What is your IWR Kit Comprised of?

[Akimbo]

I agree. Deco on a rebreather needs diligence. Plentiful compressed o2 would be better.

I concur. Rebreathers also complicates switching to air during a convulsion unless you have a BOV. Dragging a convulsing diver to the surface sucks less than letting them drown or suffer, but switching to air and waiting less than a minute for symptoms to subside makes a LOT more sense than risking a blocked airway and embolism. There is an advantage in cold water though.

I have designed and built pure O2 rebreathers strictly for decompression where commercial Oxygen supplies were limited if anyone is interested. There is much less that can go wrong with them -- no electronics, water seperators to limit caustic cocktails, fixed vertical absorbent canister that dramatically limits channeling, and seriously rugged.

Speaking of cold water and respiratory heat loss, is anyone interested in counter-current heat exchangers that captures heat from exhalations to preheat inhalation gas? We used passive gas heaters like this in sat diving until active (hot water) heat exchangers swamped the concept.

 

[JohnnyC]

I'd be curious to see what you came up with Akimbo.

As far as going hypoxic and having to vent built up inert gas, etc., those are all contingent on the diver being unable to accomplish a simple loop vent/add cycle. You'd have to be near unconscious in order to fail at that. It's simple enough to vent and hit the O2 MAV. You're not doing IWR with an unconscious diver, and if you're at the point where you're doing IWR with a diver, chances are they're a diver who is able to accomplish these pretty trivial tasks. If the alternative is dead because you can't recompress, or risk a diver venting a loop on an O2 rebreather, it's a pretty easy choice.

I'd rather do IWR on an O2 rebreather than risk running out of OC O2 in an austere environment and having to do a partial IWR and subsequent rapid ascent due to an OOG scenario.

 

[Akimbo]

I'd be curious to see what you came up with Akimbo.

Let me see what I can dig up from old CAD drawing files. It was just 2D in those days. I think there is a schematic block diagram-like concept drawing I can convert. I came close to building one for a trip to Truk a few years ago.

As far as going hypoxic and having to vent built up inert gas, etc., those are all contingent on the diver being unable to accomplish a simple loop vent/add cycle.

I don't understand. IF you follow normal procedure and purge a pure Oxygen rebreather on the surface (before suspending it over the side), you can't go hypoxic. Where would the inert gas come from beyond the minute volume outgassing from dissolved tissues?

I'd rather do IWR on an O2 rebreather than risk running out of OC O2 in an austere environment and having to do a partial IWR and subsequent rapid ascent due to an OOG scenario.

Agreed. The first one I built was for inside a decompression chamber on a boat that would be weeks away from a gas supplier. Open-circuit is definitely the way to go when the patient isn't rebreather-savvy though. It's all about logistics and compromise.

 

[JohnnyC]

Yeah, I don't understand where the whole "hypoxia, loop full of inert gas" thing came from in that post. And
"only 85% even with a perfect purge" is just untrue. I easily get 99% on any of the rebreathers I've used within 3 purge cycles.

You'd have to really screw up for the loop to get hypoxic anywhere other than the surface with zero purge cycles. (Hence why the manual, Navy Rev. 7 and Dräger, stresses the need to purge the unit before use)

 

[mmadiver]

Yeah, I don't understand where the whole "hypoxia, loop full of inert gas" thing came from in that post. And
"only 85% even with a perfect purge" is just untrue. I easily get 99% on any of the rebreathers I've used within 3 purge cycles.

You'd have to really screw up for the loop to get hypoxic anywhere other than the surface with zero purge cycles. (Hence why the manual, Navy Rev. 7 and Dräger, stresses the need to purge the unit before use)

Not untrue at all. The basis for my assertion is in fact the very Navy regs you're citing - I am (was) a Navy trained diver and have spent many hours on the Drager. I forget the exact numbers (I'm happy to reach out to some friends at NEDU if need be) but even after a perfect purge sequence you will off gas inert gas into the loop over time. What NEDU determined in Drager trials is that combat swimmers were actually breathing around 85-90% (if memory serves) O2 because of off gassing - assuming you operate completely bubble less and never vent the loop, which is always the objective for a combat swimmer. Your making an assumption that many mixed gas rebreather divers make when transitioning to a pure O2 rebreather. Remember, on a -purely- O2 rebreather like the Drager there are no galvanic sensors and you have no way of knowing what your actual PPO2 is, which under normal operating parameters doesn't matter as long as you do a good purge AND as per the citing Navy manual you DO NOT conduct an O2 rebreather dive after ANY type of decompression dive.

