One dead, one missing (since found), 300 foot dive - Lake Michigan

[Vicko]

Then I stand corrected, although I cannot understand why anyone doing that sort of diving would use a suboptimal scrubber design and survive. Trying to suck gas at 21Bar through 8" of 797 Sorb is extremely hard work. Even if you use the CD Grade coarse sorb the WOB is still very high.

Michael

As I understand it he uses it because he is afraid of co2 tuneling. A lot of people died at 150m + on radials due to some sort of scruber failiure (4 last year in europe?)

 

[cathal]

Proper scrubber design can do a lot to decrease WOB. An axial scrubber usually has the gas having to go through close to 8" of sorb. Even using an 8/92 heliox the resistance caused by that amount of sorb is considerable. Radial scrubbers present a much larger surface area and much shorter distances through the sorb, in my case about 1.75" causing a great reduction in WOB. Thats why all rebreathers designed for deep use use radial scrubbers exclusively. At 200' any scrubber (using enough Helium) will work ok, at 400+' axial scrubbers can cause serious problems even using Heliox, and at 660' I don't know of anyone surviving the use of a fully packed axial scrubber.

Michael

I reckon Gas Density plays a much greater role than scrubber design. A number of people have used the rEvo down to 650ft with no WOB issues. The He% in your mix is going to play a huge role in WOB at depth as Mitchell et al recent paper demonstrated, highlighting a direct correlation between WOB and incidents:

"One of the most important influences on work of breathing in diving is the increase in density of respired gas that occurs as depth increases. Since any underwater breathing apparatus will supply gas at ambient pressure, the density of the respired gas increases in direct proportion to depth. Increases in gas density result in a parallel increase in the resistance to flow of the gas through the diver's own airways, and in rebreather diving there is also the extra effort of moving dense gas through the hoses, connectors and CO2 scrubber of the unit. Under these circumstances, the associated increase in the work of breathing can be substantial."

"QinetiQ is a UBA testing house located near Portsmouth in the UK. Over some 20 years hundreds of manned test dives have been undertaken utilising ethics committee approved protocols which incorporate graded levels of underwater work for evaluating performance of a range open-circuit, semi-closed, and closed-circuit UBA. These dives have been conducted over depths ranging from 4 to 80 m (13 to 262 ft), using a range of gases including oxygen, air, nitrox and heliox. Throughout these tests a standard set of endpoints have been used to define 'dive failure' including: (any of) equipment or monitoring failure, diver unable or unwilling to continue because of dyspnoea (shortness of breath) or exhaustion, and an end-tidal CO2 >8.5 kPa (64 mm Hg) over five consecutive breaths. The latter is indicative of significant CO2 retention to a level associated with sudden incapacitation in the diving setting (Warkander et al. 1990)."

"there is a clear signal that near a respired gas density of 6.0 g·L-1 there is an upward inflection in the risk of dangerous CO2 retention during working rebreather dives. A similar analysis of dive failures in open-circuit underwater breathing apparatus trials produced a virtually identical result."

Comparing my Dil mixes before and after studying this paper I now use a lot more He% in line with their WOB recommendations:

"For the purposes of planning rebreather dives and in the current absence of more definitive or contradictory data, it seems prudent to recommend an ideal maximum gas density of 5.2 g·L-1 and an absolute maximum of 6.2 g·L-1."

Rgds


Cathal

 

[Dan]

Then I stand corrected, although I cannot understand why anyone doing that sort of diving would use a suboptimal scrubber design and survive. Trying to suck gas at 21Bar through 8" of 797 Sorb is extremely hard work. Even if you use the CD Grade coarse sorb the WOB is still very high.

Michael

Here is my guess (as a chemical engineer), cylindrical flow path is more uniformly utilized the bed with constant gas superficial velocity across the bed. While in radial flow path, although you would have shorter travel, the gas superficial velocity would drop as the circumference area increases with radius. The outer radius of the bed may not be fully utilized. It would be different if the torus (donut) shape is rotating where the superficial gas velocity would be in tangential direction as in Rotating Packed Bed process, ANDRITZ Rotating packed-bed centrifuge RPB (the process that I developed with Andritz).

Disadvantages of RPB is it requires power to rotate the bed and a seal between rotor and the housing (though a simple labyrinth type seal can be effectively used in this case). Advantages of RPB is very high absorption efficiency (up to 10 times more efficient than static bed) and the rotation acts as impeller of a blower, reducing WOB.

The RPB can be magnetically driven (mag-drive) by electric motor to avoid mechanical seal.

 

[Bobby]

Proper scrubber design can do a lot to decrease WOB. An axial scrubber usually has the gas having to go through close to 8" of sorb. Even using an 8/92 heliox the resistance caused by that amount of sorb is considerable. Radial scrubbers present a much larger surface area and much shorter distances through the sorb, in my case about 1.75" causing a great reduction in WOB. Thats why all rebreathers designed for deep use use radial scrubbers exclusively. At 200' any scrubber (using enough Helium) will work ok, at 400+' axial scrubbers can cause serious problems even using Heliox, and at 660' I don't know of anyone surviving the use of a fully packed axial scrubber.

