Increasing PPO2 wet?

[Akimbo]

Does the current crop of eCCRs let you change PPO2 during the dive? Say you wanted to conservative 1.2 ATA deep mix for heavy work and an aggressive 1.8 ATA for decompression? If done manually in the water, what safeguards are there to prevent accidental changes? Can a dive profile can be pre-programmed? Can you set PPO2 by depth or are they smart enough to know you are on a decompression schedule?

If you can increase PPO2 during a dive, what technique is used to purge diluent gas?

Thanks in advance.

 

[DwayneJ]

I'm recently certified L2 CCR but I would say you need to think very differently about dive planning on a rebreather than open circuit...

Your bottom time is limited likely by Oxygen CNS clock first and by scrubber second. Running a high PPO2 at any stage during the dive will quickly burn through your CNS clock. As an example, you will reach 100% Oxygen CNS clock at PPO 1.3 in 180 minutes at whatever recreational depth yet (my) scrubber is good for 240 minutes.

CCR's combine PPO2 monitors with integrated HUD's to monitor PPO2.

My training affirmed PPO2 1.6 or higher is "warning level" to be breathed down to 1.3 and 2.0 is dangerous/tox event imminent.

Dwayne

 

[Mr.X]

Does the current crop of eCCRs let you change PPO2 during the dive? Say you wanted to conservative 1.2 ATA deep mix for heavy work and an aggressive 1.8 ATA for decompression? If done manually in the water, what safeguards are there to prevent accidental changes? Can a dive profile can be pre-programmed? Can you set PPO2 by depth or are they smart enough to know you are on a decompression schedule?

If you can increase PPO2 during a dive, what technique is used to purge diluent gas?

Thanks in advance.

Are you thinking about diving a eccr? With all my eccr's I fly the things manually vs. automatically. There are so many controllers out there I can't figure out what they all do, or what redundancies/safeguards they have. By going manual, and using what I know about the clock/limits I can design my own best dive. I also carry tables with me on wetnotes, and use redundant deco systems during a longish dive - extra computer and eccr -built in tables (Megalodon/Shearwater)

I also do not trust little teeny, tiny IC chips vs. my brain which is usually a better controller.

X

 

[Akimbo]

There are a considerable number of tables used in commercial diving for decompression at rest in the water well above 1.6, let alone 1.4, which predate recreational guidelines. Workload is a critical factor in Oxygen toxicity, as is physical condition. There is some evidence that frequent exposure is also an important factor. Thus far, I have not found any eCCRs that allow automatic settings to go above 1.4. I am still unclear if different PPO2s can be set for decompression or profiles programmed.

I also have not found a procedure for increasing PPO2 post ascent to decompression stops other than bleeding gas off manually (squeezing bags or blowing gas out your nose after setting a higher PPO2). Ideally, you would want to bump PPO2 up after your first or second deep stop so there is time to slow the physiology down, assuming you are after a lower PPO2 on the bottom due to anticipated work load.

I have heard that there have some military tweaks that supported this capability, but have not been able to find anything commercially here or in Europe so far.

 

[Mr.X]

There are a considerable number of tables used in commercial diving for decompression at rest in the water well above 1.6, let alone 1.4, which predate recreational guidelines. Workload is a critical factor in Oxygen toxicity, as is physical condition. There is some evidence that frequent exposure is also an important factor. Thus far, I have not found any eCCRs that allow automatic settings to go above 1.4. I am still unclear if different PPO2s can be set for decompression or profiles programmed.

I also have not found a procedure for increasing PPO2 post ascent to decompression stops other than bleeding gas off manually (squeezing bags or blowing gas out your nose after setting a higher PPO2). Ideally, you would want to bump PPO2 up after your first or second deep stop so there is time to slow the physiology down, assuming you are after a lower PPO2 on the bottom due to anticipated work load.

I have heard that there have some military tweaks that supported this capability, but have not been able to find anything commercially here or in Europe so far.

You can manually tweak the ppo2 in some of the eccr's to go beyond 1.6. with all sorts of irritating alarms, HUD displays going off - of course dependent on the ambient pressure at depth. With my KISS I could tweak it almost anyway I wanted as the handsets are simple meters and didn't give you hullabaloo. Just sit on the injection valve.

