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Requesting Validation in My Understanding of a Concept from Mark Powell's "Deco For Divers"

Discussion in 'Decompression Theory' started by Ryan Neely, Feb 22, 2021.

  1. wedivebc

    wedivebc CCR Instructor Trainer ScubaBoard Supporter

    Excuse my less learned take on this but if you are using a graph to illustrate the relation ship between two elements then would it matter if the graph was stated in SI units or lollipops?
  2. Angelo Farina

    Angelo Farina Marine Scientist

    # of Dives: 1,000 - 2,499
    Location: Parma, ITALY
    Non-SI units are simply unacceptable in a scientific publication...
    Lollipops (or, better the number of lollipops) is a correct SI unit.
    A measurement is made of two parts, the number and the unit. Both must be present and coherent.
    The problem of the charts in the original post is that the vertical axes did carry numbers but no units...
    What is the additional cost for the publisher ensuring that all charts in his book are properly labelled according to accepted standards?
    Publishers get money for purchasing their products, so I expect to get something good.
    When I purchase some equipment or tools on Amazon or the like, I complain, ship back and ask for a refund if the good received is not compliant to standards or minimum quality.
    The OP was probably a “normal” diver, so the target customer for this book. Indeed, he was confused by charts without proper SI units, so definitely he did find a fault in the book he purchased.
    As I find this a severe fault, if it was my money, I had immediately complained and asked for a refund.
  3. Duke Dive Medicine

    Duke Dive Medicine ScubaBoard Supporter Staff Member ScubaBoard Supporter

    If it was a scientific publication, I'd probably judge it a little more strictly. But, it's not a scientific publication, it's an informational book. My "soft" scientific mind can excuse the author not labeling a graph axis and choose not to throw the baby out with the bath water, so to speak.

    BTW, the scientific method is (or should be) the same, regardless of one's field of endeavor.

    Best regards,
    RyanT and Neilwood like this.
  4. Ryan Neely

    Ryan Neely ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 50 - 99
    Location: Akeley, MN USA
    Thanks for the validation, folks! My brain sort of slipped sideways when he started talking about tissue loading at the surface, so I'm glad to hear I've understood correctly.

    As for the efficacy of the book lacking in explicit SI units, it has sometimes made the graphs confusing to correlate with the text, but from an introductory standpoint for non-scientists, the book is fabulous.
    Angelo Farina likes this.
  5. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
    I could not disagree more.
    Chill, Angelo. I'm a hard scientist too....PhD, mechanical engineer, fluid mechanics and applied math, physical oceanography.
    Many peer-review publications. Editor for many years of a major journal.
    Your over-the-top opinion is just yours. Please don't say "we" and please stop denigrating anything other than physics.
    TrimixToo, RyanT and fisheater like this.
  6. Brett Hatch

    Brett Hatch ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 100 - 199
    Location: Monterey
    This argument over units is absolutely hilarious. The meaning of the chart has precisely nothing to do with choice of units -- if Powell had chosen psi, the numbers on the axis would be multiplied by about 15, but the image and meaning would be identical. But here, the units are either atm or bar, which are not meaningfully distinct units in this context, because they only differ by 1%. How many pixels high are the bars on that chart? If I were presented a side-by-side comparison of the exact same chart scaled down by 1% to accommodate units of atm, I would need a magnifying glass to determine which one is which. And still, the meaning would be identical. Mr. Farina, you might want to give the book a chance, it was very helpful to me.

    @Ryan Neely I hope that between the chart, the text, and this thread, Powell's point is clear. I would paraphrase it as simply: helium is a faster gas than nitrogen. Here is a chart that demonstrates this fact, and just to be sure the reader is not confused by what's happening in the slower compartments, he is reminding us that He is starting from a PP of 0, but N is starting from a PP of about 0.79 (under the reasonable assumption that the diver has been breathing air sufficiently long to have all of their tissues completely saturated by air).

    So yes, I agree with @dmaziuk that a little horizontal bar at a height of 0.79 would make it easier to see a small gain in N vs a larger gain in He, even in the slower compartments.

