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

[Ryan Neely]

As the title suggests, I'm working my way through Mark Powell's Deco for Divers. It's a fabulous book that makes for a great introduction to decompression theory.

I'm closing in on the end of the book and have reached the section where he discusses trimix. In this section, there is a graph he uses to illustrate the different rates at which helium and nitrogen on-gas and off-gas.

I think I'm understand what he's saying here, but the graph seems to be missing a key element which would make understanding what is happening here more . . . intuitive.

So, first, here's the graph:

Graph from Deco for Divers

• Ryan Neely
• Feb 22, 2021

This is a graph pulled from Mark Powell's Deco for Divers which illustrates the different rate...

Next, here is the excerpt from the book illustrating the purpose of the graph:

". . . Figure 85 . . . represents tissue loadings for helium and nitrogen. These loadings are for a 60m dive for 40 minutes using 20/40. . . . We can see that compartments one to three are saturated with helium and compartment four is very close. In contrast only compartment 1 is saturated with nitrogen and all the others are at a lower level of saturation. We can also see that in most of the compartments the level of helium loading is higher than the level of nitrogen loading. The exception to this is the slower compartments eleven to sixteen. The reason for this is that all compartments already have a nitrogen loading from being saturated with nitrogen at the surface, but do not have any helium loading. [Emphasis mine.]"

I was confused by this at first because the excerpted explanation that accompanies the graph seemed to be suggesting (correctly) that tissues were already loaded with nitrogen from being on the surface, which my mind took to mean saturated. This immediately sent my mind spiraling down a rabbit hole, trying to understand how a tissue already saturated from breathing air (as any normal human being would) at the surface wouldn't already be an issue for any dive.

I realize now this was not correct.

What I think is missing from the graph that would have made my understanding easier is a label along the Y-axis which indicates that the numbers there represent partial pressure (either in atm or bar).

Because the graph represents the end of the dive's bottom time, it now appears to me that the slower tissues (especially tissue sixteen) is still sitting at a partial pressure of close to 0.79 atm or bar (or whatever the necessary unit for what the graph represents).

I just want to make sure I'm not misunderstanding something here, because (as I mentioned) I was starting to go down a tangent that completely upended my entire understanding of diving.

Any additional thoughts would be helpful.

Thank you.

 

[Duke Dive Medicine]

Hi @Ryan Neely ,

I had to dig back into my own copy of the book to double-check - yes, the y axis on those graphs likely represents the partial pressure of dissolved gas in bar, since Mark is from the UK. At those pressures bar and atmospheres are practically equivalent (1 atm = 1.01325 bar).

Best regards,
DDM

 

[GF99/99]

I would agree the y-axis units are atm's. I had to study this for a second too. But to verify the ppHe and ppN at 60m with a 20/40 would be 2.8 atm for each gas and this correlates to what is show on the bar graph for the fast tissues. However interestingly in theory the ppHe and ppN for a 20/40 on surface would be 0.4 atm but obviously a person would be breathing normal air and only switch to the 20/40 when they put there reg in just before going under.

 

[fisheater]

We are all saturated with nitrogen at the surface, at one atm of pressure of 79% nitrogen and virtually unmeasurable % of helium. That's our normal starting point.

When we start to dive we increase the saturation of nitrogen because of the increased pressure. We increase the saturation of helium if we start to breath a significant percentage of it, whether at the surface or under increased pressure.

The difference is that we're always breathing nitrogen and only breath helium when it's added to our diving gas.

 

[dmaziuk]

He should have added a thick horizontal line at "normal" N2 loading = 0.79. (With the caveat that it's only "normal" at sea level in good weather and so on and so forth.)

 

[Angelo Farina]

@Ryan Neely , your assumptions are correct.
These graphs appear to have a vertical axis in bars.
However, as a scientist, when I find a book or a paper carrying charts without SI units clearly exposed, I tend to evaluate the whole book or paper as substantially crap.
I do not own this book, and having seen that it contains charts without proper SI units exposed, I will never buy it, nor recommend it.
Sorry for my drastic position, but not exposing the SI units is something I cannot bear at all!

 

[Duke Dive Medicine]

@Ryan Neely , your assumptions are correct.
These graphs appear to have a vertical axis in bars.
However, as a scientist, when I find a book or a paper carrying charts without SI units clearly exposed, I tend to evaluate the whole book or paper as substantially crap.
I do not own this book, and having seen that it contains charts without proper SI units exposed, I will never buy it, nor recommend it.
Sorry for my drastic position, but not exposing the SI units is something I cannot bear at all!

Angelo, with due respect to your accomplishments, Deco for Divers is anything but crap, and I would humbly agree with you that that's a pretty harsh judgement for someone who hasn't read the book. I have no stake in this and I've never met the author, but Mark Powell's book has done more for divers' understanding of decompression theory than just about any other resource out there. I'm a scientist too, and my own copy is underlined, bookmarked, dog-eared, and coffee stained from use. I personally highly recommend it for anyone looking to better understand why we do what we do.

Best regards,
DDM

 

[tursiops]

I don't see any major problem with those two figures, for three main reasons:
(1) they are just two of many figures in that chapter, and you'll find the y-axis IS labeled as ATM/BAR on many of them, where needed, even to Angelo's specifications
(2) the inset legend on the second of those two figures explicitly says P...and Powell uses ATM/BAR throughout the book, so there is no ambiguity
(3) it is a graph of "tissue loadings," which Powell consistently describes as pressure in ATM/BAR.

All three of these reasons involve the context for the two graphs, not focusing on just the two graphs.
Cherry-picking nearly always causes problems.

 

[dmaziuk]

I do not own this book, and having seen that it contains charts without proper SI units exposed, I will never buy it, nor recommend it.

It is called "Deco for Divers", not "Deco for Engineers", not "Deco for Computer Scientists". There were a couple of glaring holes in the parts I was interested in, too, but overall it's OK. For divers -- I wouldn't buy it for myself, I borrowed it on interlibrary loan instead.

 

[Angelo Farina]

Angelo, with due respect to your accomplishments, Deco for Divers is anything but crap, and I would humbly agree with you that that's a pretty harsh judgement for someone who hasn't read the book. I have no stake in this and I've never met the author, but Mark Powell's book has done more for divers' understanding of decompression theory than just about any other resource out there. I'm a scientist too, and my own copy is underlined, bookmarked, dog-eared, and coffee stained from use. I personally highly recommend it for anyone looking to better understand why we do what we do.

Best regards,
DDM

I respect your opinion, which is based on having read the whole book, which I did not.
But here probably we see the difference between an hard scientist (Physics, Engineering) and a “soft” scientist (biology, medicine, natural sciences, etc.).
For us hard scientists something not adhering to the mandatory publication standards is a crap publication. This does not involve any judgement on the scientific value of the author, and probably is mostly a fault of the editor or the publisher, not of the author. Indeed I will never spend money of my University purchasing sub-standard publications.
Doing this would even be considered a crime here, as I am wasting public money purchasing a scientific publication which does not meet the standards.
Of course anyone can spend his private money as he likes, so probably for people not using this book for their public-funded research work, this book can still be considered money well spent.
We scientists should protest vehemently with the publisher whenever we find these unacceptable omission in publications we purchase.
In some cases I had to send back books or scientific journals to the publisher, and asking for refund, because the content was sub standard, and I could not justify the expense to my administration.
Serious publishers take in great account these rejections, and often release a new version of their product after correcting the faults.
But if no one complains, these sub-standard books will continue to be published, edition after edition, without correcting the problems.