To understand why, lets conduct a thought experiment: You surface from a deco dive with a signifigant on-load of inert gas having omitted deco. If you descend again in an O2 CCR (using the Drager for this example) you will continue to off gas into the loop. O2 is added into the Drager one of two ways, via an ADV or manually. Typically divers just rely on the ADV to maintain the loop volume - as the diver metabolizes the O2 in the loop, the volume shrinks, the bag hits the ADV and O2 is added. However, if your loop is full of inert gas due to a poor purge or off gassing from a previous dive you may well maintain a completely acceptable loop volume but be hypoxic because you've metabolized all the O2; and since you have no O2 sensors you have no way of knowing it.

So in short, never dive an O2 rebreather (without galvanic cells) after any other type of mixed gas diving, including air, and certainly NEVER use them for IWR. I suppose if you absolutely had to use one for IWR I would breathe it like an SCR - five breath cycles, vent the loop, five breathe cycles, vent...

EDIT: I've been out of the Navy for awhile and it looks like they have instituted some new guidelines for no-deco Mixed gas to O2 rebreather diving to facilitate SDV operations...All of the above is still true for decompression diving however.

 

[JohnnyC]

Couple things. People deco on O2 rebreathers after mixed gas diving ALL THE TIME. Many many many more hours are spent running CCR's as O2 rebreathers on deco than NEDU has ever tested. In fact, I'd be willing to bet that divers spend more time breathing a loop FO2 of 100% in one year, than in all of the NEDU's testing since WWII. Many people have dedicated O2 decompression rebreathers, often civilian analogs to the Dräger units. The fact is that civilian divers spend an order of magnitude more time on the loop than anybody in the Navy does, both through sheer number of divers, and amount of time spent per dive. They're not dropping dead from switching to O2 only rebreathers on decompression.

The NEDU studies you're talking about, produced by Butler and Thalmann during the 80's-2000's along with Harabin and others? You're misunderstanding what those studies were about, what their conclusions were, and how procedures were adjusted. First, all of the purge cycles during initial testing were required to hit a MINIMUM of 95%. Second, post-1983 there were some recommendations made to purge to a lower FO2 that would only be high enough to prevent hypoxia, in order to prevent oxygen toxicity events due to long high PO2 exposures. The finding was that by dropping the FO2 from "nearly pure" to 74% decreased the O2 toxicity levels for an 80min, 30ft dive from 4% to 0.1%. Thirdly, another study found that an FO2 of 82%, what equates to an inadequate purge, still did not require any additional purging and addition to prevent a hypoxic loop. Lastly, the 30 minute purge cycles to prevent dilutional hypoxia have no basis in fact. It was a holdover from the WWII OSS days and its pedigree is obscure at best. Your "inert gas expanding loop volume of death" theory doesn't hold water. The additional volume is minimal. Procedural recommendations were changed because dilutional hypoxia doesn't exist in the volume you think it does. In fact, the procedure was changed so that there would be zero potential for bubble venting. This was requested by a NSW corpsman btw. If what you assert was true, this change in procedure would be impossible as loop volume increases would necessitate purging the loop regularly.

The vast majority of that has little to do with using an O2 rebreather for IWR, but it does disprove your arguments against their use.

A properly purged rebreather will hit purity of drive gas. Every time. How do I know? Because 1) I've done it, a lot, and so do many other guys on a regular basis, 2) the US Navy purge procedures call for a minimum 95% O2 purge, which in actuality results in a higher FO2 (of course there's always one guy that screws it up.) And 3) read any rebreather manual and look at calibration procedures for units that aren't capable of calibrating with a head-only type of kit. You literally purge the unit until there is nothing but O2 in the loop in order to affect an O2 calibration. I've personally plugged in offboard O2 into an ADV and run deco completely free of electronics with just ADV addition of O2, exactly like any O2 rebreather.

