what kind of dives you do in what circumstances and if you'll be able to get up in case of failure to your bcd/wing. In tropical water diving (no or very thin neoprene wetsuit) you'll probably have no issues (unless you are taking an incredible amount of gas with you). In cold water diving, specifically deeper cold water diving (lots of gas) you might want to re-evaluate the use of a wetsuit, or come up with other solutions.

Cheers