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What computers are you using for tech dives?

Discussion in 'Technical Diving Specialties' started by Cyborg Pirate, Aug 27, 2017.

  1. dmaziuk

    dmaziuk Orca

    Tom Ahola likes this.
  2. KenGordon

    KenGordon Rebreather Pilot

    Unless you can examine the code you are guessing. You could try lots of test cases and narrow down what causes difference, but even then you will be pretty much guessing. You could have a reference implementation, guess why it differs, change it and test if the guessed change causes it to match and so suggest that what you changed must be a property of the target implementation.

    My point is that Erik Baker did not put forward a design for a dive computer. He put forward a design for table generating software. The gap between the two invalidates the claim that GF is some kind of standardised thing which ought to always be the same, it might be, it might not. It depends on the hidden assumptions of the programmers.
    scubadada likes this.
  3. scubadada

    scubadada Diver Staff Member ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Philadelphia and Boynton Beach
    My impression is that the implementation of Buhlmann ZH-L16C with GF is a little different on my Nitek Q than it is on a Shearwater. Nothing dramatic, but a little different. Of course, it is also a little different than software, like MultiDeco.
    Michael Guerrero likes this.
  4. Tom Ahola

    Tom Ahola Angel Fish

    I asked for references to the source code or algorithm description. I would be interested to see these because when I have run comparisons between different Bühlmann GF implementations there are large differences between implementations in decompression results with some cases of dive profiles. Some code or math would reveal why they are different. As it was implied here that these algorithms are not proprietary, there must be somewhere this information. I have seen the java code and, of course, the Fortran code of Erik Baker. But people do not dive with these, do they?

    Quoting a part of Erik Baker's text here that explains that there is room for proprietary implementation in the Bühlmann GF:

    ”The next part of the decompression sequence is the most difficult since there
    are several different ways to approach the problem. This is where the

    PHILOSOPHY of the decompression programmer comes into play. What I mean

    by this is that the programmer must make certain decisions about what this

    program is being used for, what kind of divers will be using the program,

    and whether the decompression programming sequence will be either complicated

    yet flexible or straightforward but not-very-flexible."

    I tried to make a signature now , as it seems to have caused some concern for some that I did not have one before. I hope it appears below after I hit Post.
    northernone likes this.
  5. 100days-a-year

    100days-a-year Solo Diver

    # of Dives: 5,000 - ∞
    Location: NE Florida
    I applaud your new signature.
    sigxbill and PfcAJ like this.
  6. admikar

    admikar DIR Practitioner

    # of Dives: 100 - 199
    Location: Bosnia and Herzegovina
    Aaaaaand....what are you waiting for? :poke:

    I'm from Europe and I'm a beginner. Made mistakes here, but never have been bullied. Your post irritated even me, let alone a lot of experienced folks here. If I made a statement like yours in my club, I would be allocated to cleaning ****holes for 6 months.

    rongoodman likes this.
  7. Dr Simon Mitchell

    Dr Simon Mitchell Medical Moderator Staff Member


    I would nominate isobaric counter-diffusion (IBCD) as the most commonly misunderstood and misrepresented topic in diving medicine. I will try to explain the relevant aspects of it here.

    First, let's be clear that the gas switch we are interested in technical diving is from a high helium mix (eg heliox or trimix) to a high nitrogen mix (eg air or nitrox) during decompression. Although switches in the other direction are interesting for theoretical reasons there is no reason to do them in diving. Discussing them is irrelevant, and just serves to confuse things. Similarly, discussing a situation in which you are breathing air and then put trimix in your drysuit is interesting for theoretical reasons, but it is of no practical relevance to technical diving and once again, it just confuses things. I reiterate that the switch we are interested in is from a high helium mix (eg heliox or trimix) to a high nitrogen mix (eg air or nitrox) during decompression.

    The reasons a "helium to nitrogen" gas switch is made during decompression are twofold. First, in open circuit diving it saves helium which is becoming increasingly expensive. This is not really a consideration in rebreather diving. Second, based on the physical properties of the gases it has been perceived that it may accelerate decompression. This relies on the helium leaving the tissues faster than the nitrogen enters them. Since helium is small, it has higher diffusivity than nitrogen, though flux through tissue is also influenced by gas solubility in the tissue (which varies for any gas according to the tissue composition). As many of you know recent work has cast considerable doubt about the difference between helium and nitrogen kinetics in tissues of relevance to dives of the typical duration we do. But let's assume for the purposes of this discussion that helium does diffuse out of a tissue into the blood faster than nitrogen diffuses from the blood into the tissue.

    This process of helium diffusing out of the tissue and nitrogen diffusing into the tissue after a helium to nitrogen switch is called counter-diffusion, and the fact that it will occur after a gas switch even if the diver sits at the same depth (same ambient pressure) is where the term "isobaric" comes from. In reality, these switches are rarely isobaric in technical diving because we are usually in the process of ascending.

