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DumpsterDiver emergency ascent from 180'

Discussion in 'Advanced Scuba' started by 2airishuman, Aug 9, 2017.

  1. stuartv

    stuartv Seeking the Light ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Lexington, SC
    9,818
    5,626
    Fair enough. But, I think it's a fair question, that I haven't seen an answer to on SB.

    Is there a failure mode for, say a SP Mk 25 or an Atomic 1st stage, whereby the IP is solid pre-dive, but the reg still fails catastrophically during the dive?

    If the answer it no, not when the reg is serviced and maintained correctly, then the question is, okay, but is there such a failure mode when the reg is serviced incorrectly? Say, by re-using a part that should have been replaced? Or is there such a failure mode when the reg is not serviced in a timely fashion (say, not serviced for 10 years)?

    I'm not trying to make it piston vs diaphragm. I am just trying to better understand the pros and cons of each. The only piston vs diaphragm discussion will be my own, in my own mind, after I learn as much as I can about the pros and cons of each. My diving career is short, so far, and this failure of DD's is the first time I (in my short time) have seen real discussion of such a failure. I didn't know it could happen. All I have ever really seen people talk about is HP seat failure and I have understood that to be a failure that could happen with piston or diaphragm.
     
    Last edited: Aug 18, 2017
    uncfnp likes this.
  2. uncfnp

    uncfnp Solo Diver ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: North Carolina
    6,712
    5,546
    Same here. I DIY my pistons and rely to a large extent on use AND the pressure checks. I also own diaphragm regs, in fact they are my main recreational regulators, that I also plan to self service in the near future. So this issue is very pertinent to me personally and I too would like to know if routine IP checks will catch an aging/deteriorating diaphragm. This is a very serious question for me and not just popcorn fodder.
     
    Last edited: Aug 18, 2017
  3. stuartv

    stuartv Seeking the Light ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Lexington, SC
    9,818
    5,626
    From what I have gathered, routine IP checks would not catch that. But, normal maintenance, done correctly, using new parts as appropriate, would prevent it from being a concern. And routine IP (and leak) checks would catch the other problems you might have.

    What we seem to also know is that if you don't do the maintenance often enough, or you do it wrong, you could have a solid IP and still have this type of catastrophic failure with a diaphragm reg.

    What I still want to know is, if you have a Mk 25 (for example) and you don't do the maintenance often enough, or you do it wrong (wrong to include possibly not replacing a part that you should have), is it possible that the piston reg would still show a solid IP, then just go bang on the next dive?

    We now know that it CAN happen with a diaphragm. Can it with a Mk 25?
     
  4. 2airishuman

    2airishuman Solo Diver

    # of Dives: 100 - 199
    Location: Greater Minnesota
    2,634
    1,890
    Well, I'll use the example of an Aqua-Lung Calypso IV. Older, bulletproof piston reg. You could look at it and do the sort of thought experiment you're suggesting and it sure doesn't seem like anything could possibly go wrong during a dive that wouldn't manifest as an uneven IP or slight leak beforehand.

    Except it did. On one dive. The HP seat came loose and stuck to the end of the piston, blocking the airflow. There was a recall and they added a retainer collar for the HP seat.

    The point being (says 2airishuman while donning his "engineering management" hat) that you can't think your way out of these problems completely. This is the sort of thing that happens once in 20,000 hours of use. The only way you can find these problems reliably is by tracking an installed base of thousands of units over the course of many years and looking at all the failures. That is why I am such a fan of established designs like the Conshelf that have millions of hours on them in all kinds of dive conditions. The defects are known and understood. In the case of the Conshelf the usual culprit for a surprise failure are the little HP seat actuator pin being bent by a tech and then bent back and reused so as to avoid having to order a replacement. There was also a reg well past its service window that suffered an HP seat delamination during a dive, so that the rubber seal kept the orfice shut while the rest of the seat moved up and down.

    Nobody is smart enough to think about these failures. We just get people who say you can't have a turret on a tech dive, because the extra o-rings are a possible failure point. What they're missing is that the failure probability for those o-rings is lost in the roundoff error because it's 1/1000 of the failure rate of the assembly as a whole for reasons that no one can anticipate (or where there's no viable mitigation strategy if they have).