You don't need galvanic oxygen sensors to obtain a loop FO2 of 100%, and it's easily within the realm of literally every diver to accomplish a loop FO2 of 100% every time. I could take any CCR on the market, O2 or mixed gas, and guarantee you a full O2 purge. It's a simple thing to do. Maintaining that over long decompression periods is just as trivial (we do it all the time), and a diver doing IWR is more than capable of doing so as long as they're conscious. If your assertions were even close to a concern, a simple purge cycle would quickly remedy any issues, and even the newest of divers are capable of that.

Listen to Akimbo. I'm just a guy on the internet. He was a Navy diver. A true navy diver, not just a combat swimmer. You don't need to believe me however, his pedigree far eclipses mine. Read FK Butler's "Closed-circuit diving in the U.S. Navy" if you're interested in a brief synopsis of all of the information. It's easier than individually quoting the studies. If you have verifiable information in rebuttal I'd like to hear it, but I have a hard time believing that your information is better than that which comes directly from the guys that actually ran all of the studies for the NEDU.

The discussion is about IWR in austere environments where a trip to a chamber is impossible. So in short, an O2 rebreather is a perfect application, especially paired with a full-face mask and BOV connected to an off-board low FO2 break gas to mitigate the risks of oxygen toxicity. Is OC easier? Sure, but that's not what's in question.

 

[ajduplessis]

Plenty O2 and FFM

 

[tbone1004]

@mmadiver the implication there is that the volume of the inert gas you will be expelling exceeds that of the O2 that you are metabolizing. Obviously a combat swimmer who is moving will consume infinitely more O2 than someone who is doing IWR, but I don't believe the volume of inert gas that you can really offgas would exceed that of the loop volume unless it was pretty extreme circumstances.
Simple solution is to just do a loop flush.
If you dive a CCR properly, and you subscribe to air breaks, which you should, then you park the CCR with no gas in the loop and switch to air/nitrox/whatever for 5 minutes. When you go to come back on the loop, you will pretty much have to do a loop flush procedure to get the ppO2 back up, so that's full exhale of what's in your lungs, full inhale from O2 mav/adv, and repeat however many times you choose, but 3x should be enough. You're not going to go hypoxic in 20 minutes when you aren't moving, even with inert gas coming out.

Obviously your experience as a combat diver where you can't make any bubbles at all is different, but for IWR it's some variant on 20 on, 5 off, and if you stay on the loop, you have to flush, if you get off the loop, you still have to flush

 

[Akimbo]

I forget the exact numbers (I'm happy to reach out to some friends at NEDU if need be) but even after a perfect purge sequence you will off gas inert gas into the loop over time. What NEDU determined in Drager trials is that combat swimmers were actually breathing around 85-90% (if memory serves) O2 because of off gassing - assuming you operate completely bubble less and never vent the loop, which is always the objective for a combat swimmer.

Good point; but in my experience it takes a lot of effort, skill, and motivation (like not being detected by people that want to kill you) to keep the system that tight. We did periodic informal checks with hand-held O2 monitors and found that it stayed at 97%+ during chamber Sur-D-O2 runs -- where outgassing would be higher than during an IWR. However, you make an important point. I do believe that a bag purge during an IWR using a "dumb" O2 rebreather is worth suggesting. A little testing can easily confirm what is adequate without the complexity and cost of adding Oxygen monitoring.

I suspect that everyone agrees that open circuit O2 for IWR is preferable, especially considering that the convulsion risk is higher (on most IWR tables). The logistics of operating in remote locations where their generating systems can only produce around 95% O2 and charge $2+/Ft³ is a factor that favors dumb O2 rebreathers, even if limited for decompression and IWR.

 

[mmadiver]

Couple things.

Ok, a lot to unpack here. First by way of Bona Fides to substantiate my position. I'm actually a Navy EOD and Diving Officer and still work in that capacity as a reservist, in fact just last month I was at NEDU... So, not "just a combat diver" as you say - and why the tone BTW?

To ensure we've got a foundation for this discussion can you describe your experience? The framing of some of your comments (below) leads me to believe you are a military trained diver?

This was requested by a NSW corpsman btw. he US Navy purge procedures call for a minimum 95% O2 purge, which in actuality results in a higher FO2 (of course there's always one guy that screws it up.)

