Rebreather designs and buoyancy control strategies

[RichardC]

Between five and ten litres difference depending on the rig.

A Meg, for example, which would be the same as a Liberty or Hammerhead, has about 8 litres of "non-floodable" free volume in the can (depending on which can and which scrubber). The rEvo has zero "non-floodable" internal volume.

I'm a real fan of gag straps on any rig. Learned on the LAR-V back in the Stone Age with one and then to the FGT-1D. Feel naked without one.

John, we are talking at cross purposes. You said, in response to my post, that the flooding of the scrubbers on a rEvo would make a great deal of difference compared with merely a fully deflated lung, which was my original point. I say it would not because only, at most, one extra litre of gas volume would be lost. In other words, in terms of buoyancy, a flooded rEvo and an empty one are much the same.That statement is true so the flooding issue is not relevant. But we are talking of a rEvo where the gag strap is not in use.

I am not saying that a Meg or whatever will have to flood in order to lose buoyancy, I am saying that if you take a full breath from it and then exhale that breath outside the unit, the lungs are now empty and and buoyancy of the unit will be lost just the same. The unfloodable volume is not unbreathable volume is it?

So what happens to you on your unit on the surface depends on the detail of your incident. Let's say you become hypoxic, pass out and lose your DSV. If you happen to lose it after exhaling into the unit then great, you probably won't lose all of the buoyancy of the lungs, at least not quickly. If you happen to take a full breath from the unit and lose the DSV, then when you breath out to atmosphere you are no better off than on the deflated rEvo. No flooding necessary, empty unit either way.

A rEvo with a gag strap or your unit with a gag strap, now that is a different story. Much more secure, we agree on that.

The point I am trying hard to make is that OTS units are no guarantee of retaining lung volume on the surface and there is no substitue for being sufficiently buoyant by other means.

 

[St John the Diver]

John, we are talking at cross purposes. You said, in response to my post, that the flooding of the scrubbers on a rEvo would make a great deal of difference compared with merely a fully deflated lung, which was my original point. I say it would not because only, at most, one extra litre of gas volume would be lost. In other words, in terms of buoyancy, a flooded rEvo and an empty one are much the same.That statement is true so the flooding issue is not relevant. But we are talking of a rEvo where the gag strap is not in use.

I am not saying that a Meg or whatever will have to flood in order to lose buoyancy, I am saying that if you take a full breath from it and then exhale that breath outside the unit, the lungs are now empty and and buoyancy of the unit will be lost just the same. The unfloodable volume is not unbreathable volume is it?

So what happens to you on your unit on the surface depends the detail of your incident. Let's say you become hypoxic, pass out and lose your DSV. If you happen to lose it after exhaling into the unit then great, you probably won't lose all of the buoyancy of the lungs, at least not quickly. If you happen to take a full breath from the unit and lose the DSV, then when you breath out to atmosphere you are no better off than on the deflated rEvo. No flooding necessary, empty unit either way.

A rEvo with a gag strap or your unit with a gag strap, now that is a different story. Much more secure, we agree on that.

The point I am trying hard to make is that OTS units are no guarantee of retaining lung volume on the surface and there is no substitue for being sufficiently buoyant by other means.

Maybe we're talking past each other.

The point I was (and am) making is that the -total buoyancy lost- in the two systems varies greatly from one to the other. Not simply the counterlung volume. That's going to be similar. But the total system buoyancy change, which includes the internal volume of the rig, needs to be taken into account.

It's a significant difference.

It's harder for the diver if he's exhaled into the rig before popping out the DSV. His lungs are empty which is bad news from both a buoyancy and survival standpoint. Unfortunately that's likely, as we teach to blow into your rig and then close the DSV in case you need to do off-loop surface swimming (not recommended but sometimes required). That's to both preserve that nice bag-o-mix that you've blended for yourself and to preserve buoyancy. The issue is that if you exhale into the loop and then forget step B (close DSV) you're shooting yourself in the foot as you dump all of the gas out of the counterlungs (and the interior of a rEvo) and are now sinking without anything in your lungs. Try holding your breath OUT sometime.

I've seen dozens of students take a breath, close the DSV holding their breath, and then start sinking immediately upon talking to me on the surface in training as the gas they were holding in is exhaled. I let them make that mistake once, as a learning opportunity. Always in the pool.

Surface-Swim drill 101:

Fill wings adequately.
Exhale into the loop.
Close DSV.
Then remove DSV.


