Question about pressure graph (M-value relationship)

[EFX]

Hi nohappy. The graph shows "compartment inert gas pressure" and not "total pressure". We defined the green line as the point where the tissues are saturated. That line has a slope of 1 and defines the equation y = x. If the x axis was total gas pressure (O2 + inert gasses) then y = 0.79x. But the x axis is partial pressure of inert gas at ambient pressure. Tissues saturated with inert gas will have the same pressure as the inert gas at ambient pressure.

GF-Lo and GF-Hi both define a percentage of supersaturation for a particular TC. On the green line supersaturation is 0% of the m-value. On the blue line supersaturation is 100% of the m-value. Therefore, you are correct when you say that TC inert gas pressure equals ambient at a GF of 0%.

Edited for accuracy.

 

[Narcoz]

Hi

I just started reading deco for divers and stumbled across the exact same question as this post.
I was wondering if you had in the meantime found an explanation for this discrepancy...

My observations are the following:

1) In the book, except for the fact that the line is drawn at y=x, everything else seems to make sense.

- (Page 35) - "if we stay at depth long enough, then the tissues will become saturated and the tissue tension will be equal to the inspired gas pressure at ambient pressure". - So far so good

- (Page 35) - "Point B on the diagram represents the point where he tissue tension and inspired inert gas pressure at ambient pressure are equal" - Again, makes sense.

- (Page 32) - "Tissue 2's nitrogen tension is now higher than the inspired nitrogen pressure and so it starts too to off gas although the nitrogen tissue tension is not yet higher than ambient pressure." - Clearly we're in agreement that you start to off gas when the tissue pressure exceeds the inspired inert gas pressure, but not necessarily the total absolute ambient pressure.

- The book also agrees that saturation is defined as when the pressure of inert gas in tissues reaches that of the inert gas partial pressure for a particular depth.

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So overall the book is consistent with regards to that issue, except, again for the fact that the green line seems to be drawn at y=x, where x clearly specifies ambient pressure, not inert partial pressure at ambient depth.

Now regarding the Understanding M-Values paper, that simply does not make sense to me. You are right that the slope is explicitly declared as being equal to 1. The x-axis is defined as "Ambient pressure, absolute", which as far as I understand means total pressure exerted by sum of all gases (1ATA a sea level, 2 ATA at 10m etc.).

To sum up, I think deco for divers has it right and that the green line, being just a conceptual diagram is drawn at y=x for simplicity, and that the M-Values paper simply has it wrong.

 

[J-Vo]

There is a conversation going on in the Shearwater forum about correcting the issues with this graph.

ceiling/GF

 

[KenGordon]

There is a conversation going on in the Shearwater forum about correcting the issues with this graph.

ceiling/GF

Not exactly that graph, similar ones with poor details. The one in the book is correct.

 

[EFX]

The graph that Nohappy in the first post of this thread comes from page 52 of the first version of the book "Deco for Divers". Mark Powell uses a similar but more simple graph on page 41. He explains point B which lies right on the ambient pressure line this way: "Point B on the diagram represents the point where the tissue tension and inspired inert gas pressure at ambient pressure are equal, that is the tissue is saturated."

It is not total pressure at ambient pressure but inspired inert pressure at ambient pressure. The fact that the slope is 1 is because both the x and y axis are inert gas pressures. The labling on the graph is incomplete. The x axis on the graph should be labled "Inert gas pressure at ambient pressure".

 

[Cichlidgoob]

This message string helped me a lot. Thanks for taking the time to explain some of the details.