People deco on O2 rebreathers after mixed gas diving ALL THE TIME.

Who? and what O2 CCR are they using? I'm also an avid technical CCR diver and have dove around the globe and participated in some expeditions (albeit relatively basic ones) and have never seen it done or advocated for - and certainly not in the military.

Your "inert gas expanding loop volume of death" theory doesn't hold water. The additional volume is minimal. Procedural recommendations were changed because dilutional hypoxia doesn't exist in the volume you think it does. In fact, the procedure was changed so that there would be zero potential for bubble venting. This was requested by a NSW corpsman btw. If what you assert was true, this change in procedure would be impossible as loop volume increases would necessitate purging the loop regularly.

Not quite, as your are not comparing apples to apples here. Transitioning to an O2 rebreather from ambient surface conditions or in the case of SDV, with minor on-gas from MK16 exposure is going to result in a small but measurable quantity of inert gas in the loop. That quantity has been determined to be negligible assuming a proper 2-min purge. However, what we are discussing is using an O2 rebreather following a decompression dive. There are no tables to determine how much you are going to offgas into the loop following a decompression dive and you have no way of knowing if you are getting into a hypoxic situation on a dumb O2 rebreather. A personal vignette to illustrate the point - I was conducting a Drager/combat swimmer dive and at about 25mins into the dive my buddy went unconscious from hypoxia. We had stopped, gave each other the OK, I signaled for a peek and ascended to the surface to shoot a bearing, when I descended about 20 seconds later he was unconscious. The CCR was working fine and he had adequate loop volume, and O2 was tested and determined good - How is it then someone can go unconscious on a pure O2 CCR dive 25 minutes in?? If it was a bad purge it would of got him immediately, right? He was subsurface for the entire 25 minutes. Mull that over and I'd like to hear your thoughts on how that could happen, then Ill explain our findings.

A properly purged rebreather will hit purity of drive gas. Every time. How do I know? Because 1) I've done it, a lot, and so do many other guys on a regular basis, 2) the US Navy purge procedures call for a minimum 95% O2 purge, which in actuality results in a higher FO2 (of course there's always one guy that screws it up.) And 3) read any rebreather manual and look at calibration procedures for units that aren't capable of calibrating with a head-only type of kit. You literally purge the unit until there is nothing but O2 in the loop in order to affect an O2 calibration. I've personally plugged in offboard O2 into an ADV and run deco completely free of electronics with just ADV addition of O2, exactly like any O2 rebreather.

Have you dove an true O2 rebreather, are you certified on any? Your applying your experience on mixed gas CCRs to pure O2 rebreathers and they are not the same - even if you are using a mixed gas CCR as an O2 rebreather there are some subtle differences between it an a true O2 rig like the MK25. Most notably is the counter lung size...the drager loop volume is tiny to accommodate breathing it down (metabolizing all the O2 in the loop) so that you can ascend bubble less. When you dive a drager you truly do keep it at a minimum loop volume, not even a full lung load, and are always on the edge of activating the ADV - if you don't the WOB is crap, especially when vertical. Having such a small loop volume is great, except when you introduce inert gas (off gassing) into that small quantity and then it becomes a game of percentages. That small pocket of inert gas may be just enough to keep the volume adequate and as you metabolize the remaining O2 can quickly run into an issue.

You don't need galvanic oxygen sensors to obtain a loop FO2 of 100%, and it's easily within the realm of literally every diver to accomplish a loop FO2 of 100% every time. I could take any CCR on the market, O2 or mixed gas, and guarantee you a full O2 purge. It's a simple thing to do. Maintaining that over long decompression periods is just as trivial (we do it all the time), and a diver doing IWR is more than capable of doing so as long as they're conscious. If your assertions were even close to a concern, a simple purge cycle would quickly remedy any issues, and even the newest of divers are capable of that.

Yes, flushing a rebreather with 100% O2 is easy. However, as you breath it how do you know what your PPO2 actually is if you are off gassing into the rig? Assuming you've done deco, I'm sure you've noticed how difficult it can be to maintain a perfect 1.6 at 20ft on a CCR - in fact it's impossible without regular flushing of the loop. Which is precisely what I advocated for in my above post if you are going to use a O2 CCR with no O2 monitor for IWR.