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Is the Mk VI / SE7EN really that dangerous

Discussion in 'Rebreather Diving' started by jfe, Dec 26, 2018.

  1. jfe

    jfe Barracuda

    # of Dives: 200 - 499
    Location: Qatar / South Africa
    I'm confused and at odds with myself concerning the Mk VI / SE7EN. I have, through forums and discussion with many other brand CCR divers heard the VI/7 is bound to kill you and the RB with the most recorded fatalities, prone to fail unconditionally etc.
    However I do like a lot of things I've read and researched about the 7 and frankly the unit seems well build and constructed for the kind of diving I intend to use it for i.e. wreck and photography. No one making these claims could have given me concrete evidence supporting their claims, on the contrary, even the spreadsheet on this forum showing RB incidents does not support these claims that the MK VI / 7 is the rebreather of death, as some would call it.

    So here I have a few questions:
    Who, diving and not diving this unit, have first hand experience a failure during a dive, be it in person or witnessed?
    Why, if you share above negative claims, do you have it?
    Is there a comparisson in failure rate MK VI / 7 vs other brands?
  2. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Port Orchard, WA
    Never heard anyone say its "dangerous"

    I have heard many complaints that it isn't user serviceable, and with all of the various electronic checks there is a non-zero chance it will find a "problem" and prevent you from diving. Whether those problem(s) are real or not I don't know. If you are close to a service center then it might be just fine for you. If you have to ship it back to the factory to do even basic servicing then perhaps look at a unit that can be field serviced/repaired.
  3. 688ClassRebreather

    688ClassRebreather Angel Fish

    # of Dives: 1,000 - 2,499
    Location: Maryland, USA
    To be completely up front, I am a Poseidon instructor trainer so I have a clear bias in favor of the Poseidon. I also dive the inspo and will be an instructor on it as well in the near future.

    Let's start with the biggest concern in your post, the fatality rate on Poseidon rebreathers. I have looked and I have not been able to find any data to support this claim. If somebody has the data I would appreciate pointing me in the right direction. The closest this I could find is the rebreather fatality database maintained by Deep Life. That database has its own problems, but it is the most comprehensive I have found. The current version of their spreadsheet has a section titled "Particularly Dangerous Products," and the Poseidon is not listed there although several others are. Of the 493 fatalities in the database, only 8 are listed as Poseidon MK VI/Se7en. (Some other units are listed with greater frequency, but I do not want this to turn into a bashing contest where everyone feels obligated to jump in to defend their CCR.) With the large number of incidents listed as unknown (138) or Scant Data (5), I am not sure these numbers are conclusive or even helpful. It is also important to note that these numbers are not normalized to account for the number of each model of rebreather out there, or for the number of dives made on each unit. However, I do not see any evidence that the Poseidon is inherently dangerous.

    The second major concern you raised is the failure rate. Again, without hard data we are left with opinions based on personal observation and anecdotes. I have certainly seen my share of faults with the Poseidon. I have seen several units fail right out of the box. Poseidon has told me there was a problem with over tightening some connections on the top of the head resulting in cracking. Since they corrected this issue I have had much better luck with new units. I have not seen this issue with any unit made in the last two years. I have also seen several failures due to corrosion on the battery terminals. I think putting the battery on the outside of the unit was a bad decision from the start. Now that I understand the issue and how to clean the terminals so that it does not happen, and I include it in my courses, I have not seen this issue in a while. Finally, I have seen issues with cables wearing and causing the display to lose connectivity. While the electronics module continues to maintain PO2 when this happens, loss of the primary display can be frightening and is completely unacceptable.

    I have been upfront with the negatives about this unit, so let's take a minute to look at the other side. The unit does 55 tests as part of the startup procedure. I do not know of any other unit that does so many.

    I have seen units fail some of these tests, but I don't think that is a bad thing. I have even seen a case where another diver (not one of my students!) got repeated failures during startup indicating that his fraction of O2 was too low. He was frustrated and mad. He was convinced the analysis on the tanks was correct and the problem was with the way the Poseidon measured the PO2. He even stated that it was unacceptable that the Poseidon would not let him dive in this condition when he was sure he was correct, and that if he had his preferred unit with him at the time he would be able to dive it. The other unit would let him make the decision, not make the decision for him. Upon examination of the unit, we quickly identified that he had the dil and O2 bottles reversed. While this was an extreme example, from my experience most of the failures in the startup process highlight real issues that can be easily corrected. The subsequent dive is safer because of fixing them. While it has not always been the case, lately the reliability of the Poseidons I have been diving with has been nearly perfect. That includes a period of intense diving with about 15 Poseidons each making several dives a day over several weeks. It has been a long time, at least a year, since I have seen a Poseidon miss a dive due to an error on startup or during the dive.

