Rebreather flood.

[Warren_L]

Any harm in just filling the unit, loop, et al, with clean, fresh, water to see how much it holds?

Not really I suppose, but I'd do it after my scrubber's been exhausted, or do it with the canister empty, but then you'd have to account somehow for the volume of the scrubber as well and how much water it can absorb. But I have an Evolution, which uses a smaller scrubber canister than the Inspo so I wouldn't be able to answer definitely for the Inspo.

 

[padiscubapro]

Very true, I'm sure there are many who dive with more gas in the loop than is required, however, in answer to the original question, I presumed it was asked in context of a properly configured rig, which of course, would be weighted for a diver using the rig properly. Regardless of tidal volume, it still seems buoyancy loss due to flood would still be in the 10 lb range (provided the 4 litre airspace assumption in the canister and hoses is accurate). Loss due to tidal volume is transitory unless one stops breathing altogether.

I agree with you on the "tidal" volume point, but I dont think having a few extra liters of gas in the loop diving it wrong.. Alot divers like to be able to take a deep breath at will, its not as effifcient in using oxygen but its generally safe (its still much less than the maximum capacity which wouldnt be safe)....

My guess is that on units with an ADV divers tend to fill the loop with a relatively deep breath, the breathe more shallower throught the dive, I dont think it would be off base to say that its probably 75% of the persons maximum inhale.. so in all liklihood probably an excess of ~2liters or so for most people is a good guess.. so about 4.5lbs additional loss of buoyancy.. now in the 14lb range..


Another thing to consider is at the time of failure, how a person's buoyancy is adjusted.. SOmeone doing an ascent might let the cl fill much more than normal, giving them the positive lift they want instead of in the wing (since now you can sdjust buoyance handless), if something caused a failure here there would be a large drop in buoyancy than "normal" this is where there is a potential danger..

I know in general, once I am going to start an ascent I usually have a bit more gas in my cl and a bit less in my wing so I have better control without having to use my hands to control buoyancy as often.. I know I just need to loosen my lips to vent excess gas.. I have better control this way, but I do have a slightly elevated risk of being really negative should something casue a catostrophic failure of the loop.

 

[Warren_L]

Some good points. I don't think that having more than minimum loop volume is necessarily wrong, but at some point, if more than a certain amount, it does start to become a problem. I do the same thing on ascent as naturally, the counterlung volume will begin to increase and I don't necessarily vent to the point where minimum loop volume is attained right away. But typically during a dive once I've reached depth, I'll try to stick reasonably close to minimum as possible. Usually I can gauge by taking a slightly deeper than normal breath until the point I feel the ADV starting to kick in.

 

[padiscubapro]

Some good points. I don't think that having more than minimum loop volume is necessarily wrong, but at some point, if more than a certain amount, it does start to become a problem. I do the same thing on ascent as naturally, the counterlung volume will begin to increase and I don't necessarily vent to the point where minimum loop volume is attained right away. But typically during a dive once I've reached depth, I'll try to stick reasonably close to minimum as possible. Usually I can gauge by taking a slightly deeper than normal breath until the point I feel the ADV starting to kick in.

Warren,
I have never liked the term minimum loop volume.. Its means so many different things to different people, and even the agencies that teach this term (ANDI doesnt use this term) can't make up their mind what it means

Take these 2 statements from Richard pyle

Richard Pyle said in response to a post By Gordon S. on Wednesday, July 26, 2000 4:16 PM
Put another way, I think most divers should generally maintain loop volume near
the minimum point such that a full inhale just barely triggers the ADV.

An excerpt of an article by Richard Pyle orignally published in
Proceedings of the Rebreather Forum 2.0. 26-28 September, 1996. Redondo Beach, CA, pp. P45-P67.
I define "minimum" loop volume as that volume of gas occupying the rebreather loop
when the counterlung(s) are completely "bottomed-out", and the diver has completely
exhaled the gas from his or her lungs. Conversely, "maximum" loop volume is the volume
of gas in the breathing loop when the counterlung(s) are maximally inflated, and the diver
has maximally inhaled gas into his or her lungs. Although the magnitude of the difference
between these two volumes, ([Vmax] [Vmin]), will vary from one rebreather design to another,
it will always be non-zero.

no wonder why so many people are confused.. I can find many more examples, but these are two that clearly show the difference how people perceive the term in the way it is applied.. One is really a good concept, and one is a clear definition of the term, and moth mean different things...

At ANDI we teach "Optimal" breathing loop volume, That is the point at which you can take a CONFORTABLE breath, This allows the smallest change in buoyancy when ascending, uses as little diluent gas as possible to make up the loop volume, wastes aslittle gas as possible when doing flushes, and uses the least amount of oxygen to maintain a specific po2..

 

[Warren_L]

The first definition is what I do - a full breath just starts to trigger the ADV.

 

[padiscubapro]

The first definition is what I do - a full breath just starts to trigger the ADV.

I take a "confortable" breath if I use an ADV... If I did a "full" breath (Or for that matter a slightly deeper inhale) I would trigger the adv..

Maintaining the po2 with this loop volume uses the least amount of oxygen (better control as well) and diluent....

 

[SilverSurfer7]

it would seem it would swing 21-26 lbs. if completely flooded.

Thats a huge swing. Not good.

 

[webhead]

Think of say an empty lift bag you might carry with you. At depth, if you fill it with water, will your buoyancy change? Same sort of thing.

:huh: If I have an empty lift bag and fill it with water, while underwater, the buoyancy doesn't change. Why would the water in the lift bag weigh more than the water outside the bag?

I think the key to this question is determining how much gas in the loop gets replaced with water. That volume of gas lost is lost buoyancy which can be significant. But just because you can hold 15 liters of volume in your CLs does not mean if you completely flood the unit that you will get 15 liters times the weight of water in negative buoyancy. You lose positive buoyancy equal to the amount of gas lost or water displaced.