With the tech/recreational breathers - I think it's a simple issue of compliance/liability not to make a adjustable controller that if used improperly would kill you with high PPO2. I am thinking that the Inspiration and Meg had a limit of something like 1.4? I'd have to look. I can't remember what the original Cis Lunar MK 4P controller did. Nor, the Carleton/BioMarine MK 15 - seemed like 1.3 with + , or - 1 indicators.

X

 

[DwayneJ]

There are a considerable number of tables used in commercial diving for decompression at rest in the water well above 1.6, let alone 1.4, which predate recreational guidelines. Workload is a critical factor in Oxygen toxicity, as is physical condition.

Unlike open circuit with travel mixes and bottom gas which builds to a max ppo2 value as you descend, a ccr is running at a optimum ppo2 for most of the dive - PPO2 1.3 at 60', at 90', at 120' and all the way back to the surface until you can be 100% O2/PPO2 1.0 at the surface.

NOAA (not a recreational agency) tables state a constant ppo2 of 1.4 would give you 150 minutes max CNS exposure and a constant ppo2 of 1.6 45 minutes max exposure - As a real world example, my 70 minute dive at 70' was fine at PPO2 1.3 with no deco obligation. Bouncing ppo2 around between 1.4 and 1.6 would give you somewhere between 45 and 150 minutes max exposure although a computer with ppo2 monitoring could track this. If I had ran with a higher PPO2 than, I could have exposed myself to a possible tox event.

If a ccr gives you incredible depth and bottom time without (and with) deco because of optimal use of PPO2, why would you want to screw around with oxygen toxicity given a CNS Tox event at depth is most likely fatal?

Dwayne

 

[Akimbo]

Unlike open circuit with travel mixes and bottom gas which builds to a max ppo2 value as you descend, a ccr is running at a optimum ppo2 for most of the dive - PPO2 1.3 at 60', at 90', at 120' and all the way back to the surface until you can be 100% O2/PPO2 1.0 at the surface.

NOAA tables state a constant ppo2 of 1.4 would give you 150 minutes max CNS exposure and a constant ppo2 of 1.6 45 minutes max exposure - …

I fully understand that eCCR sensors measure (and therefore control) PPO2 rather than percentage of the mix. Your definition of optimum is far too limited. It has long been recognized that optimum PPO2 is relative to work load and environmental conditions. For simplification and legal liability, recreational organizations have settled on a very, if not overly, simplified model.

Some of the eCCRs I have found in my research since the original post allow user-set PPO2, up to 1.4 ATA. I have not found any that allow it to be optimized for safer deep mixes (PPO2) and faster decompression. There is a huge body of evidence, both in the US and Europe, that in-water decompression in a relaxed state can very safely go much higher than 1.4. There is also reason to believe that 1.4 is on the high side under heavy work loads and longer for durations.

Obviously, the application I am investigation is outside the majority of recreational eCCR dives and will have a double-lock decompression chamber onboard and be used for Sur-D-O2 as well as be available for emergency treatment. Also all divers will have undergone oxygen tolerance testing and have many years of mixed gas experience.

 

[rjack321]

Regardless of the wisdom of nuking your CNS and lungs on high ppO2s in a CCR...

O2 sensors are already out of their element in a humid gas path at >ppO2 1.0 Which is why most sensor manufacturers already hesitate to warranty or allow them for rebreather use. Some expressly forbid it.

Cranking up the ppO2 to something like 2.0+ is going to push the sensors way beyond their tested territory. They are likely to fail quite fast. Because they fail "low" your true fO2 will be too high. Even if they don't fail, their calibration will almost surely go completely non-linear. You could probably calibrate them in a pressure pot using something like the narced@90 sensor testing head. But you won't know the stablility of the calibration and its likely to vary in stocastic ways.

Bottom line trying to run a CCR at ppO2s beyond ~1.4 is outside the technological capacity of the tool to consistently and reliably execute. Hence the alarms and bells built into the electronics.

If you really want deco ppO2s as high as you say you do, talk your DSO into doing the deco in that on-site chamber.

