Rebreather designs and buoyancy control strategies

[dumpsterDiver]

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A ScubaBoard Staff Message...

This discussion about rebreather designs and buoyancy control strategies has been split off from the Accidents and Incidents thread about Rob Stewart's death


For the SB Moderator corps,

Storker
SB Moderator

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Seems that this vulnerability would be hammered in training. I would think the student would be given a flooded unit or valid mock up of one to practice bail out and ascent and bouyancy control ... ..??? These things are even more dangerous than I thought.

 

[St John the Diver]

Seems that this vulnerability would be hammered in training. I would think the student would be given a flooded unit or valid mock up of one to practice bail out and ascent and bouyancy control ... ..??? These things are even more dangerous than I thought.

No, because those people selling and training people on the more vulnerable rigs don't make a big deal about pointing out their products weak spots in comparison to the competition. For the most part the BMCL rigs (internal counterlungs, not the JJ-style Iungs) have very weak flood tolerance and a very fast vent-rate with an open DSV above the counterlung level. That describes the rEvo and SF2 as well as the Mark-15 and a few others.

The T-Piece rigs with external counterlungs are FAR less vulnerable to this.

 

[Tassi Devil Diver]

A few observations having been diving a rEvo for four years now.

I have totally flooded my rEvo, (my mistake, practising bailout to off board in shallow water, hands were cold and did not completely close off BOV) I had no issue with buoyancy and only noticed I had flooded the unit when I went to go back on the loop (after around 10 minutes) and I noticed that the cells were going haywire. The unit was full to the brim with water.

I religiously wear the gag strap, on the surface from time to time I pull the mouth piece away from mouth (with gag strap still in place) briefly to yell instructions to the boat without closing the BOV for these brief moments, but have never noticed any woosh of gas out the mouthpiece as I do this.

 

[RichardC]

The T-Piece rigs with external counterlungs are FAR less vulnerable to this.

Surely this statement is only true if the diver happens to lose or remove the DSV after exhaling into the unit. If the diver inhales from the unit immediately prior to losing the DSV or shouting to the boat or whatever, then it doesn't matter which type of counterlungs are installed as they are now empty (or nearly so). As soon as the diver exhales to atmosphere then the buoyancy due to lung volume is lost. Of course in the normal course of events the diver just needs to inhale air to restore buoyancy, but if they can't...

I would argue that it is quite likely that someone in a stressful or compromised situation may well inhale from the unit first. All the more reason to be properly buoyant and not reliant on the counterlungs at the surface, whatever unit you have.

 

[rjack321]

Surely this statement is only true if the diver happens to lose or remove the DSV after exhaling into the unit. If the diver inhales from the unit immediately prior to losing the DSV or shouting to the boat or whatever, then it doesn't matter which type of counterlungs are installed as they are now empty (or nearly so). As soon as the diver exhales to atmosphere then the buoyancy due to lung volume is lost. Of course in the normal course of events the diver just needs to inhale air to restore buoyancy, but if they can't...

I would argue that it is quite likely that someone in a stressful or compromised situation may well inhale from the unit first. All the more reason to be properly buoyant and not reliant on the counterlungs at the surface, whatever unit you have.

Has nothing to do with inhaling or exhaling. The external lungs are more flood tolerance on the surface than the internal lungs listed because part of the counterlung is at the water's surface or very shallow. The deeper the counterlung(s) the more rapidly the gas inside will exit if you are on the surface with an open DSV.

 

[RichardC]

Has nothing to do with inhaling or exhaling. The external lungs are more flood tolerance on the surface than the internal lungs listed because part of the counterlung is at the water's surface or very shallow. The deeper the counterlung(s) the more rapidly the gas inside will exit if you are on the surface with an open DSV.

You miss my point. Flood tolerance was not the issue I was addressing. I am concerned with loss of buoyancy on the surface.

It has already been said that flooding is not necessary for loss of buoyancy. In the event of an open loop and insufficient wing/suit buoyancy, the gas will be squeezed out of the counterlungs and buoyancy lost, even if no water enters. In fact, it makes little difference from a buoyancy point of view if the lungs are empty or contain water, since either way water is replacing the gas volume.

So, in the case of shoulder mounted lungs, the same condition can be arrived at if the diver inhales before losing or removing the loop. My point being that the supposed buoyancy retaining benefit of shoulder mounted lungs on the surface can be easily defeated if the diver inhales from the loop and then exhales into the air, for whatever reason.

You can't rely on counterlungs for buoyancy on the surface if you become incapacitated, whatever the unit. You need to be fully buoyant by other means.

 

[St John the Diver]

Surely this statement is only true if the diver happens to lose or remove the DSV after exhaling into the unit. If the diver inhales from the unit immediately prior to losing the DSV or shouting to the boat or whatever, then it doesn't matter which type of counterlungs are installed as they are now empty (or nearly so). As soon as the diver exhales to atmosphere then the buoyancy due to lung volume is lost. Of course in the normal course of events the diver just needs to inhale air to restore buoyancy, but if they can't...

