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My Journey into UTD Ratio Deco

Discussion in 'Advanced Scuba Discussions' started by CAPTAIN SINBAD, May 11, 2018.

  1. huwporter

    huwporter DIR Practitioner

    # of Dives: 1,000 - 2,499
    Location: Sydney, Australia.
    The GF target value for each stop is a peak value, which means in practice the GF quoted for a stop is the GF that you experience on first arriving at the stop. It'll decrease as you off-gas during each stop.

    So, for my example GF 100/70 profile above, after ascending from the bottom at 70m you hit GF100 (i.e. 100% of the permissible oversaturation in the leading compartment) at 24m. You then have to wait at 24m until you have off-gassed sufficiently that when you ascend to the next stop you will experience a GF of 95 on arrival at 21m. Then you need to wait at 21m until you have off-gassed sufficiently to hit GF of 90 on arrival at 18m, and so on.

    (Note that there are a lot of details about how to implement gradient factors that aren't explicitly laid out in any of the papers and so software creators have to decide for themselves - this blog post talks well about the challenges: https://thetheoreticaldiver.org/wordpress/index.php/2017/11/02/why-is-buhlmann-not-like-buhlmann/ - so some software may differ from this description, but the above is a pretty standard method of approaching the issue.)
  2. rjack321

    rjack321 Captain

    # of Dives: 1,000 - 2,499
    Location: Port Orchard, WA
    I guess this why I am challenged to think of this as plausible. Arriving at 24m with GF100, you have to keep moving to 69.9m to keep the GF at 100 in order to continue to offgas (in theory). In my mind you can't stay at 24m until you've offgassed to GF95-ish to be able to jump up to 21m and achieve GF100 again. While there may be a way to implement this, conceptually once your leading GF has fallen from 100% to 99% you aren't supposed to be offgassing anymore. But I don't know anyone diving 100/100 either which (should) have the same issue but is obviously do-able.
  3. victorzamora

    victorzamora Solo Diver

    I think you've misunderstood the M-value line. It's not that 100% is the "minimum" threshold of off-gassing. It's the opposite, it's the MAX level of offgassing before you get bend the model.

    GF1 means you're offgassing, but barely. GF100 means you're on the verge of "bending" the model.
  4. rjack321

    rjack321 Captain

    # of Dives: 1,000 - 2,499
    Location: Port Orchard, WA
    100% in the context of GF low means you're riding the m-values on at least one compartment all the way up.

    CAPTAIN SINBAD Divemaster Candidate

    # of Dives: 100 - 199
    Location: Woodbridge VA
    Altitude diving is a very specialized form of diving which requires specific training. If you are going to dive at an altitude then seek localized training. (Notes from my UTD Class)

    I hope this will put to rest one of the burning :gas: issues on scubaboard.
    decompression likes this.
  6. huwporter

    huwporter DIR Practitioner

    # of Dives: 1,000 - 2,499
    Location: Sydney, Australia.
    Yeah, what victorzamora said. I think you may be confusing 100% or GF100 with the point where you start off-gassing; GF100 is experienced where a tissue is so supersaturated compared to the instantaneous ambient pressure that it has hit 100% of the maximum 'allowed' in the Buhlmann model at that point in time, or 100% of the M-value. Buhlmann says you cannot safely off-gas faster than this.

    You still off-gas below GF100 by definition so long as you have a gradient between tissues and inspired partial pressure. (You can can even be off-gassing below GF0 as the gradient factor is calculated from ambient pressure not inspired gas inert partial pressure - see below)

    GF Low, in the context of how it is programmed in to AFAIK every desktop Buhlmann implementation, influences only where you place the first stop, not all the way up. The other stops are determined by stepping through the range between GFLo and GFHi depending on how many stops you are going to end up doing between the first stop and surface. Riding the M-values - i.e. maxing out permissible supersaturation at every stop, and hitting GF100 at the start of every stop - would be GF100/100.

    The gradient factor you experience during each point of an ascent varies with changing depth, but also with off-gassing during each stop; as you off gas, you get further from the maximum permissible supersaturation at that point in time, so the 'gradient factor' drops until you reduce ambient pressure again by moving up to the next stop.

