Questions Regarding Deep Into Deco (Revised & Updated) by Asser Salama

[Ryan Neely]

I'm currently reading the aforementioned book. I've made it through Page 45. The book is dense and tightly packed with information. Now, for full transparency: I am not a technical diver, nor am I planning to become one anytime soon. (I think it would be fun and challenging to learn and use new skills like this, but my current lifestyle and dive buddy will likely never allow for that route for me.) I am reading the book for recreation and theory only. I mention this because it is highly possible that the questions I have from the text may come from a mere knowledge gap that I otherwise would have discovered elsewhere (in a course, for example).

Here are my questions:

Several times Salama has referenced alveolar water vapor pressure. During these mentions, he provides a number (0.0493) but does not explain where that number comes from or offer a label (such as bar or atm). My assumption had been that the number actually referred to the specific alveolar water vapor pressure within the lungs at sea level, but I cannot find a reference to corroborate this assumption.

Can anyone help me understand what his number is and where it comes from?

Second, at the end of Chapter 5, Salama is discussing Isobaric Counterdiffusion. During this discussion, he provides two equations to illustrate the rule of fifth. When solving the equations, Salama appears to move a decimal point without explanation. One of those equations is as follows:

(0.25 * 4 * 0.015) + (0.55 * 4 * 0.067) = 0.015 + 0.1474 = 1.624 bar

Admittedly, it has been a few years since I've dealt with advanced maths, but I'm confused over the decimal point shift.

Can anyone help me understand this shift as well?

Thanks in advance!

 

[John C. Ratliff]

One of my pet peeves is when people put together equations and don’t include the units. Without the units, it is difficult to say what he meant. The fact that it came out to 1.624 bar indicates that at least one unit was “bar” for barometric pressure, but what the others are is unknown from just the equation. It is apparent that whatever they are, they cancel each other out. And, yes,

0.015 + 0.1474 does not equal 1.624 (whether it’s bar or something else). This was not only bad mathematical expression, but a wrong multiplication and addition. Now, I did some of this long-hand, and found that the decimal was wrong above.

BUT, the second operation was also wrong if your figures are correct. I got out my TI-83Plus calculator and went through the calculation, and came out differently too.

(0.55 x 4 x 0.0678), if multiplied out, equals 0.1.

(0.25 x 4 x 0.15), when multiplied, is reduced to (1 x 0.15), which equals 0.15 (rounded by my calculator to 0.2). (Hint: 0.25 x 4 is 1; 1 x anything is itself.).

Ad the two together, and

0.1 + 0.15 equals 0.25.

So something is really “off” with these figures and equations.

SeaRat

 

[dmaziuk]

Several times Salama has referenced alveolar water vapor pressure. During these mentions, he provides a number (0.0493) but does not explain where that number comes from or offer a label (such as bar or atm). My assumption had been that the number actually referred to the specific alveolar water vapor pressure within the lungs at sea level, but I cannot find a reference to corroborate this assumption.

Can anyone help me understand what his number is and where it comes from?

The one I have is 47 mm Hg or 0.0627 bar, I forget which source it came from.

This one gives 45 mm Hg: Alveolar Gas Equation - StatPearls - NCBI Bookshelf

Exactly how he got to 0.0493 depends on how he is using in what units: e.g. Baker in "Decolessons" does everything in fsws and uses 2.042 (1 bar ~= 33 fsw i.e. 2.042/33 = 0.061878788)

Edit: maybe he's doing in meters of Hg?

 

[Ryan Neely]

@John C. Ratliff I agree that a lack of units is maddening, however there may still be an error in your calculations. I get the same numbers as Salama when working longhand, except for the misplaced decimal.

@dmaziuk That is also the number I have found when searching PubMed. Since he doesnt provide units, I cant be certain.

Perhaps, in the end, it doesn't matter overly much. Factoring his smaller number into other equations he uses may appear to build in a level of conservatism (although, it will be minor).

Really, I'm just curious where the number comes from.
 

 

[dmaziuk]

Perhaps, in the end, it doesn't matter overly much. Factoring his smaller number into other equations he uses may appear to build in a level of conservatism (although, it will be minor).

It doesn't really make sense because it's easier to work in bar than fsw (the round-off) and I fail to see how any other unit would be useful in any practical context.

Maybe it's just as accurate as that decimal point shift?

 

[dmaziuk]

PS out of curiousity, what does he have PCO2 and Respiratory Quotient?

 

[Ryan Neely]

PS out of curiousity, what does he have PCO2 and Respiratory Quotient?

He doesn't list either for this specific equation. He's working ppHe and ppN2 in a trimix breathing mix.

 

[happy-diver]

Are you sure you're not really Richard Ayoade man



or is Richard Ayoade not really he


Was a good guy Asser Salama did you read his mags

TECH DIVING MAG (TDM), the real TECHNICAL DIVING MAGAZINE (TDM, 12/2010 - 09/2018)

Here's all 32 of them so people can be on ScubaBoard with more

 

[lermontov]

Second, at the end of Chapter 5, Salama is discussing Isobaric Counterdiffusion. During this discussion, he provides two equations to illustrate the rule of fifth. When solving the equations, Salama appears to move a decimal point without explanation. One of those equations is as follows:

Help me understand the rule of fifths for IBCD

This great article gives you a quick and dirty understanding of the rule of fifths as a way to avoid Isobaric Counterdiffusion when diving Helium. Hello, The rule of fifths is a classic example of 'bro-science' that somehow became accepted as fact and continues to be perpetrated to this day...

scubaboard.com

FYI here's simon Mitchel take on rules of fifths post #21

 

[Ryan Neely]

Are you sure you're not really Richard Ayoade man

View attachment 822956

or is Richard Ayoade not really he


Was a good guy Asser Salama did you read his mags

TECH DIVING MAG (TDM), the real TECHNICAL DIVING MAGAZINE (TDM, 12/2010 - 09/2018)

Here's all 32 of them so people can be on ScubaBoard with more

I really wish I was Richard Ayoade. I've worked hard to tone down my enthusiasm because he is my idol. (Seriously, if I could take a Russian vacation with Greg Davies and watch him being throttled by tree branches, that would be the best day ever.)