    Edit to add, after re-reading the OP: One more thing that might be confusing here, is what exactly is meant by the word "saturation." I do not have the book in front of me, so cannot cite the page where Powell defines it, but I would recommend re-reading that definition. The key part being that saturation is not a fixed state, it is a relationship between the gas you are breathing, the ambient pressure, and the gas dissolved in your blood and tissues. Specifically, it is the equilibrium state where breathing that gas at that depth results in no change in tissue loading, because the tissues are already at the same partial pressure as the breathing gas.

    So by walking around living our lives on the surface for awhile (say, a few days), all tissues will achieve saturation -- equilibrium -- with an N load of about .79. As soon as any of the relevant variables change (ambient pressure increase by going underwater, pressure decrease by going up a mountain or in an airplane, or breathing a different gas), we will no longer be in equilibrium, so can no longer say that we are saturated. But, in a few minutes the fastest tissues will reach saturation, a few hours for the middle tissues, and days for the slowest tissues which, if I recall correctly, are modeled with half-times of something like 6 hours.
    sabbe and DiveClimbRide like this.
  7. KenGordon

    KenGordon Rebreather Pilot

    Anyone who wants to know more about deco could do much much worse than join one of the web based course/workshop events Mark has been running. If you go to his website and send him an email I am sure he will send details Dive-Tech: TDI Nitrox, Helitrox, Trimix and technical diver training courses with instructor Mark Powell I think he has two levels of course, so neither beginners nor more knowledgeable people need worry about being in the wrong cohort.

    I think, but could easily be wrong, that the format is a weekly web based session with the course running several weeks. There are sessions timed appropriately for the various timezones.

    I have read Deco for Divers and am a technical person, but had several “oh, now I see...” moments doing in person training.
  8. Neilwood

    Neilwood Loggerhead Turtle

    # of Dives: 50 - 99
    Location: Scotland
    I have to say that, irrespective of any comments regarding units, Deco for Divers is a book that ALL divers should try to read.

    It is not a technical or scientific manual (which he readily admits), it is an attempt by a very experienced technical diver to explain what are often VERY technical and scientific concepts in a concise way that lay people can understand and, to my mind anyway, he does the job superbly. I can forgive any small errors such as not marking up a chart 100% correctly on the basis that, when read with the text, it explains the concepts he is discussing very well (probably the best explanations that I have seen). The example tables mentioned (which IIRC are a series showing a progression) do a brilliant job of representing the process without too much by way of technical mumbo jumbo - he could have spent pages explaining the scientific detail but that is not what his book is about.

    There is a reason this book is recommended so often by a range of people from scientists working in the field of decompression through experienced technical divers to simple recreational divers like myself.
  9. Wibble

    Wibble Barracuda

    # of Dives: 500 - 999
    Location: UK
    What book would you recommend? Or is that reading scientific papers?

    Deco For Divers is an excellent book for divers to read. It doesn't pretend to be in-depth, but it covers all the things divers care about.

    All the units are SI - Bar, metres, seconds, litres. This is what technical divers and scientists use.

    Some of the diagrams are hard to interpret and need some effort on the part of the reader, for example the explanation of the effects of Gradient Factors. That's because the subject is complex and hard. Mark Powell, the author, has done lots of presentations on this so a trawl of YouTube will net you some decent talks.
    Storker and Angelo Farina like this.
  10. Angelo Farina

    Angelo Farina Marine Scientist

    # of Dives: 1,000 - 2,499
    Location: Parma, ITALY
    Of course, my opinions are just MINE. I have very harsh opinions about the whole "scientific publications" world, I find that most publishers are really ripping off scientists of their work, not paying the authors for their author's rights, asking to other scientists to peer-review for free, and finally charging universities and libraries of absurd costs for subscribing to journals or purchasing books.
    I am a strong supporter of the Free Science movement, and a daily user of SciHub.
    Going back on topic, I find that in this case the publisher/editor are to be blamed. Not the author. A good publisher/editor should ensure that all charts are properly labelled.
    But if no one raises the point and complains, asking their money back, these predatory publishers will continue in their bad behaviour, amassing money in change of a crap job done.
    Again, all this is my personal position, I do not want to evangelize anyone to follow my own ideas...
    But here we are in a public forum, the "right" place where everyone is free to share his opinions...

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