Rebreather flood.

[padiscubapro]

ONe thing to keep in mind is that you should not be basing this calculation on counterlung volume, but rather loop volume. When diving rebreathers in general, you want do dive with what we call minimum loop volume, which is essentially your lung volume (which is part of the loop), empty (or near empty during inhilation) counterlungs, and non-compressible air spaces (hoses, scubber canister, etc). So factoring full counterlung volume is not that really relevant.

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.

what is a reasonable estimate for someone is about 4l of cl volume and 4l of dead space or about an 18lb swing.. newer RB divers tend to have more volume in their lungs than an experienced diver, From observation and experience howmuch weight a RB diver drops from his rig, double that of an experienced diver is a reasonable estimate (8 l instead of 4, so 12lin total or about 27 lbs)

 

[Warren_L]

But keep in mind also that the tidal volume is typically 0.5l (or maybe a bit more), and unless the diver has drowned and inspired water and lost the rest of the lung volume, only the excess air in the counterlungs would play a factor in the event of a flood. Assume for the moment that we are talking about a reasonably experienced RB diver diving with minimum loop volume. If the diver is neutral and a flood occurs, counterlungs full of water will not contribute at all to any loss of buoyancy. And for an inexperienced RB diver, even if he were to be holding an excess volume equal to his tidal volume in the counterlungs (say approximately 0.5 litres), this would only contribute about a loss of 1 lb in buoyancy. And if the other dead air spaces are as you say, 4 litres, completely flooded you would lose another 9 lbs. So I cannot really see more than a 10 lb loss in buoyancy with a complete flood (assuming the diver has not drowned with lungs full of water).

 

[Warren_L]

If we consider the air spaces within the breathing loop as 3 parts, it may be easier to visualize what might happen during a flood. So, if we consider a normal diver, weighted to be neutral, no flood, assuming tidal volume of 0.5 litres:

During inhale:
Scrubber, hoses, and other non-compressible air spaces: 4 litres (8.9 lbs buoyancy)
Counterlungs: 0 litres (0 lbs buoyancy)
Diver's lungs: 4 litres (8.9 lbs buoyancy)

During exhale:
Scrubber, hoses, and other non-compressible air spaces: 4 litres (9 lbs buoyancy)
Counterlungs: 0.5 litres (1.1 lbs buoyancy)
Diver's lungs: 3.5 litres (7.8 lbs buoyancy)

So now, consider a loop flood.

During inhale, consider loss of gas and buoyancy change:
Scrubber, hoses, and other non-compressible air spaces: -4 litres (-8.9 lbs buoyancy)
Counterlungs: 0 litres (0 lbs buoyancy)
Diver's lungs: 4 litres (8.9 lbs buoyancy)
Net shift in buoyancy: -8.9 lbs

During exhale, consider loss of gas and buoyancy change:
Scrubber, hoses, and other non-compressible air spaces: -4 litres (-8.9 lbs buoyancy)
Counterlungs: -0.5 litres (-1.1 lbs buoyancy)
Diver's lungs: 3.5 litres (7.8 lbs buoyancy)
Net shift in buoyancy: -10.0 lbs

But note in the situation when flood occurs during exhale, as soon as the diver inhales again, the buoyancy loss due to tidal volume is restored.

So am I missing something here?

 

[caveseeker7]

During inhale:
Diver's lungs: 4 litres (8.9 lbs buoyancy)

During exhale:
Diver's lungs: 3.5 litres (7.8 lbs buoyancy)

So am I missing something here?

Yes, you're breathing very, very shallow. 0.5 litres per breath?
Hypercapnia would bite me in the arse so fast it would make my head spin.
I move more than 3 litres, and that's not a really deep drawn breath.
So Joe's assesment of 4 litres gas in the bags seems about right, at least for me.