Michael

Michael,
I can tell you, from personal experience, that this is not the case. I've made many dives below 600 feet with axial scrubbers. I've also over breathed a radial scrubber below 600 feet and had to bail out to clear CO2. The higher gas density at depth makes it easier to breath through the much shallower bedding of a radial, compared to an axial.

 

[Stoo]

Much less the temperature down there. Isn't the water temp near freezing at that depth there this time of year? Can you imagine have a free flow on your first stage or really anything important freeze up after 10 minutes at that depth?

The Lakes typically remain around 40° at depth year round. In the early season, it's normal that shallow water is colder than deep. Rebreathers are much less of a concern in cold water as far as freezing, but the life of the sorb is reduced.

 

[Johnoly]

DAN co-authors review other people's reports and the editor compiles them into pithy lessons learned.....

Not always.............In my humble opinion {for legal purposes}- DAN selectively chooses to bury detailed reports on highly public accidents. They were given PBC county medical examiner's report just weeks after the Skiiles RB accident and decided to bury it. {in my humble opinion}. I think DAN does a great job with what they release, but you are not getting the whole story.

 

[uwxplorer]

One piece of information that may or may not be relevant here is that the stock rEvo'ss are constant mass flow units, i.e. they bleed O2 at a constant rate, as long as the IP (which is fixed, not ambient pressure + delta) is over the ambient pressure by a set margin (I believe it is 2 atm but I may be wrong on that). Go deep enough and that flow stops, and not only that, but the solenoid or the manual addition valve cannot add any O2, since the IP is fixed at a constant value, which again, once it is equal or lower than the ambient pressure, prevents gas from exiting the first stage.
To dive deeper than that, the O2 first stage needs to be modified and reverted back to a differential pressure regulator, where the IP is above the ambient pressure at all time (by a constant amount). The modification is simple, and consist in replacing a metal face plate by a membrane. This makes the rebreather a purely electronic unit, where O2 is added by action of the solenoid only (or manual addition valve, if needed).
My unit has a set IP of 12 bar, and I believe it will not deliver O2 (at least the constant mass flow part) below 90 m (i.e 295 ft, 10 ata). This might differ from unit to unit.
I am assuming that the divers were aware of this (they were deep trimix certified, I assume) and had their units were modified to be purely electronic. A solenoid failure can then lead to rapid pO2 drop if not carefully monitored.
I am bringing this up, as the IP can creep (up or down) and I know at least one diver who experienced pO2 drop at 250 ft because its IP had decreased to a low value. He noticed that, tried a few things and decided to abort the dive. Had he not monitored his pO2 (assuming that the CMF and solenoid did their job faithfully), the end could have been much less innocuous...

 

[cathal]

One piece of information that may or may not be relevant here is that the stock rEvo'ss are constant mass flow units, i.e. they bleed O2 at a constant rate, as long as the IP (which is fixed, not ambient pressure + delta) is over the ambient pressure by a set margin (I believe it is 2 atm but I may be wrong on that). Go deep enough and that flow stops, and not only that, but the solenoid or the manual addition valve cannot add any O2, since the IP is fixed at a constant value, which again, once it is equal or lower than the ambient pressure, prevents gas from exiting the first stage.
To dive deeper than that, the O2 first stage needs to be modified and reverted back to a differential pressure regulator, where the IP is above the ambient pressure at all time (by a constant amount). The modification is simple, and consist in replacing a metal face plate by a membrane. This makes the rebreather a purely electronic unit, where O2 is added by action of the solenoid only (or manual addition valve, if needed).
My unit has a set IP of 12 bar, and I believe it will not deliver O2 (at least the constant mass flow part) below 90 m (i.e 295 ft, 10 ata). This might differ from unit to unit.
I am assuming that the divers were aware of this (they were deep trimix certified, I assume) and had their units were modified to be purely electronic. A solenoid failure can then lead to rapid pO2 drop if not carefully monitored.
I am bringing this up, as the IP can creep (up or down) and I know at least one diver who experienced pO2 drop at 250 ft because its IP had decreased to a low value. He noticed that, tried a few things and decided to abort the dive. Had he not monitored his pO2 (assuming that the CMF and solenoid did their job faithfully), the end could have been much less innocuous...

I believe your original hypothesis is more likely i.e. one of them got into difficulty and the other due to the emotional bond labored intensively to try save them. What the original difficulty was I don’t know.

 

[rjack321]

Not always.............In my humble opinion {for legal purposes}- DAN selectively chooses to bury detailed reports on highly public accidents. They were given PBC county medical examiner's report just weeks after the Skiiles RB accident and decided to bury it. {in my humble opinion}. I think DAN does a great job with what they release, but you are not getting the whole story.

Oh I agree with you that there's reporting bias
I was just saying they are not an investigatory agency

 

[rjack321]

My unit has a set IP of 12 bar, and I believe it will not deliver O2 (at least the constant mass flow part) below 90 m (i.e 295 ft, 10 ata). This might differ from unit to unit.

This is Revo's official position. The practical depth limit is 2 ata above the IP.

In reality O2 flows all the down to the IP, its just getting slower and slower the closer you get to the IP.

fO2 drop in the loop at 300ft is pretty slow though. Going hypoxic at 300ft because your IP "only" 180psi is going to take longer than your bottom time.