I would argue that it is quite likely that someone in a stressful or compromised situation may well inhale from the unit first. All the more reason to be properly buoyant and not reliant on the counterlungs at the surface, whatever unit you have.

It actually does. The difference is in that the actual scrubber bed will very rapidly flood on a BMCL unit while the entire plenum containing the scrubber stays dry in a OTS counterlung unit. That's an enormous difference in lost gas volume. Pop your DSV out if your mouth on a rEvo and you lose everything in both counterlungs and the scrubber beds. Do the same on, say... A Hammerhead and your can stays dry. Even though there's a scrubber in there, there's lots of open volume left dry and of course the actual scrubber bed has quite a bit of interstitial space.

Flood tolerance and lack of loss of all buoyancy go hand in hand. Lose all the gas and you both flood the scrubber bed and lose more buoyancy. Protect the scrubber bed and you both keep the powder dry and more than halve the lost buoyancy with the open DSV scenario. Good engineering plays in more than one way. The rEvo is about the worst rig built from a flood and buoyancy loss tolerance standpoint.

 

[RichardC]

. That's an enormous difference in lost gas volume. Pop your DSV out if your mouth on a rEvo and you lose everything in both counterlungs and the scrubber beds.

An enormous difference John? How many litres? Perhaps (being generous) one litre of gas out of a total of seven or eight. If you empty the counterlungs on any unit without alternative buoyancy you will sink fast if you exhale to air, period! If that were not the case how would you get down under normal circumstances?

Flooding is a red herring, which will only occur if you are already on the way down with your loop out, by which time you are in big trouble anyway, especially if you are incapacitated.

I was arguing that the empty lungs sinking scenario is quite possible on any unit and that sufficient alternative buoyancy is the only way to be sure of staying up if you become incapacitated.

I have noticed a tendancy to compare the rEvo without a gag strap with other units that have all safety measures in place, which is biased. This is like saying "if I defeat this safety device. it's not safe"; well Duh!

The point about the gag strap is that it is rEvo's solution to the flooding issue, above or below the surface. Rapid loss of buoyancy (and ultimately flooding) will not occur if your loop stays sealed in your mouth and a correctly fitted gag strap should ensure this. It is arguably safer than an OTS unit without a strap, where you can lose the loop when the lungs are empty.

I know it is suggested that in this incident a gag strap was not used, so in that scenario a safety device was defeated, which is a crucial point and any post about loss of loop buoyancy should be prefaced with that caveat, or other less well informed people (say lawyers or the press maybe) might drop into this thread and be misled.

 

[reefrat]

I have a Drager gag strap/safety mouthpiece on the way that I will be trying out with my explorer. It seems to me that a gag strap and a BOV should go together very well in practice although I still have no intention to remain on the loop on the surface. I get it that staying on the loop as long as possible after a deep/deco dive to breathe a high PO2 has benefits, but I'm not doing those dives and I suspect the average CCR RB diver also does plenty of recreational profile dives as well as the tech ones.
Staying on the loop on the surface, in my case and my opinion, is prolonging the risks of being on the loop at a time when the risks are heightened and there is no benefit. So as soon as I break the surface I fill my wing and lay back, close the loop (OC on the BOV) put the BOV under my chin and then put it back in my mouth when climbing the ladder.

 

[St John the Diver]

An enormous difference John? How many litres? Perhaps (being generous) one litre of gas out of a total of seven or eight. If you empty the counterlungs on any unit without alternative buoyancy you will sink fast if you exhale to air, period! If that were not the case how would you get down under normal circumstances?

Flooding is a red herring, which will only occur if you are already on the way down with your loop out, by which time you are in big trouble anyway, especially if you are incapacitated.

I was arguing that the empty lungs sinking scenario is quite possible on any unit and that sufficient alternative buoyancy is the only way to be sure of staying up if you become incapacitated.

I have noticed a tendancy to compare the rEvo without a gag strap with other units that have all safety measures in place, which is biased. This is like saying "if I defeat this safety device. it's not safe"; well Duh!

The point about the gag strap is that it is rEvo's solution to the flooding issue, above or below the surface. Rapid loss of buoyancy (and ultimately flooding) will not occur if your loop stays sealed in your mouth and a correctly fitted gag strap should ensure this. It is arguably safer than an OTS unit without a strap, where you can lose the loop when the lungs are empty.

I know it is suggested that in this incident a gag strap was not used, so in that scenario a safety device was defeated, which is a crucial point and any post about loss of loop buoyancy should be prefaced with that caveat, or other less well informed people (say lawyers or the press maybe) might drop into this thread and be misled.

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.

Pop your DSV out on any of the above rigs and hold the DSV over your head for a few seconds and you maintain that volume dry, and can recover the loop.

Do that with a rEvo and you lose all of the gas, flood the scrubber, and wet the sensors with cocktail.

Totally different design paradigm. Totally different outcome. On one system you only lose the counterlung gas. On the other you lose both the counterlung gas and all of the rest of the internal gas within the rigid area of the rug as well.

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.