Rebreather designs and buoyancy control strategies

[oya]

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.

This is silly.

 

[oya]

unlike open circuit they aren't starting off heavy and then lightening constantly as they consume gas.

And if you bailout, but were perfectly neutrally buoyant with 5 full tanks?

I assume you deliberately flood the unit to add ballast? Or do you prefer to have to spend hours swimming straight downwards to maintain your stop depth?

 

[St John the Diver]

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?

Absolutely and "It certainly should be".

Most rebreathers these days are provided with appropriate wings. There are exceptions and I've seen some very inadequate wings installed on rigs in the field. You definitely need to consider the rig in a flooded state when picking what you're going to wear.

 

[St John the Diver]

And if you bailout, but were perfectly neutrally buoyant with 5 full tanks?

I assume you deliberately flood the unit to add ballast? Or do you prefer to have to spend hours swimming straight downwards to maintain your stop depth?

Having several (many) dozen practice sessions doing this (teaching Mod 3 trimix with full deco on bailout as a drill)....

Never been an issue. That's with an 80 and two 40's or two 80's. Just squeeze down the good old Weezle a bit in the suit and turn on the heat vest. Open the OPV on the counterlungs and vent them. No biggie.

Been there, done that. Many many times.

PS: deco bottles are cheap: feel free to snap them to the line and let them go when they are empty or of no value (deep mix when you reach a shallow stop) if you're fighting them.

There's more than one way to dive.

There was a diver who died on the Doria a while back who was wing-centric and not suit-centric for bouyancy. The scenario was that the stupid pull-knob on his lower wing exhaust valve got wrapped under his harness when he was strapping it and when he tightened his waist strap the wing dump was being pulled open. He jumped into the water and began faffing with a camera next to the boat (bad plan). Lost bouyancy and added wing gas. Lost more and added more. Did this until he ran out of diluent. Sank and not only couldn't add wing gas, he couldn't add diluent to his counterlungs. Couldn't breathe and apparently couldn't find his bailout regulator. Apparently didn't contemplate adding drysuit air or adding 02 to the loop to have volume to breathe.

He was already dead when he drifted down past past a friend of mine who was on his 40 foot stop.

Think.

In any event, this is a segue from the subject.

 

[mmadiver]

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.

Have to disagree with this. Why would you buoyancy compensate with a drysuit and loop? Relative to your wing, a drysuit is a massive, irregular shaped, gas space that is far more difficult to manage. Put minimum gas in your suit to avoid squeeze and be done with it. Similarly, you should have only the necessary amount of gas in your loop to maintain the best WOB and PPO2. Using your loop for buoyancy compensation would result in constant swings in your PPO2 - Again, dial in your loop to optimal WOB and PPO2 and be done with it, only adding gas to maintain.

 

[oya]

Have to disagree with this. Why would you buoyancy compensate with a drysuit and loop? Relative to your wing, a drysuit is a massive, irregular shaped, gas space that is far more difficult to manage. Put minimum gas in your suit to avoid squeeze and be done with it. Similarly, you should have only the necessary amount of gas in your loop to maintain the best WOB and PPO2. Using your loop for buoyancy compensation would result in constant swings in your PPO2 - Again, dial in your loop to optimal WOB and PPO2 and be done with it, only adding gas to maintain.

Nope. Totally wrong. Instead of adding the amount of gas required to offset buoyancy into an airspace specifically designed for that function you should instead use the same amount of gas by, evidently in part, way overinflating your loop.

Because a guy knew a guy once that screwed up using his wing without noticing.

Oh, also, buying a suit heater is mandatory. Because your thermal protection will no longer operate as thermal protection in this configuration.

 

[Wookie]

Having several (many) dozen practice sessions doing this (teaching Mod 3 trimix with full deco on bailout as a drill)....

Never been an issue. That's with an 80 and two 40's or two 80's. Just squeeze down the good old Weezle a bit in the suit and turn on the heat vest. Open the OPV on the counterlungs and vent them. No biggie.

Been there, done that. Many many times.

PS: deco bottles are cheap: feel free to snap them to the line and let them go when they are empty or of no value (deep mix when you reach a shallow stop) if you're fighting them.

There's more than one way to dive.

There was a diver who died on the Doria a while back who was wing-centric and not suit-centric for bouyancy. The scenario was that the stupid pull-knob on his lower wing exhaust valve got wrapped under his harness when he was strapping it and when he tightened his waist strap the wing dump was being pulled open. He jumped into the water and began faffing with a camera next to the boat (bad plan). Lost bouyancy and added wing gas. Lost more and added more. Did this until he ran out of diluent. Sank and not only couldn't add wing gas, he couldn't add diluent to his counterlungs. Couldn't breathe and apparently couldn't find his bailout regulator. Apparently didn't contemplate adding drysuit air or adding 02 to the loop to have volume to breathe.