    Dunno if this might help visualise it - this is a representation of the in-the-moment gradient factor experienced over time for an example GF 20/85 ascent, where the first stop, driven by the leading tissue exceeding 20% of permissible supersaturation, occurs at 45m, and ascent to surface is allowed when the leading tissue won't exceed 85% of permissible supersaturation:


    Conversely this is the in-the-moment gradient factor experienced over time for our example GF 100/70 ascent after the same depth and bottom time: First stop is triggered when the leading tissue gets to 100% of permissible supersaturation, and final ascent to surface is allowed when the leading tissue at that point won't exceed 70% of permissible supersaturation:


    Note that these charts only show the gradient factor (i.e. the % of permissible supersaturation) for the single leading tissue at that point in time. The other tissues all have their own gradient factors at each point in time but these aren't shown because the other tissues are all lower. Early in the ascent, the leading tissue is predominately a faster tissue, and later in the ascent it'll be predominately a medium/slow tissue.

    (And obvious disclaimer, all this just explaining how the Buhlmann model works, we shouldn't confuse all this talk of tissues with anything that is actually happening in the body, just that this model seems to be a way of representing an idea of what may be going on in a way that seems to be useful in terms of not actually getting bent, mostly...)

    (Edit: And, oh, by the way - in case anyone questions how these charts show the 6m stops going "below GF0" - what this means is the tissues are no longer supersaturated compared to ambient pressure (= 1.6ATA). The example diver is still 'off gassing' even though GF is less than zero because they are breathing oxygen in these examples and therefore the offgassing gradient between tissue and inspired gas is steeper than the gradient between tissues and ambient pressure.)
    Last edited: Jun 15, 2018
  7. boulderjohn

    boulderjohn Technical Instructor Staff Member

    # of Dives: 1,000 - 2,499
    Location: Boulder, CO
    It does not put it at rest for me.

    From whom do you seek this training? From someone from another agency?

    Does UTD not teach anything about altitude diving?

    Do UTD divers never dive at altitude on their own?

    Why has this changed from the time I was in UTD and we were told very specifically and very emphatically that altitude does not matter and nothing different needs to be done when diving at altitude?
    shoredivr likes this.
  8. RainPilot

    RainPilot completey delusional scientology snowflake Staff Member

    # of Dives: I just don't log dives
    Location: UAE
    A lot of agencies have changed a lot of views over the years. It seems that there is a different approach to altitude diving in UTD now than back then.
    rjack321 likes this.
  9. Dan_P

    Dan_P DIR Practitioner

    # of Dives: 1,000 - 2,499
    Location: Scandinavia
    As a diver, I personally dive at altitude (once in a blue moon) and make adaptations compared to sea level diving of course, but UTD doesn't have a formal class towards it - I'm ok with that. I've sought knowledge outside of the formal coursework and materials that UTD offer, including with other agencies.

    As a professional, I don't feel that the lack of a formal "Altitude Diver"-class in the UTD portfolio has an impact (not one diver ever asked me for it).

    In either case, for extreme altitude, I need to seek training outside of commonly availably certification courses anyway (they max at 3.000m - though I don't know if any exist that cover higher altitudes.) and I'm in agreement with a policy that divers who want to dive the specialised type of diving that altitude diving is, should seek adequate knowledge first.

    I am not aware of any previous policy of UTDs that conflicts with this, so I can't offer any answers in that regard either.
  10. PfcAJ

    PfcAJ Orca

    # of Dives: 5,000 - ∞
    Location: St Petersburg, Fl
    I don’t think you can expect every agency to be all things for all people. If they don’t have training in altitude stuff that’s fine. Seek it elsewhere. For years GUE didn’t have ccr training. If you wanted that you had to go somewhere else.

    The real issue is that you (tragically) got crap information earlier. Everyone should keep this example in mind when saying things like “agency doesn’t matter! It’s all about the instructor!” Agency and curriculum DOES matter.
    rjack321 likes this.

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