Things are always more complex than they seem. This is why GOOD rebreather training takes time, a good intelligent student, and an instructor who actually knows all of these details and who incorporates them into training.

 

[RichardC]

Maybe we're talking past each other.

The point I was (and am) making is that the -total buoyancy lost- in the two systems varies greatly from one to the other. Not simply the counterlung volume. That's going to be similar. But the total system buoyancy change, which includes the internal volume of the rig, needs to be taken into account.

It's a significant difference.

It's harder for the diver if he's exhaled into the rig before popping out the DSV. His lungs are empty which is bad news from both a buoyancy and survival standpoint. Unfortunately that's likely, as we teach to blow into your rig and then close the DSV in case you need to do off-loop surface swimming (not recommended but sometimes required). That's to both preserve that nice bag-o-mix that you've blended for yourself and to preserve buoyancy. The issue is that if you exhale into the loop and then forget step B (close DSV) you're shooting yourself in the foot as you dump all of the gas out of the counterlungs (and the interior of a rEvo) and are now sinking without anything in your lungs. Try holding your breath OUT sometime.

I've seen dozens of students take a breath, close the DSV holding their breath, and then start sinking immediately upon talking to me on the surface in training as the gas they were holding in is exhaled. I let them make that mistake once, as a learning opportunity. Always in the pool.

Surface-Swim drill 101:

Fill wings adequately.
Exhale into the loop.
Close DSV.
Then remove DSV.


Things are always more complex than they seem. This is why GOOD rebreather training takes time, a good intelligent student, and an instructor who actually knows all of these details and who incorporates them into training.

John, we were talking past each other, but now I think we agree on quite a lot.

I have been talking all the time (obviously not very clearly) specifically about the incident that is the subject of the thread i.e. some crisis on the surface.

I think we can agree that, with wing/suit fully deflated and whatever the residual buoyancy of our units, we will both be weighted such that we will be roughly neutral or a little bit negative when we have a comfortable, breathable loop volume (depending on kit, stages etc.). This means that if we vent/lose/exhale just our counterlung volume we would both be at least 6kg (13lbs) negative, unless we inflate something.

On the surface I think we also agree that if we have marginal, less than 6kg, lift from our wing or suit, then we would begin to sink if we vented/lost our counterlung volume. Your nice teaching scenarios above (presumably on your own choice of unit) show that the loss of that volume can happen and it certainly can on mine. This means that in a marginal buoyancy situation, any diver on any unit could end up with their faces under water very quickly if a mistake is made and that volume is lost.

No flooding is required here and it could even happen if the counterlungs are emptied, the DSV is closed and the then diver exhales, talks, shouts to the boat or whatever.

Of course a competent and aware diver shouldn’t let this happen and could do something about it if it did, but in the case of a compromised or unconscious diver this could well be fatal and a diver in such a state may well not inflate the wing/suit properly.

The bottom line is that if we achieve enough lift from our wing/suit this will not happen on any unit. If we don’t, it could.

If you have a gag strap you are better off on any unit if you become incapacitated. It may keep your loop intact long enough to be rescued.

 

[St John the Diver]

John, we were talking past each other, but now I think we agree on quite a lot.

I have been talking all the time (obviously not very clearly) specifically about the incident that is the subject of the thread i.e. some crisis on the surface.

I think we can agree that, with wing/suit fully deflated and whatever the residual buoyancy of our units, we will both be weighted such that we will be roughly neutral or a little bit negative when we have a comfortable, breathable loop volume (depending on kit, stages etc.). This means that if we vent/lose/exhale just our counterlung volume we would both be at least 6kg (13lbs) negative, unless we inflate something.

On the surface I think we also agree that if we have marginal, less than 6kg, lift from our wing or suit, then we would begin to sink if we vented/lost our counterlung volume. Your nice teaching scenarios above (presumably on your own choice of unit) show that the loss of that volume can happen and it certainly can on mine. This means that in a marginal buoyancy situation, any diver on any unit could end up with their faces under water very quickly if a mistake is made and that volume is lost.

No flooding is required here and it could even happen if the counterlungs are emptied, the DSV is closed and the then diver exhales, talks, shouts to the boat or whatever.

Of course a competent and aware diver shouldn’t let this happen and could do something about it if it did, but in the case of a compromised or unconscious diver this could well be fatal and a diver in such a state may well not inflate the wing/suit properly.

The bottom line is that if we achieve enough lift from our wing/suit this will not happen on any unit. If we don’t, it could.

