Question about pressure graph (M-value relationship)

[nohappy]

Hi guys,

I'm reading "Deco for divers", but I'm confused when reading chapters about M-value and oxygen window.
Assume that we're using air in the following questions...

1.
Everyone knows the famous "Pressure graph" that describes M-value relationship.

So far my understanding is that if you stay at the same depth long enough, you'll reach "Ambient pressure line" where ambient pressure equals to compartment inert gas pressure (y=x) and that's "saturation". When you start to ascend you'll go above the "ambient pressure line", and that's "supersaturation". This is confusing me, I thought if you stay at the same depth long enough, for example, at the sea level, your compartment inert gas pressure will only reach 0.79ata. And so if you're going to plot a line that indicate saturation it should be below "ambient pressure line" (i.e. y = 0.79x). Which part did I miss??


2.
According to "Deco for divers", in the chapter about oxygen window it says that off-gas speed only depends on inert gas partial pressure gradient between tissue and ambient air. However, "forming bubble" depends on the gradient between tissue total pressure (which includes O2,CO2,water,N2,...) and the ambient pressure. Once it reaches the critical point it might start to form bubbles. And the critical point is "M-value". That confused me again, if M-value depends on compartment total pressure, why do you make y-axis "compartment inert gas pressure" instead of "compartment total pressure"? What did I miss?

 

[leadduck]

Please see "Understanding M-Values" by Erik Baker (google for the PDF).

1. Your compartment inert gas pressure converges to the inspired inert gas pressure, which is less than the ambient pressure. In your graph above, the vertical section of the black line would eventually drop below the green line again towards y=0.79x for nitrogen if you wait long enough at that depth. In your graph however the diver didn't wait for that but decided to ascend to the next stop as soon as possible.

2. The critical point M-value is not the point where bubbling begins; M-value is the point where bubbling is so strong it'll cause DCS (symptomatic bubbles). There are bubbles for tissue total pressure below M-value (silent bubbles). See Fig.2 in "Understanding M-Values" by Erik Baker. The graph usually shows only the inert gas because the others (CO2, water) cause a small constant offset and are included in the M-value. O2 is usually ignored because O2 gets metabolized right in the tissue quickly and need not be transported to the lungs to remove it from the body, so it's not that much of a concern like N2 or He.

 

[nohappy]

Thank you @leadduck, I already checked "Understanding M-Values" before the OP and still have questions.

Reply your statements as following..
1. If so, why does "Understanding M-values" says "This is important because when the inert gas loading in a compartment goes above the ambient pressure line, an overpressure gradient is created."? It should be overpressure already when it reached y=0.79x (or whatever Nitrox you're using). And if you agree that it's saturated at y=0.79x, why do they plot y=x since it should not be meaningful in this case.

2. I knew silent bubbles always exist, but that's not my question. I quote this from "Deco for divers" page 49 - "If the sum of all of the gas pressure exceeds the M-value then bubble will start to form. So each gas acts independently of the other gases for the purpose of off-gassing but they act together when it comes to forming bubbles". My question is not "when" do bubbles start to form. My question is that if M-value depends on the sum of all of the gas pressure, is it "because we ignore other minor gas in the tissue" the reason why making y-axis to be partial pressure? And yet the book mentioned it should be the total pressure that matters beforehand, not partial pressure. That confused me.

 

[elmo]

I haven't read the book for a while, but the following points come from logic.

1. This graph is conceptual rather than exact. There is no scale on either axis, so you can't say the line is y=x any more than y=1.2345x. Another thing is it's missing the time element. I believe the 'ambient pressure line' should really be 'fraction inert gas x ambient pressure' as it's supposed to indicate the saturation value. But it is correct that the line goes through the origin. Surfacing is at the dashed line.

2. Total pressure should be total inert gas pressure. Water in your blood won't bubble (unless you are literally cooking), O2 will be absorbed/metabolised, and CO2 is such a tiny fraction that it's irrelevant (if you have enough that it's a consideration for deco you have bigger issues to deal with).

 

[TrimixToo]

1. The maximum possible inert gas loading depends on what you are breathing. The .79 number is for air. It can be lower for Nitrox or higher for Trimix.

