Higher M-Values in Faster Compartments...

[DoOrDive]

Hello all.

Right, I know this isn't about Tech Divingm but this seems like the best forum to post this question... Hopefully some of you Techies will be able to help me out.

I don't understand why, in Haldenean decompression models, faster compartments have a higher M-Value, while slower compartments have a lower M-Value.

It seems to me that it should be the other way round... Surely if a compartment absorbs and releases nitrogen quickly, then the acceptable levels should be lower - otherwise, upon surfacing, a large amount of nitrogen will be released quickly...?

Have I missed something here?

Any help would be much appreciated!

Ta!

 

[texdiveguy]

A good basic starting point: http://www.tek-dive.com/portal/upload/M-Values.pdf

 

[Meng_Tze]

I think it had to with the assumption that the faster compartments/tissues have a greater tolerance for the differential pressure (M-Value) than the slower ones.

 

[mrlipis]

The M value is the maximum allowable nitrogen in a given compartment. This should not be confused with half times. A faster compartment absorbs and releases the nitrogen faster thus has a shorter half time where as the slower compartments have longer half times. I don't recall seeing a specific number associated with a M value, instead measured in half times. Keep in mind, once the compartment exceeds the M value, it becomes a Decompression dive.

While I learned this stuff I don't know how I could use it in planning a dive. My tables and computer do all that work for me. Good stuff to understand but doubtful I will actually apply it in an ordinary dive day.

Bruce

 

[DoOrDive]

Thanks for the replies everyone, but I'm still none the wiser!

I understand the meaning of halftimes and M-Values, but what the book I'm reading (the PADI Divemaster Manual) doesn't explain is why we're not at a higher risk of DCS if compartments which can absorb the highest concentration of nitrogen are also the compartments which release it the quickest upon ascending.

The whole idea of dive computers and tables and safety stops and not ascending faster the 18m per minute - is to manage the amount and speed of nitrogen released into our system, i.e. to not allow too much nitrogen to be released too quickly into our system that our bodies cannot get rid of it (through respiration), and there for, for the nitrogen to form an excessive amount of bubbles and cause DCS.

But these fast compartments with high M-Values, see to contradict that entirely...

Nick

 

[TSandM]

Doordive, try these two papers and see if they help:

http://www.amsta.leeds.ac.uk/~read/scuba/baker1.pdf

http://www.amsta.leeds.ac.uk/~read/scuba/baker2.pdf

Fast compartments tolerate greater overpressure, but profiles that exceed those gradients tend to produce Type II symptoms.

 

[mrlipis]

doesn't explain is why we're not at a higher risk of DCS if compartments which can absorb the highest concentration of nitrogen are also the compartments which release it the quickest upon ascending.

I hope TSand M chimes back in to correct me if I am wrong. Let's assume a 5 minute compartment reaches saturation and let's also assume a 20 minute compartment has also reached saturation. I don't believe either compartment has more or less nitrogen than the other compartment. If the nitrogen could measured in volume, I believe the two compartments would conatain the same volume.
The faster compartment just absorbed the nitrogen faster. Now while this is going on, all of the other compartments are absorbing nitrogen, just at different rates. It must be noted this is all still theoretical. A great theory and probably factual, but still theoretical.

Bruce

 

[Blackwood]

I hope TSand M chimes back in to correct me if I am wrong. Let's assume a 5 minute compartment reaches saturation and let's also assume a 20 minute compartment has also reached saturation. I don't believe either compartment has more or less nitrogen than the other compartment. If the nitrogen could measured in volume, I believe the two compartments would conatain the same volume.
The faster compartment just absorbed the nitrogen faster. Now while this is going on, all of the other compartments are absorbing nitrogen, just at different rates. It must be noted this is all still theoretical. A great theory and probably factual, but still theoretical.

Bruce

If the tissues are of the same size, which they won't be. Fortunately, it isn't volume with which we're concerned, but rather pressure. At any given depth, all tissues which have reached saturation will contain the same inert gas pressure. Some tissues take longer than others to reach saturation (generally considered to be either 5 or 6 half-life cycles).

The theories going back to buhlmann do suppose that the faster tissues can tolerate a greater overpressure gradient. I don't know if I've ever read the theoretical justification behind this assumption, but I would submit that, if a bubble exists, a faster tissue is more likely to get rid of it on ascent than a slower tissue. Maybe. (And it is worth noting that bubbles in fast tissues are more dangerous).

To the best of my understanding, the maximum ascent rate isn't so much to allow you to exhale what you've offgassed, but rather to prevent m-values from being exceeded by allowing sufficient time for offgassing as depth is decreased (and therefore inert gas pressure gradient is increased). That said, you DO obviously have to exhale the gas, but given that it takes something like 5 minutes for the blood (which carries the gas to and from tissues) to make a complete circuit, it's difficult for me to relate that to anything in the 60FPM range (i.e. the assumed rate in the DSAT model).

 

[TSandM]

I don't know the physics behind the assumption that fast compartments will tolerate greater overpressure gradients. I'm sure it's something in the physics or the math; perhaps Wienke's books explain it.

All compartments, when saturated, will be in equilibrium with ambient gas tension. It's just that they take different amounts of time to reach saturation. The volume, or sheer number of molecules of gas that are required to reach that tension, depends on the solubility of the gas in the given material and the volume of the material.

 

[mrlipis]

All compartments, when saturated, will be in equilibrium with ambient gas tension. It's just that they take different amounts of time to reach saturation. The volume, or sheer number of molecules of gas that are required to reach that tension, depends on the solubility of the gas in the given material and the volume of the material.

So much better said. Great links too, but holy crap that information is hard to assimilate. I think my brain is one of those slow tissues. LOL

Bruce