    The weak point in any CCR is, arguably, the O2 sensors. While the Poseidon is currently the only unit offering the solid state sensor, we will put that aside and assume everyone is still diving with galvanic sensors. Most people are, so this is a reasonable perspective. We all test our cells to PO2 of 1.0 on the boat. Unless we have a pressure pot with us, that is about as good as we can do. Most of us dive with a higher PO2, so testing at 1.0 is good but not good enough. A common "best practice" is to flush the unit with 100% O2 at 20 feet to ensure the sensors can read up to 1.6 PO2. An even better practice is to do an O2 flush at the end of the dive, but that is discussion for another thread. From my experience, and yours may very well be different, many divers miss this flush. The Poseidon does it automatically at the start of every dive. You can not forget. The Poseidon also tests the sensors frequently throughout the dive by doing a quick spray of dil onto the secondary sensor and measuring the response and response rate of that sensor and then continually comparing that sensor to the primary sensor. I personally dive with a solid state sensor as an independent check, but I have never had any issues with any of my sensors.

    Finally, instead of just listening to people venting on the internet, perhaps it is worthwhile looking at what the professionals are doing. Apparently several professionals have looked at the data and decided the Poseidons are safe and reliable despite what you may hear on the internet or from divers who use other units. Richard Pyle and Brian Greene are scientific divers who spend a lot of time at remote locations diving to 500 feet/150 meters. You can google them and see videos of them at 500 feet on the Poseidon. If it were more dangerous than other units, or had a higher failure rate, I doubt they would choose the Poseidon. Right now NOAA has authorized 3 CCRs for their divers and the Poseidon is one of the three. I have trained several NOAA divers on hypoxic trimix down to 100 meters using the Poseidon. I have recently encountered US Navy and US Coast Guard divers transitioning to the Poseidon from other units.

    In my opinion, and it is just an opinion, there are no bad units on the market today. They all have strengths and weaknesses. Many of these strengths and weaknesses are design decisions that are hard to change. It all comes down to what you like, how you feel comfortable, and your preferences. I recommend you find an instructor that you like and trust, hopefully a few instructors that you like and trust. Talk to them about specific aspects of their units and why they do or do not like them. Focus on the aspects that are important to you. The availability of good instruction, again in my opinion, as much more important than anything else. The availability of technical support is a close second. This is life support equipment and should be treated and serviced as such. You should trust your technician with your life, because you will be trusting them with your life.

    Good luck in your decision and welcome to the community. No matter which way you go, you will find CCR diving much better than open circuit diving and you will get much better photos.
    craig haymaker, jfe, JackD342 and 2 others like this.
  4. Gareth J

    Gareth J Manta Ray

    # of Dives: 1,000 - 2,499
    Location: UK
    In fairness to the Poseidon Unit, I've not heard any significant criticism that it is dangerous.

    The main criticism, from established CCR users, is that it is factory locked.
    i.e. Not upgradable, Return to Factory for almost all maintenance, pre-packed scrubbers, lack of versatility due to the factory locked nature of the unit.
    One of the issues was its ability to do mix gas diving. But there may well have been upgrades to the unit or available packages that negate this criticism.

    Some of those criticisms have been levelled at other units on the market, but not all of the criticism against a single unit.

    From the little I have read about the unit in the past, it has some clever features, that potentially improve safety. One of which was an on the fly cell calibration feature.

    As a disclaimer, I have not been trained on the unit, I have not used one, and I have not spent any time looking at them. So I might well be wrong.

    There has long been a debate (in CCR circles), is more automation improving safety, or making users lazy, less inclined to monitor the unit (trusting in god - or technology) and therefore making units more dangerous. I truly don't know the answer to that.
    I dive an AP inspiration. i.e. an eccr rather than an mccr. Thus I rely, to some extent, on the electronics to maintain the set point. To activate alarms and warnings. I do monitor the unit, but probably not as much as a diver on an mccr. I can fly the unit manually if required.