If you have a gag strap you are better off on any unit if you become incapacitated. It may keep your loop intact long enough to be rescued.

Agreed on all with the following observations:

One thing to note is that it's easier to go to that negative buoyancy state from a marginal buoyancy state with certain designs than others. rEvo (and Mark-16 and SF2 etc) lose all of the CL lift within very few seconds. OTS lung rigs tend to only vent the inhale lung rapidly and the exhale lung tends to retain a gas bubble for some time. Little things often add up into big things.

These are the little technical details that aren't mentioned clearly by most instructors teaching. It's important for a FULLY trained diver to be just as aware of the weaknesses of his system as he is of its strengths so that he can apply skill to minimize the impact of the design compromises that are part of EVERY rebreather design.

In this particular case, the divers were using one of the fastest venting, fastest flooding, and least bouyancy protective rigs built. And by all guesses had not retained the one device (gag strap) installed by the manufacturer to limit that exposure. That's just not doing your best to work around a rigs inherent design compromises. I lay that down onto the plate of poor training given by and poor safety discipline used by the associated instructor. If anything, the observation that many of Sotis's students report that they didn't even know the gag strap was as option in training is telling from a liability standpoint, considering that it's provided with the rig and that training standards require that instructors teach on unmodified rebreathers. Routinely defeating a factory standard safety device on rebreathers is bad juju. My bet is that this will be a point made in a venue before 12 good citizens before too long.

Closing thought: why do you think the USN now requires the Mark-16, which shares this same basic "flaw" with the rEvo, to be dived with a full face mask? Airway protection, pure and simple.

 

[Patoux01]

TBH, we're taught since OW to always be largely buoyant on the surface. Many instructors will teach to inflate the wing until the OPV blurps...

So sure, flooding the unit would be a shame as it ducks up some electronics and could cost a bunch, but I don't feel like it should be so much of a big deal in terms of safety... What did I miss?

 

[St John the Diver]

TBH, we're taught since OW to always be largely buoyant on the surface. Many instructors will teach to inflate the wing until the OPV blurps...

So sure, flooding the unit would be a shame as it ducks up some electronics and could cost a bunch, I don't feel like it should be so much of a big deal in terms of safety... What did I miss?

What you miss is that many rebreather divers don't routinely use their wings. That's for a number of reasons, but one is that unlike open circuit they aren't starting off heavy and then lightening constantly as they consume gas. Between a drysuit and counterlungs you've already got two variable buoyancy devices to manage. Wings become almost redundant. The other thing is that the "lure" on the surface of being comfortably positively buoyant with whatever you ascended the last ten feet with in your counterlungs gives a feeling that you're just fine... Which lasts until you forget to close the DSV and remove it to (talk, yell, spit, swim). Generally new rebreather divers don't correlate "remove mouthpiece" and "lose bouyancy and sink like a stone" perfectly because it's never been that way before.

So yes, you should add air to your wings on the surface. But.... Many don't.

Further offering that wings on most rebreathers are fed from diluent. If you're trying to make a diluent bottle run for multiple dives (common) it's often logistically unwise to squander it on inflating your wings. Truly you can use more gas into your wings on the surface than you might have used for a two hour dive at 200 feet. The supply of gas available is darned small. Third dive of the day here? 2 litre diluent bottles? Unless they changed cylinders (not terribly likely) they were probably low on diluent to start with. Wings waste precious trimix.

 

[Patoux01]

Further offering that wings on most rebreathers are fed from diluent. If you're trying to make a diluent bottle run for multiple dives (common) it's often logistically unwise to squander it on inflating your wings. Truly you can use more gas into your wings on the surface than you might have used for a two hour dive at 200 feet. The supply of gas available is darned small.

Well that's a bad reason if I ever saw one...

 

[St John the Diver]

Well that's a bad reason if I ever saw one...

Build your own rebreather and design it differently then.

Truthfully, and this is based on hundreds and hundreds of rebreather dives: I can't remember the last time I inflated my wings with one. Bouyancy is controlled by slightly varying the counterlung volume and judicious use of your drysuit. I consider my wings to be emergency inflation to lift a flooded rebreather.

 

[CuzzA]

I consider my wings to be emergency inflation to lift a flooded rebreather.

And so I guess the question is does your wing have enough lift in an emergency with a flooded unit? Is this considered when students buy a CCR and take the course?

 

[northernone]

And so I guess the question is does your wing have enough lift in an emergency with a flooded unit? Is this considered when students buy a CCR and take the course?

In my case yes.