Higher M-Values in Faster Compartments...

[Blackwood]

A'ha. It's often best to go to the horse's mouth. The OP is referring to a PADI document, so here ya go:

When pressure is reduced after some gas uptake has taken place...
... The result is a theoretical supersaturation. A certain amount of supersaturation is usually tolerable without bubble formation, but the presence of a theoretical supersaturation is generally considered by some decompression researchers to be an indication that bubble formations has probably begun. The impact of supersaturation depends on many things, including exercise, temperature, etc., and especially the duration of time it is maintained; longer exposures entail a higher risk that supersaturated gas will become bubbles.

The next element of the Haldane method is a means of interrupting (or slowing down) the ascent of a diver. The method assumes that given differential partial pressure or supersaturation can be "tolerated" in each compartment...
...At some point one or more of the compartments reaches its limit-empirically determined-of tolerable differential pressure over ambient, and at that point (depth) the diver should stop or slow down the ascent...
...As a reminder, the values of the ascent limits are empirically determined.

Haldane expressed this limit as a ratio of partial pressures...
...His value for a tolerable pressure ratio... was observed to be 2, a figure that was the same for each compartment. Later studies showed that a single set of ratios does not work well for any but short, shallow dives, nor does it work for a wide range of dives, and in time the ratios were changed to reconcile results with more recent data. The current air tables of the U.S. Navy were done with ratios (Dwyer, 1955; 1956; Workman, 1957)...

...The concept of setting the limit based on a differential pressure was introduced by Workman (1965). This amounts to about the same thing as ratios but is easier to use over a broad range of depths and times. It consists of selecting (again, empirically) a set of differential pressures which represent the maximum tolerable gas loading in a compartment at each depth. These are called M-values...

They've since been massaged, but Workman's original m-values came from empirical data. And as was wisely "spoken" on scubaboard, I'll reiterate that an unexploded goat doesn't guarantee a safe profile [paraphrased].

 

[rjack321]

I think what you're asking is why can faster compartments "tolerate" a higher M-value in the model? The logic, is there is any, is that a fast tissue can tolerate a higher overpressure gradient because enough gas is leaving the tissue fast enough to avoid creating a bubble inside. While an overpressure gradient in a slow tissue would create a bubble because even a moderate overpressure gradient will not 'vent' gas fast enough. Remember Buhlmann M-values are not actually based on the solubility of gasses, they are based on the speeds of molecules across membranes.

Fast across a membrane = hard to form a bubble at a given overpressure gradient = "tolerates" higher over pressure
Slow = harder to get out = easier to form a bubble = "tolerates" less over pressure.

 

[TSandM]

That may well be the origin of it, Richard. I guess the high overpressure gradient just doesn't PERSIST long enough in fast compartments to encourage bubble formation. On the other hand, one wouldn't think that the supersaturation amounts would be very great, as the compartment would have to pass through many "moments" of supersaturation on its way to equilibration, so there'd be significant time spent supersaturated anyway. (Boy, I didn't phrase THAT very well!)

 

[rjack321]

I don't know how close to reality it is, but its the basic premise behind "no deco" diving. A 5 min fast tissue is completely saturated in fairly short order ~20mins. On a 50min 60ft dive, "somehow" those fast saturated tissues are not bending (at least not clinically and all the time) with a direct ascent.

Its all just voodoo trying to imperfectly describe human physiology. I don't know anyone using straight Buhlmann, its all gradient factor now which essentially backs off pushing the controlling M-value so hard.

 

[TSandM]

I think, from brief discussions with German in Mexico, that he is still doing pretty straight Buhlmann and teaching it as well -- one deco gas, that being either O2 or 80%. I was really surprised to hear it.

 

[rjack321]

I think, from brief discussions with German in Mexico, that he is still doing pretty straight Buhlmann and teaching it as well -- one deco gas, that being either O2 or 80%. I was really surprised to hear it.

100/100 Buhlmann?? I have to say that's nuts. The water's toasty warm, what's the rush?

Everyone I personally know using Buhlmann in some fashion or another is defaulting to 30/85 or something like that. There are some rumors about that on big big basically saturation dives some folks are using 30/115 which theoretically should bend them like pretzels (since they are 15% over allowed pressures). But they worked that out by trial and error.

 

[AbyssalPlains]

Keep in mind, once the compartment exceeds the M value, it becomes a Decompression dive.


Bruce

Sorry, but this is incorrect. An M-value is defined as the maximum difference between ambient pressure and the partial pressure of inert gas in a compartment that that particular compartment can tolerate before overt symptoms of DCS occur. In other words, once inert gas is dissolved in a tissue, you can relieve the ambient pressure only so much before gas starts bubbling out. The higher the M-value of a compartment, the more you can relieve the ambient pressure before it starts forming gas bubbles. Once you exceed the M-value of a given compartment, for example by ascending above a certain ambient pressure, gas bubbles form in that compartment. You could say once you exceed the M-values, DCS will likely result.

 

[TSandM]

M 0 is the M value at the surface for a given compartment; if a dive results in loading such that a routine ascent and safety stop will result in you surfacing with a higher nitrogen tension that the M 0 for that compartment, the dive has become a staged decompression dive. At that point, you have a mandatory waiting period underwater before you have offgassed enough nitrogen to surface with tensions at or below the M 0 of all compartments.