 

[wedivebc]

I also have not found a procedure for increasing PPO2 post ascent to decompression stops other than bleeding gas off manually (squeezing bags or blowing gas out your nose after setting a higher PPO2). Ideally, you would want to bump PPO2 up after your first or second deep stop so there is time to slow the physiology down, assuming you are after a lower PPO2 on the bottom due to anticipated work load.

I have heard that there have some military tweaks that supported this capability, but have not been able to find anything commercially here or in Europe so far.

Let me suggest you learn how a rebreather actually works and get some experience on one before you start using it in peculiar ways

 

[Akimbo]

Perhaps I should have been more specific in my original post to indicate that this project, which is currently in the proposal & budget development stage, would be conducted by experienced mixed gas commercial divers. Several have been trained in the military with O2CCRs and eCCRs, but all would go through training on the specific eCCR selected. The project would not actually fall under the constraints imposed by regulators on commercial diving operations, so eCCRs could have saved a lot of money if we could find one to fit the required profile.

I only asked this question here to see if actual users could provide some quick guidance before wasting a lot of time and effort with sales weenies. Unfortunately, after talking with two instructors who are certified to teach virtually all of the eCCRs available (meaning between the two of them), it looks like this project will have to done mostly with ROVs and some limited surface supplied operations if it gets funded at all.

I do feel that recreational CCR divers might gain useful information from commercial operations, as we can from you. Perhaps these comments will be interesting.

Regardless of the wisdom of nuking your CNS and lungs on high ppO2s in a CCR... .

Considering that running well above 1.4 has been, and still is, standard procedure for Sur-D-O2 (Surface Decompression using Oxygen) and decompression treatment in military and commercial diving for longer than I have been alive, your assertion may be a little strong. For example, see page 14-2, 14-2.4 of the US Navy Diving Manual, probably the most conservative table in use for mixed gas surface supplied diving:

Gas Mixtures. Four gas mixtures are required to dive the surface-supplied mixed gas tables over their full range:

• Bottom Mixture - The bottom mixture may vary from 90% helium 10% oxygen to 60% helium 40% oxygen depending on the diver’s depth. The allowable range of bottom mixtures for each depth is shown in Table 14-3.
• 50% Helium 50% Oxygen - This mixture is used from 90 fsw to 40 fsw during decompression. Oxygen concentration in the mixture may range from 49 to 51 percent.
• 100% Oxygen - Oxygen is used at the 30- and 20-fsw water stops during in-water decompression and at 50, 40 and 30 fsw in the chamber during surface decompression.

Military and mixed gas commercial divers get oxygen toxicity tests, months of training, get a lot of on-the-job experience, average better physical condition (with the exception of alcohol abuse), and have support that recreational divers can’t afford or would be willing to put up with. It makes perfect sense that there is a big difference between the conventions each operates under.

…O2 sensors are already out of their element in a humid gas path at >ppO2 1.0 Which is why most sensor manufacturers already hesitate to warranty or allow them for rebreather use. Some expressly forbid it...

True, but can you blame them considering the relative legal liability? That policy is a huge improvement for galvanic oxygen sensor manufacturers since they started testing at the Navy’s Experimental Diving Unit in the 1960s. They wouldn’t warrant anything above one atmosphere! They began to loosen up a little after hyperbaric oxygen therapy (unrelated to diving) became such a big market in the early 1990s though.

All the same, military and commercial diving has been using these same sensors in very high humidity and PPO2 well before I graduated high school. Sensor life has proven to not be all that much shorter when adjusted for %-Hours — providing you don’t get them physically wet. I have seen thousands of dollars in sensors trashed because hot water from disconnecting the suit pissed all over the bell… that would be the bellman’s bad for forgetting to close the valve before unplugging from the suit.

…If you really want deco ppO2s as high as you say you do, talk your DSO into doing the deco in that on-site chamber.

Sur-D-O2 is part of the profile, but you have to make the high PPO2 water stops first. Water stops are often at or above 2.0 ATA O2. I have seen proprietary company tables, mostly French, using 3.0 on projects that went on for weeks. The majority of operations I have seen hold at 2.0 to 2.2 though.

We considered switching to an umbilical for the pure O2 stops but logistics makes it hard to get the divers on a hose for deep stops, let alone dealing with several premix banks on deck.