He was already dead when he drifted down past past a friend of mine who was on his 40 foot stop.

Think.

In any event, this is a segue from the subject.

If you are discussing DeWolf, that wasn't what happened. It was the initial report, but not the final. Having dived with him a number of times, he didn't have a knob on his dump valve, like all good tech divers.

 

[St John the Diver]

Nope. Totally wrong. Instead of adding the amount of gas required to offset buoyancy into an airspace specifically designed for that function you should instead use the same amount of gas by, evidently in part, way overinflating your loop.

Because a guy knew a guy once that screwed up using his wing without noticing.

Oh, also, buying a suit heater is mandatory. Because your thermal protection will no longer operate as thermal protection in this configuration.

I'm not going to argue about technique with you. Proper weighting is part of it. At that point trimming out neutral is easy. You hardly notice a 2 pound bouyancy change in a counterlung set. And with some 500 dives in water colder than 35 degrees in the Arctic I'm not learning much about insulation here. I'm happy with my Weezle as it can be fluffed up full or compressed about 50% and still keep me toasty. And that represents enough bouyancy delta to make diving easy. A nice double 20 Amp Hour light monkey battery pack and a heat vest make it even better. One half of the battery for my light, the other for my vest, and if the vest gets cold on the hang after 2+ hours that other half of the battery is still almost new. Unplug and swap. Only did that a few hundred times.

You're 41 according to your profile? Just thinking: I was diving a rebreather when you were two years old and still sucking a binkie. You were in elementary school when I did my first Doria trip. Maybe, just maybe, I'm competent and maybe, just maybe, I've got something of interest to offer. Maybe you're too knowledgable to be able to learn anything. I dunno. I hope I never stop learning and thinking and revising things based on new information.

Duly noting that I'm not criticizing your techniques. I'd appreciate the same courtesy to be extended equally. You might actually learn something if you pay attention, and so might I.

That's why we are here. To share, and learn.

There's more than one way to do things. Open minds learn more. Dogmatic approaches stifle life. Be open.

Use your wings, use whatever works. The point to be taken away from this is that it's easy to think you're positive on the surface with a RB and then to screw the pooch when you forget to close your DSV. If just one guy learned that by reading this thread it might be one life saved. That feels like a good days work for me.

 

[mmadiver]

I'm not going to argue about technique with you. Proper weighting is part of it. At that point trimming out neutral is easy. You hardly notice a 2 pound bouyancy change in a counterlung set. And with some 500 dives in water colder than 35 degrees in the Arctic I'm not learning much about insulation here. I'm happy with my Weezle as it can be fluffed up full or compressed about 50% and still keep me toasty. And that represents enough bouyancy delta to make diving easy. A nice double 20 Amp Hour light monkey battery pack and a heat vest make it even better. One half of the battery for my light, the other for my vest, and if the vest gets cold on the hang after 2+ hours that other half of the battery is still almost new. Unplug and swap. Only did that a few hundred times.

This made me laugh...

Also, to anybody still reading this thread, opening your DSV on the surface doesn't result in some massive catastrophic event where you suddenly plunge below the surface to your death - as it has been made to seem in this thread. In fact it's really not an issue if you are diving your equipment as intended and not relying on your loop volume for buoyancy as St John the Diver advocates.

 

[St John the Diver]

This made me laugh...

Also, to anybody still reading this thread, opening your DSV on the surface doesn't result in some massive catastrophic event where you suddenly plunge below the surface to your death - as it has been made to seem in this thread. In fact it's really not an issue if you are diving your equipment as intended and not relying on your loop volume for buoyancy as St John the Diver advocates.

I'm not "advocating" anything.

I'm OBSERVING that based on watching MANY divers, that a marginally positively bouyant rebreather diver (using whatever he is using for bouyancy control) WILL lose significant bouyancy if he removes an open DSV, this happening faster on internal back mounted counterlung rigs than on OTS counterlung rigs.

That's not an opinion it's a fact of physics.

The result will be based on whatever excess positive bouyancy when he does that, and may vary from "no big deal" to "doing the dead fly". It may be remedied by a bit of finning, hitting the BC inflator, drysuit button, or may result in sibling to the bottom. Actual mileage may vary.

I would also add that if you've done this with an OTS counterlung rig, you probably don't really "get" how different it is with an internal BMCL rig. It's vastly different. As I learned some 35 years ago on a Mark-15 when I did it. ;-). After diving the OTS lung USN Mark-XI it was eye opening.