    A simple depiction of what we assume is happening is shown in the diagram below which depicts a tissue and a blood vessel passing through it just after a gas switch from helium to nitrogen. Helium in the tissue moves into the blood faster than the nitrogen moves into the tissue as indicated by the arrow sizes.

    IBCD diagram 1.jpg

    If the assumption that this occurs held true, the potential benefit of a helium to nitrogen gas switch during decompression is illustrated in the figure below which comes from a paper on decompression from technical dives that David Doolette and I published in the journal Diving and Hyperbaric Medicine. Assume the diver has been breathing heliox 10/90 at 90m (10 ATA) and switches to breathing nitrox 10 (10% oxygen 90% nitrogen) at time zero (on the horizontal axis on the graph) whilst remaining at 90m. I know no one would do this, but we are trying to illustrate the principle of a helium to nitrogen gas switch at this stage, rather than describe reality.

    IBCD diagram 2.jpg

    You can see that immediately after the switch the pressure of helium in the tissue decreases and the pressure of nitrogen increases, but the former process takes place more quickly so that the total pressure (the line labelled "sum") of dissolved inert gas in the tissue declines even though we are still breathing 90% inert gas. The tissue becomes undersaturated with respect to ambient pressure (the horizontal line at 10ATA) and this would allow a larger ascent (decrease in ambient pressure) before the total tissue inert gas pressure exceeded ambient pressure and the tissue therefore became supersaturated. In this way, the ascent would be accelerated.

    In theory, this description of events following a helium to nitrogen gas switch should hold true for almost all tissues in the body, and such switches should not cause any problems. Indeed, they should accelerate decompression to some extent. Whether this acceleration is real or significant in practical terms is highly debatable, but one point needs to be clear: the internet obsessing about IBCD after helium to nitrogen switches as a cause of all types of DCS in technical diving (as we have seen from one commentator in particular in this thread) is misinformed. With one exception (see below), helium to nitrogen gas switches during decompression should be neutral or beneficial in relation to risk of DCS.

    The exception is the inner ear. The inner ear has a unique anatomy which makes it vulnerable to injury by counter-diffusing gases. It differs from the simple depiction in the first diagram above in that the sensitive functional inner ear tissue (which receives all the blood flow) has a relatively large reservoir of fluid immediately adjacent to it (called the perilymph). This does not have its own blood supply, and any movement of gas in and out of it must occur through the sensitive functional inner ear tissue which, as I said above, receives all the blood flow.

    During bottom time on a deep dive this reservoir of fluid absorbs helium, and becomes a helium reservoir. (Note: What follows is not strictly correct but is a conceptually adequate way of thinking about it). When we make a gas switch from helium to nitrogen, the sensitive inner ear tissue is already probably supersaturated with helium (remember, we are ascending from a deep dive here). We make the switch, which in a normal tissue should (in theory) result in faster loss of helium from the tissue than nitrogen uptake into the tissue as previously discussed. But in the inner ear there is also a "dump" of helium from the perilymph reservoir into the sensitive inner ear tissue (this helium has no other way of escaping except through the tissue). This stops the partial pressure of helium in the tissue from falling as the nitrogen arrives and the combination of the arrival of the nitrogen, and the maintenance of the helium partial pressure by helium diffusing from the “reservoir” results in a transient INCREASE in supersaturation instead of a decrease. This could be enough to precipitate bubble formation within the inner ear itself if the inner ear is already supersaturated. I have tried to illustrate this in the diagram below.

    IBCD diagram 3.jpg

    The likelihood of this being a problem is influenced to a large extent by the amount of supersaturation in the inner ear before the switch is made, which essentially means the quality of the decompression of the inner ear prior to the switch. These switches can be (and are) done with a very low incidence of problems if the decompression to that point has been adequate.

    Anyway, I hope that has helped with understanding of the problem, which frequently gets confused on internet forums. I must acknowledge the fantastic work done by David Doolette on this issue. We have collaborated on the various papers, but he is the modelling genius.

    Simon M
  8. Miyaru

    Miyaru Tech Instructor

    # of Dives: 2,500 - 4,999
    Location: EU
    The post above from @Dr Simon Mitchell is for sure interesting and very relevant, but also at least 4 years old. It doesn't describe how to prevent this from happening. In order to not hijack this thread I started a new one here: ICD - preventing Inner Ear DCS
    Would like to see your comments on that!
  9. Michael Guerrero

    Michael Guerrero Solo Diver

    Shearwater set to 70/70, though I'd like to run it at 100/70.
    Last edited: Jan 31, 2018
  10. Doc Harry

    Doc Harry Solo Diver

    # of Dives: 500 - 999
    Location: Appalachia
    What computer? I have a small laptop computer with my deco planning app.

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