2. The offgassing *rates* of the individual gases vary (as do their ongassing rates). The sum of their partial pressures vs. ambient absolute pressure is what can cause bubbles.

HTH...

 

[nohappy]

Thank you @elmo,

1. I thought of that, too. I tried to believe that it's just a conceptual line and then I read "understanding m value" online, and found that it states explicitly "the slope of ambient pressure line is 1". If the slope should be any according what gas we are using, why does the paper specifically declare the slope is 1?

2. I understood your point, but what I quoted is what I read from "oxygen window" in deco for divers. You can also read Oxygen window from wiki. "... the window is a major factor in the rate of bubble shrinkage when the subject is in a steady state...".
That indicate the sum of pO2, pCO2 (and pN2) also contribute to bubble forming. I agree that maybe their pressure are so small that we'll ignore them when calculating, but why do authors specify y-axis as "inert gas pressure" when it's just a conceptual coordinate? I mean, you don't have to not mention "all gas pressure" when you're just plotting a conceptual graph, right?

 

[EFX]

So far my understanding is that if you stay at the same depth long enough, you'll reach "Ambient pressure line" where ambient pressure equals to compartment inert gas pressure (y=x) and that's "saturation". When you start to ascend you'll go above the "ambient pressure line", and that's "supersaturation". This is confusing me, I thought if you stay at the same depth long enough, for example, at the sea level, your compartment inert gas pressure will only reach 0.79ata. And so if you're going to plot a line that indicate saturation it should be below "ambient pressure line" (i.e. y = 0.79x). Which part did I miss??

As you descend the pressure in the tissues is increasing relative to the ambient pressure (inert gas pressure in the lungs). If you stay long enough at a certain depth those tissues reach saturation. That point on the graph is represented by the black circle on the ambient pressure line. Now, when you start to ascend the pressure in the tissues is higher than ambient. This is why the black line stays above the green line. If we were to stop and stay at a shallower depth to allow equalization of the pressure between the tissues and ambient, then the point on the black line would drop to a point on the green line and stay there. The blue line represents the highest pressure in the tissues above the ambient pressure that can be tolerated without causing DCS. This is called the m-value or maximum value. Sometimes it's easier to think in terms of a pressure difference. It's not a particular pressure in the tissue but the difference in pressure between two points: the tissue and ambient (lung or water pressure at that depth). This is actually what the graph depicts.

 

[EFX]

nohappy, after re-reading your post above I think I understand your confusion. The ambient pressure line in the graph is not total pressure. It is the inert gas partial pressure at your current position in or out of the water column. The pressures are in absolute units. Therefore, the point on the green line at sea level is 0.79 ata assuming saturation. If we start the dive at sea level and dive to 66 ft the tissues will saturate not at 3 ata but 3 x 0.79 ata or 2.37 ata due to the inert gas nitrogen.

Another point of confusion is the term supersaturation. This might imply the tissues can absorb more inert gas above saturation but this is not correct. Tissues at saturation cannot absorb any more gas staying at the same depth. It refers to the pressure of the tissues being higher than the ambient pressure and thus "supersaturated" so that the flow of inert gas is out of the tissues rather than in to them. In the graph it represents the area between the m-value and ambient pressure lines.

Looking at the graph, in the region below the green line the tissues will be taking on inert gas. On the line they are saturated and gas will neither be flowing in or out of the tissues. Above the green line but below the blue line they are supersaturated and gas will be flowing out of the tissues. Above the blue line gas is flowing out of the tissues at a rate that may cause DCS.

 

[DevonDiver]

I find the video below to be quite explanatory. It's a demo of how the Shearwater Petrel operates with m-values, differential etc.... but it's a good general demo for how the m-value graph works..

 

[nohappy]

Thanks @DevonDiver, that's a clear explanation video. After watching that video and this article Gradient Factors, my question is that why is the ambient pressure line has the slope 1.0? The zone between M-value line and ambient pressure line is defined as "decompression zone". But decompression (or supersaturated) should start already at the line y=0.79x (assume using air) which should be below y=x?

I quote this from the article - "GF 0% means that there is no supersaturation occurring and inert gas partial pressure equals ambient pressure in leading compartment". Is that correct? I mean supersaturation should be already occurring before ambient pressure line (e.g. y=0.79x)?