 

[Warren_L]

I'm no physiologist, but doing a quick google search, typical tidal volumes are around 0.5 litres for an adult male for a normal breath. So all this would hinge really on what the tidal volume would be. 4 litres seems pretty high seeing as total lung volume is typically around 5 litres. So even with a deep exhale, that 0.5 litres getting up to 4.0 litres is quite a change. That's almost a gallon of gas.

BTW, how do you know what your tidal volume is? Did you measure it at 3 litres somehow?

 

[caveseeker7]

BTW, how do you know what your tidal volume is? Did you measure it at 3 litres somehow?

Yes, after you mentioned the tidal volume, it just seemed to be too low.

Placed a bucket of water in a small tub, the former full with water.
Breathed normally as I do when I dive (which is a bit deeper than I usually do as I try to vent my lungs well) a couple of times, then into a thin plastic bag. Submersed the bag, collected water in tub, then measured it.

What do you think is the reason for 5,7 ltr and 7 ltr counterlungs if the tidal volume is only 0.5 litre?
AP isn't the only manufacturer with a large sized loop, the PRISM specs say aproximately 8 ltr, the SM1600 (MK15.5) 8.5 ltr, all seem to sugest that one should move more than 0.5 ltr/breath.

 

[Warren_L]

I agree, I think my tidal volume might normally be in the 1 - 1.5 litre range. But my point is, even with a higher tidal volume, say 3 litres in your situation, this would add a drop in buoyancy of about another 7 lbs should you experience a flood. So adding to the -9 lbs in the example, still this is only -16 lbs. It's a far cry from the -25lbs or -40 lbs that have been mentioned before. And I'm not referring to other rebreathers, as the numbers given here (I didn't come up with 4 litres for the scrubber and other non-compressible air spaces, someone else mentioned it here) are for the inspiration. As well, that loss of buoyancy is only temporary too until you take your next breath (unless of course, you've drowned and inspired 3 litres of water).

 

[padiscubapro]

I agree, I think my tidal volume might normally be in the 1 - 1.5 litre range. But my point is, even with a higher tidal volume, say 3 litres in your situation, this would add a drop in buoyancy of about another 7 lbs should you experience a flood. So adding to the -9 lbs in the example, still this is only -16 lbs. It's a far cry from the -25lbs or -40 lbs that have been mentioned before. And I'm not referring to other rebreathers, as the numbers given here (I didn't come up with 4 litres for the scrubber and other non-compressible air spaces, someone else mentioned it here) are for the inspiration. As well, that loss of buoyancy is only temporary too until you take your next breath (unless of course, you've drowned and inspired 3 litres of water).

Warren you are forgetting to take one thing into account, alot of RB divers tend to have much to much gas in the loop and weight thmeselves baded on this... Especially when new...

I remember my FIRST rb dive, it was in fresh water in a drysuit.. I needed 30 lbs of led to sink... I steadily dropped weight as I learned to control the loop volume better..

in a short time I was diveing with 6-8 lbs of lead (mostly for trim)..

MY normal breathing pattern when diving is 3l+ I never measured tha actual volume but based on my dives with a Sport Kiss, where many divers can get away with the restrictor plate in the middle setting (cutting the volume by about half), I would need to get gas from the adv on every inhale and vent the OPV on every exhale..

When I am working really hard, I am near the limits of the breathing bags at their maximum setting..

 

[Twomixdiver]

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

 

[Warren_L]

Warren you are forgetting to take one thing into account, alot of RB divers tend to have much to much gas in the loop and weight thmeselves baded on this... Especially when new...

I remember my FIRST rb dive, it was in fresh water in a drysuit.. I needed 30 lbs of led to sink... I steadily dropped weight as I learned to control the loop volume better..

in a short time I was diveing with 6-8 lbs of lead (mostly for trim)..

MY normal breathing pattern when diving is 3l+ I never measured tha actual volume but based on my dives with a Sport Kiss, where many divers can get away with the restrictor plate in the middle setting (cutting the volume by about half), I would need to get gas from the adv on every inhale and vent the OPV on every exhale..

When I am working really hard, I am near the limits of the breathing bags at their maximum setting..

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.