    We don't know the specifics of DD's situation.

    While manufacturers of various regulators specify the service windows differently, the usual recommendation is teardown and service every two years or 500 dives, whichever comes first -- and inspect, leak check, and IP check every year. I find it hard to believe that a diaphragm would "just fail" within two years or 500 dives.
     
  5. stuartv

    stuartv Seeking the Light ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Lexington, SC
    9,818
    5,626
    No need for a thought experiment. I'm asking if anyone knows if a piston reg can have a catastrophic failure, with a solid IP, pre-dive, as a result of too long between services, or an incorrect service? Does know anyone know of a case where it has happened?

    I mean, if someone knows of a theoretical way that it could happen, sure, explain away. But, it's fine to give it no "thought" and just post up if you know of an example where it has happened. With the combined experience here, if nobody here knows of a case where it HAS happened, then I personally would conclude that it is pretty dang unlikely TO happen.
     
  6. 2airishuman

    2airishuman Solo Diver

    # of Dives: 100 - 199
    Location: Greater Minnesota
    2,634
    1,890
    ??? I just gave an example, I thought.
     
  7. Andrew Donn

    Andrew Donn Rebreather Pilot

    # of Dives: 1,000 - 2,499
    Location: DC
    70
    29
    Every flow-through piston regulator (like a Mk 25) has as its Achilles heel the small HP piston seal. It is a small o-ring that sees the greatest pressure differential - from tank pressure on one side to ambient on the other, in a non-sealed 1st stage it is exposed to salt and sand from the ambient pressure chamber and it sees the movement of the piston back and forth every time you take a breath. A flow-through, as opposed to a flow-by piston 1st stage can be identified by the fact that the outlet ports are on the opposite end away from the tank inlet. Regulator manufactures have tried several remedies like harder seals or different seal compounds and even different types of seals with Teflon backup rings or internal springs over the years to mitigate it. On a neglected regulator this seal along with the HP seal would more than likely be the first thing to go.

    No regulators are immune to requiring regular maintenance and I would say over the year I've seen more diaphragm regs that have survived years of neglect.
     
  8. uncfnp

    uncfnp Solo Diver ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: North Carolina
    6,712
    5,546
    Obviously I need to read up on diaphragm first stages but that was pretty much want I assumed. As to normal maintenance for my AL since service kits are hard to come by it's s concern. Now in addition to the IP, HP seat and o-ring life expectancy I now know the diaphragm has a unknown life span as well. Add to this that I have to get kits when I can find them and will not know their age and they may will sit for some time on my shelf before use.

    @OWIC647 would it be reasonable to open the stage and visually examine the diaphgragm and get some feel for its health? Or would it be as hoses and fail even without obvious defects?
     
  9. Bryan@Vintage Double Hose

    Bryan@Vintage Double Hose Instructor, Scuba

    680
    886

    Get in touch with me as I'm 99% sure I have the service kits for your regulators.


    Not much of the HP diaphragm is visible when they regulator is operational. Don't let the concern snowball...The HP diaphragm is one of the toughest working parts of the regulator. Anything with moving parts CAN fail but the probability of this part doing so is extremely remote.
     
    uncfnp likes this.
  10. halocline

    halocline Solo Diver

    9,003
    3,405
    Yes, regulators are all mechanical devices and as such, can fail at any time, although it's pretty unlikely. On a MK25 (or any other balanced piston) the HP seat could break, causing an immediate massive IP spike. On some of the early MK15s, they used a terrible IP adjustment system that caused the knife edge of the piston to cut into the seat, occasionally causing it to break apart or the piston to get impaled on the seat. That was a quick recall....

    Also any regulator can develop a sudden catastrophic leak by virtue of a blown o-ring or hose, then there is the tank valve o-ring, burst disc, etc. So anyone who really dives as if his life depends on his regulator is an unsafe diver.

    Regarding DD's 'failure' keep in mind that for years he has posted videos and/or told stories about all kinds of very unusual dive events of his. I would take it all with a grain of salt.
     

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