    I would suggest if you are looking to move to CCR diving. You look for a unit that can be supported locally, that has a good reputation for reliability, maintainability, and good overall support. Also, will it meet your future needs, CCR is an expensive investment, changing units is a very expensive deal.

    Also - number of deaths on a particular unit is not indicative of its risk. Number of units in service, type of diving being undertaken is more representative.
    Similarly, when CCR units became readily available, there was a massive spike in incidents, as users, and training agencies learnt how to train with and use the new technology. Despite there now being significantly more units on the market, and more secondhand units circulating, the number of incidents has dropped significantly.
    jfe, rjack321 and 688ClassRebreather like this.
  5. jfe

    jfe Barracuda

    # of Dives: 200 - 499
    Location: Qatar / South Africa
    Thanks to all responding, as I said it's for my peace of mind as part of my research.
  6. wetb4igetinthewater

    wetb4igetinthewater Instructor ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: Seattle
    What are in those tests? Or what are the tests that the unit conducts that other rebreathers don't?
  7. JohnnyC

    JohnnyC PADI Pro

    # of Dives: I'm a Fish!
    Location: United States
    The Poseidon has some interesting architecture. I enjoy telling @RainPilot that he’s gonna die, but the truth is, it’s a perfectly acceptable rebreather for most people. It has its pluses and minuses like any other unit.

    I don’t like the automation involved, I don’t like their upgrade path, but I understand why they do both. I don’t like the idea of one fallible cell being the sole checksum against another fallible cell. Simply blowing dil across the face every once in a while won’t necessarily show a problem. 3-cell voting logic has its own share of issues, but it gives you another check for cell health. I would be much more comfortable diving with a solid state sensor, or even a third galvanized sensor, in order to validate loop contents. The Poseidon methodology is simply to tell you to bail out.

    I’m not sure how someone could swap dil and O2 bottles like mentioned above. None of my units will go together correctly if I did that. Not that I don’t believe the anecdote, but it’s hard to believe someone could make that type of mistake, and still put their unit together. If that’s possible with the Poseidon, that’s a reason I would be wary. I have to purposely try hard to screw up building any of my units.

    The battery architecture doesn’t give me good vibes. I get that the idea is you get the battery you’re certed for, and you can simply travel with your battery to any center with a Poseidon, but I’m not diving a unit that I’m not responsible for, and the idea that you can just rock up with a dongle and hop on a unit that you have zero pedigree with just doesn’t give me the warm fuzzies. For some people, this is the sole reason they went with Poseidon. They like traveling without a unit and not having to worry about it.

    Name dropping people who dove a unit is pretty pointless. Sami Paakkarinen dives a JJ, Patrik Gronqvist dives a Meg and they both recovered bodies out of the Plurdalen. Matt Matthes dives a Meg and an Op2ima and found some of the deepest passage in the Pit. Mikko Passi took his SF2 I to the back of the Thai cave to get those boys. People are doing all sorts of crazy dives on all sorts of units so don’t buy one just because someone else dives one. Get the unit that is right for you.

    Personally, I dive an SF2, a Pelagian, and used to dive a Meg. Poseidon was never even an option, not the least of which was the terrible stateside service options when I was looking into rebreathers. Point is, there are lots of good, solid, safe units on the market. You need to define what your needs and wants are first, and aside from one friend, all my other CCR diving friends have gone with other units for various reasons. That doesn’t mean the Poseidon is bad btw.
    sunnyboy and taimen like this.
  8. taimen

    taimen ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Europe
    What is going on with the Poseidon solid state O2 sensor these days? Are they actually shipped out with new units or is it still in some sort of beta testing project?
    Any real world experiences with it so far?
  9. michael-fisch

    michael-fisch ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: Finally Lake City FL
    My main gripe with the MK6/MK7 is the number of courses and battery options that have to be bought after each course in order to dive 10M deeper. At a rough guess you'll spend more money on courses and special batteries as you advance to 100M dives, than the unit cost new at full retail price. Almost every other rebreather doesn't actively limit you in your diving after you buy it.

  10. taimen

    taimen ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Europe
    Isn't it possible to buy a Seven in tech configuration and with the tech battery directly?

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