Are rebreathers getting safer over time?

[tbone1004]

I’m confused here. All the galvanic O2 sensors I have used read PPO2 directly. No pressure calculation was required unless the cell was on the surface and the gas was bled off the chamber or gas stream through a flow meter.

Oxygen monitors inside bells and chambers consisted of a Teledyne sensor, potentiometer for calibration, and an analog millivolt meter with a custom scale… nothing more than that beyond some wire. That is why inside sensors were considered more accurate once calibrated. A typical 0.3 ATA at 1000' would only read 1% when bled off and read on a sensor at the surface.

touche, my apologies. Was reading about the sensors reporting FO2.

Kyle, I agree with you regarding the descent, descents are easy, the ADV's are mechanical so they take care of that. It's the ascents that are the problems because you have to manually add O2 to keep your PO2's high enough if you have a quick ascent. I think the problem with the breathers on autopilot is the solenoids are limited by the delay, so it is difficult for them to keep up because they have been programmed not to anticipate that to prevent oxtox. So you have a quick ascent and you just hit the button a few times on the way up to play catchup

 

[descent]

There is a sobering story about how hard it is to program a computer to give the correct dose.

 

[Akimbo]

… Going up is the real problem and it's harder for a rebreather to keep the PPo2 in a safe zone during this time...

True, but it is a lot easier now with slower ascent rates. I worked with some prototype GE and Westinghouse rebreathers in the 1970s while in the Navy. Some fairly primitive analog circuitry drove the pure Helium diluent and pure O2 solenoids. They often set off alarms during shallow test dives, like less than 100', when ascending at 60'/minute.

I am more confident in modern digital circuits and programming than I am with batteries, solenoids, and sensors. One advantage of manual O2 addition is you are consciously aware of adding gas and can hear it. If you hear gas after letting off on the spring-loaded dead-man valve you know to reach back and close the HP O2 valve until you figure out the next step. The disadvantage is you still have the same mixing and volume change issues as the automatic system at a time when you are often distracted by other tasks.

To the other point, a few liters of volume in your canister and bags doesn’t allow much time before things go seriously wonky. That is the single biggest complication. Just the mixing mechanics under varying pressures is challenging. All my exposure to eCCRs has been for deep saturation diving in the Military for a primary and for bailout for commercial diving.

 

[muzikbiz22]

There is a sobering story about how hard it is to program a computer to give the correct dose.

Actually, interesting and quite fascinating. My simplified take away is that this thing was just WRONG in about 50 different ways. Software, human error, bad design, programming, coding, etc. This was also the 80's, which was eons of time ago. Computers and their multitudes of applications, to some extent, were just "learning to walk".

 

[descent]

Actually, interesting and quite fascinating. My simplified take away is that this thing was just WRONG in about 50 different ways. Software, human error, bad design, programming, coding, etc. This was also the 80's, which was eons of time ago. Computers and their multitudes of applications, to some extent, were just "learning to walk".

The system was seen as flawed in retrospect. While it was in use, everybody involved trusted it completely.

Technology has changed. The people that integrate the technologies are as human as ever.

 

[tbone1004]

The system was seen as flawed in retrospect. While it was in use, everybody involved trusted it completely.

Technology has changed. The people that integrate the technologies are as human as ever.

and with that they were placing blind trust in an unproven system. Technology is a bit different than it was in the 80's, and that was a fully analog system. It is important to remember the past to ensure we don't make the same errors in the present and future, but we can't bring up ancient history when we are at least 3 major steps ahead of the technology curve than they were back then. This is similar to us saying nuclear reactors can't be trusted because of Chernobyl. We are leaps and bounds ahead of where they were, we have systems in place to prevent this type of accident from taking place again, but we can't stay that skeptical of everything or nothing will ever be gained.

 

[victorzamora]

To be fair, from what you described so far, we still have not yet necessarily veered outside of the realm of simple counting... and when you refer to things like machine learning, I get a bit skeptical. The methods do sound disproportionately more sophisticated as compared to what the problem being solved appears to be. But, I most likely just do not understand enough about the mechanics of rebreather operation, and there is more to it than what has been described so far in this thread. I would be interested in better understanding where the complexity lies...

The mechanics of rebreather operation are actually incredibly simple. There's very little to them. My "machine learning" reference was for tuning PID gains, and it's probably the direction I'd go. The scenario tbone painted was woefully inaccurate, and is a serious case of him trying to justify a rebreather to himself....but let's expand on it. Your sensor senses that your PO2 is low, right? It's "too low" relative to your required parameters. Well, first of all how do you set "too low"? You want to be breathing PO2 of 1.0...is 0.9999999 okay? How about .9999? How about .99? 0.9? What about 0.85? Where do you draw that line? Same thing on the high side. So, before you've even decided to fire off the solenoid to inject more oxygen you have a complex decision to make. Yeah, it's simple in that you can arbitrarily say "0.9"....but is that good enough in practical terms for the diving you're doing? I dunno. Now, the solenoid fires after you've arbitrarily set that min PO2 limit. How much does it fire? For how long? When does the O2 sensor reading matter next? Well, that's another issue that sounds simple but isn't. If you inject too close to the sensors, there's no mixing. If you inject before the scrubber (pretty common) then it takes a while to get mixed air to the sensor again....so it's sensing low PO2 a lot and will keep dumping O2 in if your response time is too low. Too high, and you'll never get your PO2 right. You want instant information, which can't happen due to the challenges above. So, now you've shot off an arbitrary amount of O2 after arbitrarily choosing between trade-offs and you're waiting a fairly arbitrary amount of time before you go through that whole cycle again. Having the machine decide how much O2 to add is a great thing if you can get the PID settings right, or get the machine to tune itself. PID is just a way of calculating how much "correction" to apply.

Unclear why you think there is no way, could you please elaborate?

What you say might very well be true, but those are some pretty strong statements you made about certain things being impossible, and I think it would be fair fto ask for a little more justification. When you say that a diver can predict something, but a rebreather can't, what scenario are you referring to?

Physics. Same for descent and ascent. You and I are swimming along at 35ft in a cave before the vertical passage to 115ft appears in front of us. Heck, it's 180ft+ in Eagle's Nest....but let's stick to 115ft. You're descending from about 2ATA to 4.5ATA. If you're at a "setpoint" of PO2=1.0 at 2ATA, you're breathing EAN50. Take that breathing gas to 115ft (over the course of 1 minute) and you're breathing a PO2 of 2.3 and you've got a tox risk. There are ways of mitigating that, but it requires human input which isn't being discussed. Human input is you set the setpoint VERY low initially and add tons of dil on your way down before upping your setpoint. This is experience, not a rebreather decision. It would take a LONG time of a VERY metered descent to keep your PO2 and setpoints in line happily. The diver predicting the descent is because they see the cave turn downwards, they see the moray eel 50ft below them when they decide to chase it, the wreck they're penetrating has led them to a stairwell. They see ascents/descents coming before they happen. Rebreathers don't.

This is interesting, do you happen to know more detail, how the eCCR screwed up when the diver lost focus? Just curious about the limitations of existing systems. In what circumstances do they fail if left unsupervised, and how do they screw up? Do they fail to automatically maintain a setpoint on rapid depth changes, or in some other situation?

They were following a passage and ascended a little too quickly while screwing with some gear issues they were having. PO2 started getting very low. Another diver did the opposite. I heard him discussing that he started messing with his computer alignment (slave, not master) and before he knew it his PO2 was 1.8.....exact scenario as posited above.

What you are describing sounds like an analogy to a fully automated rebreather that pretty much runs "on autopilot", but with a manual override that lets you instantly take over... would you be comfortable with that throughout the dive, or only during the times when you are not making depth changes?

The analogy I'm describing isn't theoretical but is exactly how many (dare I say most?) eCCR divers use their units. Some high-profile divers in a high-profile training agency said it's the only way they'd ever feel comfortable diving a rebreather (and they dive rebreathers almost exclusively).

I am always amazed by how the cruise control in my Sprinter keeps the desired speed pretty accurately in spite of going up or down steep hills. It anticipates and responds to those stressors pretty well. After all, that's what we're talking about here: cruise control for your rebreather.

So let's take your Sprinter and make that cruise control analogy a little more accurate in terms of rebreathers. What if it took your Sprinter, loaded to the teeth with cave gear, 5 seconds to get input feedback? How about 15 seconds? 30 seconds? Your Sprinter's cruise control works well because the PID gains are tuned well and it all gets instant feedback. When you start going down a steep hill, it's not sitting there accelerating aggressively while the computer is WAITING for "appropriate" input. It knows. The speedo shows it immediately. You have a resolution on that system infinitely higher than on a rebreather.

 

[KenGordon]

There is a sobering story about how hard it is to program a computer to give the correct dose.

That is the example product they tell you about at the start of a medical device software course.

It is why the regulations focus on the soft design management issues and not the technical stuff like actually getting the correct dose.

It is not at all typical of any modern software in safety critical applications.

The problem with a rebreather is the software does not have enough facts to be perfect, the cells may lie. The Mk 6 and 7 have a way round that which I think is cool but they are distrusted by many. As a result the normal mode of operation is terribly susceptible to cell issues. I think these swamp any other issue. We do see people actually die as a result of cell issues, not as a result of AP or Shearwater being unable to implement simple machine control.

Manually running a machine with dead cells will also fail. Perhaps it could be argued that the user would be better placed to notice they were bad. Personally I think computers are better at the boring, repetitive stuff than people.

 

[tbone1004]

Physics. Same for descent and ascent. You and I are swimming along at 35ft in a cave before the vertical passage to 115ft appears in front of us. Heck, it's 180ft+ in Eagle's Nest....but let's stick to 115ft. You're descending from about 2ATA to 4.5ATA. If you're at a "setpoint" of PO2=1.0 at 2ATA, you're breathing EAN50. Take that breathing gas to 115ft (over the course of 1 minute) and you're breathing a PO2 of 2.3 and you've got a tox risk. There are ways of mitigating that, but it requires human input which isn't being discussed. Human input is you set the setpoint VERY low initially and add tons of dil on your way down before upping your setpoint. This is experience, not a rebreather decision. It would take a LONG time of a VERY metered descent to keep your PO2 and setpoints in line happily. The diver predicting the descent is because they see the cave turn downwards, they see the moray eel 50ft below them when they decide to chase it, the wreck they're penetrating has led them to a stairwell. They see ascents/descents coming before they happen. Rebreathers don't.

deleted due to inaccurate statement

 

[The Chairman]

You have a resolution on that system infinitely higher than on a rebreather.

'Infinitely higher' is a poor choice of words. I bet that integrating depth and the change of depth could really resolve the problem here. Gasoline cars integrate MAP and TPS sensors to determine load pretty instantaneously. Why? The 02 sensors can't be relied upon to keep up with hard accelerations or deceleration. Sounds a lot like what we have here.

For those not familiar with automotive components, MAP= Manifold Air Pressure and TPS= Throttle Positioin Sensor.